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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Bioorg Med Chem. Author manuscript; available in PMC 2010 July 1.
Published in final edited form as:
PMCID: PMC2695835
NIHMSID: NIHMS113254

2-Oxoamide inhibitors of phospholipase A2 activity and cellular arachidonate release based on dipeptides and pseudodipeptides

Abstract

A series of 2-oxoamides based on dipeptides and pseudodipeptides were synthesized and their activities toward two human intracellular phospholipases A2 (GIVA cPLA2 and GVIA iPLA2) and one human secretory phospholipase A2 (GV sPLA2) were evaluated. Derivatives containing a free carboxyl group are selective GIVA cPLA2 inhibitors. A derivative based on the ethyl ester of an ether pseudodipeptide is the first 2-oxoamide, which preferentially inhibits GVIA iPLA2. The effect of 2-oxoamides on the generation of arachidonic acid from RAW 264.7 macrophages was also studied and it was found that selective GIVA cPLA2 inhibitors preferentially inhibited cellular arachidonic acid release; one pseudodipeptide gave an IC50 value of 2 μM.

Keywords: Dipeptides, Inhibitors, 2-Oxoamides, Phospholipase A2, Pseudodipeptides

1. Introduction

Phospholipase A2 (PLA2) enzymes catalyze the hydrolysis of the sn-2 ester bond of glycerophospholipids producing free fatty acids and lysophospholipids.1,2 The PLA2 superfamily currently consists of fifteen groups and many subgroups of which a number of enzymes differ in primary sequence, structure and catalytic mechanism.1 However, the three predominant types of phospholipase A2 (PLA2) found in human tissues are the cytosolic (such as the GIVA cPLA2), the secreted (such as the GV sPLA2), and the calcium-independent (such as the GVIA iPLA2) enzymes. GIVA cPLA2 preferentially hydrolyzes membrane phospholipids containing arachidonic acid (AA), which is converted to a variety of proinflammatory eicosanoids.3 Therefore, inhibiting AA release is of great therapeutic relevance for the development of new anti-inflammatory drugs. In many cases the activity of GIVA cPLA2 has been shown to be dependent on or linked to the activity of sPLA2.46 GVIA iPLA2 appears to be the primary phospholipase for basal metabolic functions within the cell, and perhaps has additional functions in specific cell types; however its role in inflammation is still unclear.1,2,7,8

The various classes of intracellular and extracellular PLA2 inhibitors have been summarized in recent review articles.911 Our laboratories have developed a novel class of GIVA cPLA2 inhibitors designed to contain the 2-oxoamide functionality and a free carboxyl group.1220 2-Oxoamides based on γ-aminobutyric acid (compound AX006, Figure 1) and the non-natural amino acids γ-(S)-norleucine or δ-(S)-norleucine (compounds AX062 and AX109, Figure 1) are potent inhibitors of GIVA cPLA2 presenting in vivo anti-inflammatory and analgesic activity.12,13,17 In addition, the 2-oxoamide ethyl ester derivative AX048 (Figure 1), which in vitro inhibits both GIVA cPLA2 and GVIA iPLA2, presents a potent anti-hyperalgesic effect.15 Most recently, we have reported that an amide based on γ-(R)-norleucine is a selective inhibitor of GV sPLA2.20

Figure 1
Known 2-oxoamide inhibitors and inhibitors designed for this work.

To extend our studies on the inhibition of phospholipase A2 by 2-oxoamides, we synthesized a variety of 2-oxoamides based on dipeptides and pseudodipeptides and we studied their in vitro activity on three human PLA2 classes: GIVA cPLA2, GVIA iPLA2, and GV sPLA2. Furthermore, to further understand the role and specificity of 2-oxoamide inhibitors in cells, we studied the effect of various 2-oxoamides on arachidonic acid release from RAW 264.7 macrophages.

2. Results and discussion

2.1. Design of inhibitors

Dipeptides are considered to be δ-amino acid analogues, because of the distance between the N- and C-terminal groups (Figure 1). Ether pseudopeptide derivatives may be also considered δ-amino acid derivatives. Thus, in the present work we chose L-norleucine and L-valine as starting materials to synthesize a series of derivatives that contain the 2-oxoamide functionality and an amide group or ether group to replace the two methylenes of the δ-amino acid derivative. The replacement of the peptide bond with a suitable surrogate can increase the stability of peptides toward enzymatic hydrolysis, prolong the half-time of peptide action and improve the transport of peptides into cells.21 In addition, using amide bond isosters is a convenient way to elucidate the role of the amide unit in protein-ligand interactions and thus, to understand the structure-biological activity relationship. The methylene ether group constitutes an interesting amide bond surrogate, since the calculated Cαi-Cαi+1 distance of Ψ [CH2O] pseudodipeptides (3.7 Å) is almost the same as that in a dipeptide (3.8 Å).22 In the past, ether pseudopeptides have been studied such as renin23 and cAMP-dependent protein kinase24 inhibitors, as agonists of substance P,25 and as analogues of the antidiuretic drug desmopressin.26

2.2. Synthesis of inhibitors

N-Protected L-norleucine (1a) and L-valine (1b) were coupled with methyl and ethyl glycinate using 1-(3-dimethylaminopropyl)-3-ethyl carbodiimide (WSCI)27 as a condensing agent in the presence of 1-hydroxybenzotriazole (HOBt) (Scheme 1). Removal of the Boc group, followed by coupling with 2-hydroxyhexadecanoic acid yielded 2-hydroxyamides 3a–c, which were oxidized either by the Dess-Martin method28 or by the NaOCl/AcNH-TEMPO method29,30 to the target compounds 4a–c. Following similar reactions, tert-butyl ester derivatives 8a,b were prepared as depicted in Scheme 2, starting from Z-protected norleucine and valine. 2-Oxoamides 9a,b, containing a free carboxyl group, were obtained by treatment of 8a,b with trifluoroacetic acid.

Scheme 1
Reagents and conditions: (a) HCl.H-Gly-OR2, Et3N, WSCI, HOBt, CH2Cl2; (b) 4N HCl/Et2O; (c) CH3(CH2)13CHOHCOOH, Et3N, WSCI, HOBt, CH2Cl2; (d) Dess-Martin reagent, CH2Cl2; (e) NaOCl, AcNH-TEMPO, NaBr, NaHCO3, EtOAc/PhCH3/H2O 3:3:0.5, 0 °C.
Scheme 2
Reagents and conditions: (a) HCl.H-Gly-OBut, Et3N, WSCI, HOBt, CH2Cl2; (b) H2, 10% Pd/C, EtOH; (c) CH3(CH2)13CHOHCOOH, Et3N, WSCI, HOBt, CH2Cl2; (d) Dess-Martin reagent, CH2Cl2; (e) NaOCl, AcNH-TEMPO, NaBr, NaHCO3, EtOAc/PhCH3/H2O 3:3:0.5, 0 °C; ...

The synthesis of pseudopeptide derivatives is described in Schemes 3 and and4.4. Boc-L-norleucinol (10), obtained by reduction of Boc-L-norleucine,31,32 reacted with ethyl bromoacetate using sodium hydride in the presence of 18-crown-622 to produce ether 11 (Scheme 3). Removal of Boc group, followed by coupling and oxidation yielded 2-oxoamide 13. Pseudodipeptide derivatives 17 and 18 were synthesized by similar procedures (Scheme 4). However, for the synthesis of tert-butyl ester derivative the reaction of Z-L-norleucinol with tert-butyl bromoacetate was carried out under phase transfer conditions.33,34

Scheme 3
Reagents and conditions: (a) BrCH2COOEt, NaH, 18-crown-6, THF; (b) 4N HCl/Et2O; (c) CH3(CH2)13CHOHCOOH, Et3N, WSCI, HOBt, CH2Cl2; (d) Dess-Martin reagent, CH2Cl2.
Scheme 4
Reagents and conditions: (a) BrCH2COOBut, Bu4NHSO4, 50% NaOH, C6H6; (b) H2, 10% Pd/C, EtOH; (c) CH3(CH2)13CHOHCOOH, Et3N, WSCI, HOBt, CH2Cl2; (d) Dess-Martin reagent, CH2Cl2; (e) 50% TFA/CH2Cl2.

2.3. In vitro inhibition of GIVA cPLA2, GVIA iPLA2 and GV sPLA2

All the inhibitors synthesized in this work were tested for inhibition of GIVA cPLA2, GVIA iPLA2 and GV sPLA2 using mixed micellar assays. Details for the assays have been published previously.12,13,16,17 The results of the inhibition are presented in Table 1 using either percent inhibition or XI(50) values. Initially, the percent inhibition of each PLA2 at 0.091 mole fraction of inhibitor was determined. XI(50) values were estimated when the percent inhibition was higher than 90%. The XI(50) is the mole fraction of the inhibitor in the total substrate interface required to inhibit the enzyme by 50%. Data for the reference 2-oxoamide inhibitors AX006 and AX048 are included in Table 1 for comparison.

Table 1
Inhibition of PLA2 by 2-oxoamides based on dipeptides and pseudodipeptides.a

Dipeptide-based 2-oxoamides 9a and 9b containing a free carboxyl group inhibited GIVA cPLA2 without affecting the activity of the other intracellular enzyme GVIA iPLA2. This observation is in full agreement with our previous report, that amino acid-based 2-oxoamides containing a free carboxyl group are selective inhibitors of GIVA cPLA2.16,17

Methyl ester 4a based on dipeptide Nle-Gly and tert-butyl ester 8b based on dipeptide Val-Gly, inhibited both GIVA cPLA2 and GVIA iPLA2, showing a small preference for GIVA cPLA2. The methyl ester 4b based on dipeptide Val-Gly lost the activity against both GIVA cPLA2 and GVIA iPLA2. To the contrary, the tert-butyl ester 8a and the ethyl ester 4c, both based on Nle-Gly dipeptide presented potent inhibition of GVIA iPLA2 with XI(50) values of 0.011 and 0.020, respectively. At the same time, both 8a and 4c inhibited very weakly GIVA cPLA2 and GV sPLA2.

A bioisosteric replacement of the amide bond in compounds 4c, 8a and 9a by an ether group, resulted to the structurally related compounds 13, 17 and 18. Pseudodipeptide derivative 18, selectively inhibited GIVA cPLA2, as expected, due to the free carboxyl group. This derivative is the most potent inhibitor of GIVA cPLA2 [XI(50) 0.017] identified in the present work and its potency is comparable to that of the reference 2-oxoamide inhibitor AX006 [XI(50) 0.024].15 Interestingly, both ethyl ester 13 and tert-butyl ester 17 preferentially inhibited GVIA iPLA2. In particular, derivative 13 is a potent inhibitor of GVIA iPLA2 with a XI(50) value of 0.017, while high mole fraction (0.091) of the inhibitor caused only 52% inhibition of GIVA cPLA2.

Among the compounds tested in this study, only two esters (compounds 8b and 13) were able to reduce GV sPLA2 activity (around 80% inhibition at 0.091 mole fraction). GV sPLA2 utilizes a catalytic histidine to activate a water molecule as the nucleophile in phospholipid hydrolysis. Although there is no serine nucleophile in GV sPLA2, the 2-oxoamides may resemble the substrate phospholipids or the transition state such that they would bind to the GV sPLA2 active site and inhibit the enzyme.

Both the intracellular enzymes GIVA cPLA2 and GVIA iPLA2 share the same catalytic mechanism using active site serine as the nucleophilic residue. Thus, compounds designed to inhibit GIVA cPLA2, may show cross reactivity with GVIA iPLA2. Very interestingly, ethyl and tert-butyl esters 4c, 8a, 13 and 17, based on Nle-Gly or the corresponding ether pseudodipeptide preferentially inhibit GVIA iPLA2. Up to now, we have shown that 2-oxoamide esters may inhibit both GIVA cPLA2 and GVIA iPLA2 either with similar potency or with a preference for GIVA cPLA2.16 However, we identified for the first time in the course of the present work several 2-oxoamides, such as 8a or 13, which show higher inhibition against GVIA iPLA2 than GIVA cPLA2. It seems that the replacement of the amino acid unit of a 2-oxoamide inhibitor by a dipeptide unit may shift the selectivity in favour of GVIA iPLA2. Since there is a lack of potent and selective inhibitors for GVIA iPLA2,35 the findings of the present study may help in designing 2-oxoamides presenting selectivity for this particular PLA2.

2.4. Ex vivo inhibition of cellular arachidonic acid release

Arachidonic acid (AA) is the ideal metabolic indicator of GIVA cPLA2 activity, since it is the product of GIVA cPLA2-mediated catalysis. In addition to some of the new 2-oxoamides based on dipeptides and pseudodipeptides that were synthesized in this work, a series of potent 2-oxoamide in vitro inhibitors of GIVA cPLA2, previously reported by us,12,13,16,17,19,20 were further tested in a cellular ex vivo system to investigate whether they reduced cellular AA production. For this purpose, RAW 264.7 macrophages were preincubated with 25 μM concentrations of the inhibitor prior to treatment of the cells with Kdo2-Lipid A. Subsequently, AA release was quantitated from the supernatants.

The results of our studies for twenty one 2-oxoamides are summarized in Table 2. Note that some of the compounds led to an actual increase or activation of AA release which is not unusual in this kind of assay due to detergent or other non-specific effects. Those compounds giving an activation of AA release were not evaluated further. Compounds 18, 19,12 20,12 25, 26, 3016 and 35,17 which contain a free carboxyl group and are potent and selective inhibitors of GIVA cPLA2, induce a significant reduction of AA release. Derivatives 26 and 30, based on γ-norleucine, cause the highest inhibition percentage in AA release. Both strongly inhibit GIVA cPLA2. Comparison between compounds 20 and 26 indicates the importance of the long aliphatic chain. Compound 19, based on γ-aminobutyric acid and compound 25, based on γ-leucine, cause significant inhibition, though to a lesser extent, of AA production. Moderate inhibitors of GIVA cPLA2 result to a lower inhibition of AA release (compounds 21,17 2820). Compound 35,17 a δ-norleucine derivative, and compound 18, based on an pseudodipeptide, which also can be considered as a δ-amino acid analogue, lead to a considerable AA reduction in cells. Both molecules are strong inhibitors of GIVA cPLA2.

Table 2
Effect of 2-oxoamide inhibitors on Kdo2-Lipid A stimulated AA release from RAW 264.7 macrophages.

Although production of arachidonic acid can be catalyzed by other phospholipases as well, such as GVIA iPLA2 and GV sPLA2, most of the non-selective inhibitors of the three phospholipases tested here, did not show a significant inhibitory effect on AA release. Compounds 23,17 24,15 27,20 29,16 3120 and 36,20 carry an esterified carboxyl group. In compound 32,19 the ester group has been replaced by a sulfonamide group. They all inhibit GIVA cPLA2 to a high extent, though not selectively, since they inhibit also both GVIA iPLA2 and GV sPLA2.

Dipeptide inhibitor 8b reduced the cellular release of AA by 30%, while derivative 18 reduced AA production by 68%. Interestingly, the 2-oxoamide inhibitor 18, which contains a free carboxyl group, leads to more than double the AA release inhibition, indicating the importance of selectivity of an inhibitor against GIVA cPLA2.

2.5. IC50 values for arachidonic acid inhibition

Dose response curves were measured for the seven synthetic inhibitors that reduced AA release the most (see Table 2) in order to calculate the IC50 values. All of these contained free carboxylic acids and were specific for GIVA cPLA2. The IC50 values for these seven compounds are summarized in Table 3. The individual dose response curve for the inhibition of AA release from RAW 264.7 cells stimulated with Kdo2-Lipid A in the presence of 18 is depicted in Figure 1. Inhibitors 19, 25, 26 and 30 all display IC50 values of approximately 25 μM whereas 20, 35 and 18 display values of 10, 7 and 2 μM, respectively. Interestingly, inhibitors 35 and 18, which contain the carboxylic acid functional group spaced at the δ position relative to the 2-oxoamide moiety, demonstrated the lowest IC50 values at 7 and 2 μM, respectively. In contrast, the five γ linked inhibitors, 19, 20, 25, 26 and 30, all displayed higher IC50 values at 25, 10, 25, 25 and 25 μM. These data suggest that inhibitors with δ structural spacing display more effective AA release inhibition than γ structural spacing within this inflammatory cellular model. Of course, this specificity may relate to the ability of the compound to be taken up by the cell and other factors, but these results demonstrate that the oxoamides are able to inhibit within the cellular milieu.

Table 3
IC50 values for the inhibition of arachidonic acid release.

3. Conclusions

In conclusion, we have synthesized new 2-oxoamides based on dipeptides and pseudodipeptides and we have confirmed that derivatives containing a free carboxyl group are selective GIVA cPLA2 inhibitors and also preferentially inhibit AA release intracellularly. 2-Oxoamides ethyl and tert-butyl esters, based on the dipeptide Nle-Gly or the corresponding ether pseudodipeptide preferentially inhibit the other major intracellular enzyme GVIA iPLA2. Three of them are potent inhibitors of GVIA iPLA2 with XI(50) values between 0.011–0.020. We demonstrated that selective GIVA cPLA2 inhibitors inhibit AA release in RAW 264.7 macrophages and a 2-oxoamide based on a pseudodipeptide inhibits AA release with an IC50 value of 2 μM.

4. Experimental Section

4.1. General

Melting points were determined on a Buchi 530 apparatus and are uncorrected. Specific rotations were measured at 25 °C on a Perkin-Elmer 343 polarimeter using a 10 cm cell. NMR spectra were recorded on a Varian Mercury (200 MHz) spectrometer. All amino acid derivatives were purchased from Fluka Chemical Co. Electron spray ionization (ESI) mass spectra were recorded on a Finnigan, Surveyor MSQ Plus spectrometer. TLC plates (silica gel 60 F254) and silica gel 60 (70–230 or 230–400 mesh) for column chromatography were purchased from Merck. Visualization of spots was effected with UV light and/or phosphomolybdic acid and/or ninhydrin, both in EtOH stain. THF was dried by standard procedures and stored over molecular sieves or Na. All other solvents and chemicals were reagent grade and used without further purification. Inhibitors 25 and 26 were prepared according to procedures described in the literature.13,17

4.2. Synthesis of 2-Oxoamide Inhibitors

4.2.1. General method for the coupling of 2-hydroxy-hexadecanoic acid with amino components

To a stirred solution of 2-hydroxy-hexadecanoic acid (0.27 g, 1.0 mmol) and hydrochloride amino component (1.0 mmol) in CH2Cl2 (10 mL), Et3N (0.3 mL, 2.2 mmol) and subsequently 1-(3-dimethylaminopropyl)-3-ethyl carbodiimide hydrochloride (WSCI) (0.21 g, 1.1 mmol) and 1-hydroxybenzotriazole (HOBt) (0.14 g, 1.0 mmol) were added at 0 °C. The reaction mixture was stirred for 1 h at 0 °C and overnight at room temperature. The solvent was evaporated under reduced pressure and EtOAc (20 mL) was added. The organic layer was washed consecutively with brine, 1N HCl, brine, 5% NaHCO3, and brine, dried over Na2SO4 and evaporated under reduced pressure. The residue was purified by column-chromatography using CHCl3–CHCl3/MeOH 99:1 as eluent.

4.2.1.1. Methyl 2-((S)-2-(2-hydroxyhexadecanamido)hexanamido)acetate (mixture of diastereomers) (3a)

Yield 68%; White solid; mp 78–80 °C; 1H NMR (200 MHz, CDCl3) δ 7.53–7.45 (m, 1H), 7.40 (d, J = 8.6 Hz, 1H), 4.59–4.43 (m, 1H), 4.15–4.06 (m,1H), 4.02–3.94 (m, 2H), 3.71 (s, 3H), 1.89–1.53 (m, 4H), 1.23 (br s, 28H), 0.98–0.80 (m, 6H); 13C NMR (50 MHz, CDCl3) δ 175.1 (174.8), 172.7 (172.5), 170.1 (170.0), 72.0 (71.9), 52.4 (52.7), 52.2, 41.1, 34.8 (34.6), 32.0 (31.8), 29.6, 29.5, 29.3, 27.6, 27.5, 25.0, 22.6, 22.3, 22.2, 14.0, 13.8. Anal. Calcd for C25H48N2O5: C, 65.75; H, 10.59; N, 6.13. Found: C, 65.49; H, 10.78; N, 6.01.

4.2.1.2. Methyl 2-((S)-2-(2-hydroxyhexadecanamido)-3-methylbutanamido)acetate (mixture of diastereomers) (3b)

Yield 73%; mp 109–111 °C; 1H NMR (200 MHz, CDCl3) δ 7.34–7.15 (m, 2H), 4.43–4.27 (m, 1H), 4.10–3.96 (m, 3H), 3.75 (s, 3H), 2.24–2.08 (m, 1H), 1.87–1.63 (m, 2H), 1.26 (br s, 24H), 1.05–0.91 (m, 6H), 0.88 (t, J = 6.6 Hz, 3H). Anal. Calcd for C24H46N2O5: C, 65.12; H, 10.47; N, 6.33. Found: C, 64.98; H, 10.68; N, 6.18.

4.2.1.3. Ethyl 2-((S)-2-(2-hydroxyhexadecanamido)hexanamido)acetate (mixture of diastereomers) (3c)

Yield 64%; White solid; mp 77–79 °C; 1H NMR (200 MHz, CDCl3) δ 7.31–7.07 (m, 2H), 4.59–4.42 (m, 1H), 4.25–4.06 (m, 3H), 4.02–3.95 (m, 2H), 1.97–1.51 (m, 4H), 1.24 (br s, 31H), 0.97–0.80 (m, 6H); 13C NMR (50 MHz, CDCl3) δ 174.9 (174.5), 172.4 (172.1), 169.7 (169.5), 72.1 (72.0), 61.5, 52.5 (52.7), 41.3, 34.6 (34.8), 31.9, 29.7, 29.4, 29.3, 27.6, 25.0, 22.7, 22.3, 14.1, 13.9. Anal. Calcd for C26H50N2O5: C, 66.35; H, 10.71; N, 5.95. Found: C, 66.19; H, 10.99; N, 5.89.

4.2.1.4. tert-Butyl 2-((S)-2-(2-hydroxyhexadecanamido)hexanamido)acetate (mixture of diastereomers) (7a)

Yield 69%; White solid; mp 50–52 °C; 1H NMR (200 MHz, CDCl3) δ 7.37–7.26 (m, 1H), 7.23–7.08 (m, 1H), 4.58–4.45 (m, 1H), 4.15–4.03 (m, 1H), 3.97–3.81 (m, 2H), 1.98–1.52 (m, 4H), 1.44 (s, 9H), 1.24 (br s, 28H), 0.98–0.79 (m, 6H); 13C NMR (50 MHz, CDCl3) δ 174.9 (174.6), 172.4 (172.1), 168.6 (168.7), 82.2, 72.1 (72.0), 52.9 (52.5), 42.0, 34.8 (34.6), 32.1, 31.8, 29.6, 29.5, 29.3, 27.9, 27.6, 27.5, 25.0, 22.6, 22.3, 22.2, 14.0, 13.8. Anal. Calcd for C28H54N2O5: C, 67.43; H, 10.91; N, 5.62. Found: C, 67.28; H, 11.08; N, 5.47.

4.2.1.5. tert-Butyl 2-((S)-2-(2-hydroxyhexadecanamido)-3-methylbutanamido)acetate (mixture of diastereomers) (7b)

Yield 81%; White solid; mp 81–83 °C; 1H NMR (200 MHz, CDCl3) δ 7.25 (d, J = 8.8 Hz, ½H), 7.16 (d, J = 8.8 Hz, ½H), 6.96 (t, J = 4.8 Hz, ½H), 6.88 (t, J = 4.8 Hz, ½H), 4.34–4.22 (m, 1H), 4.18–4.03 (m, 1H), 3.92–3.81 (m, 2H), 2.23–2.05 (m, 1H), 1.84–1.48 (m, 2H), 1.42 (s, 9H), 1.21 (br s, 24H), 0.93 (t, J = 5.8 Hz, 6H), 0.84 (t, J = 6.6 Hz, 3H); 13C (50 MHz, CDCl3) NMR δ 174.7 (175.0), 171.7 (171.4), 168.8 (168.6), 82.4, 72.3 (72.0), 58.1 (57.9), 42.0, 34.9 (34.6), 31.9, 30.8 (30.7), 29.7, 29.6, 29.4, 29.3, 28.0, 25.0, 22.6, 19.3, 18.1, 14.1. Anal. Calcd for C27H52N2O5: C, 66.90; H, 10.81; N, 5.78. Found: C, 66.65; H, 10.98; H, 5.62.

4.2.1.6. Ethyl 2-((S)-2-(2-hydroxyhexadecanamido)hexyloxy)acetate (mixture of diastereomers) (12)

Yield 69%; Waxy white solid; 1H NMR (200 MHz, CDCl3) δ 7.03–6.92 (m, 1H), 4.25–3.95 (m, 6H), 3.65–3.42 (m, 2H), 1.83–1.53 (m, 4H), 1.25 (br s, 31H), 0.95–1.79 (m, 6H); 13C NMR (50 MHz, CDCl3) δ 174.2 (173.9), 170.8 (170.7), 73.2 (73.1), 72.2 (71.8), 68.0 (68.1), 61.0, 48.5 (48.8), 34.9 (34.7), 31.8, 31.0, 29.6, 29.5, 29.4, 29.3, 28.1, 24.9, 24.8, 22.6, 22.5, 14.0, 13.9. Anal. Calcd for C26H51NO5: C, 68.23; H, 11.23; N, 3.06. Found: C, 68.04; H, 11.34; N, 2.91.

4.2.1.7. tert-Butyl 2-((S)-2-(2-hydroxyhexadecanamido)hexyloxy)acetate (mixture of diastereomers) (16)

Yield 77%; Oil; 1H NMR (200 MHz, CDCl3) δ 7.05 (d, J = 8.8 Hz, 1H), 4.17–3.96 (m, 2H), 3.93 (s, 2H), 3.73 (br s, 1H), 3.57 (dd, J1 = 9.6 Hz, J2 = 3.6 Hz, 1H), 3.45 (dd, J1 = 8.8 Hz, J2 = 3.6 Hz, 1H), 1.90–1.55 (m, 4H), 1.45 (s, 9H), 1.23 (br s, 28H), 0.98–1.78 (m, 6H); 13C NMR (50 MHz, CDCl3) δ 174.2 (174.0), 170.1, 82.0, 73.2, 72.2 (71.7), 68.4 (68.6), 48.7 (48.9), 34.7 (35.0), 31.9, 31.0, 29.6, 29.5, 29.3, 28.2, 28.0, 24.9, 24.8, 22.6, 22.5, 14.0, 13.9. Anal. Calcd for C28H55NO5: C, 69.23; H, 11.41; N, 2.88. Found: C, 69.01; H, 11.59; N, 2.73.

4.2.2. General method for the oxidation of 2-hydroxy-amides. Method A

To a solution of 2-hydroxy-amide (1 mmol) in dry CH2Cl2 (10 mL) Dess-Martin periodinane was added (0.64 gr, 1.5 mmol) and the mixture was stirred for 1 h at room temperature. The organic solution was washed with 10% aqueous NaHCO3, dried over Na2SO4 and the organic solvent was evaporated under reduced pressure. The residue was purified by column-chromatography using CHCl3 as eluent.

4.2.2.1. (S)-Methyl 2-(2-(2-oxohexadecanamido)hexanamido)acetate (4a)

Yield 85%; White solid; mp 65–67 °C; [α]D = −21.8 (c 0.5 CHCl3); 1H NMR (200 MHz, CDCl3) δ 7.50 (d, J = 8.4 Hz, 1H), 6.74 (t, J = 5.4 Hz, 1H), 4.49–4.37 (m, 1H), 4.17–3.95 (m, 2H), 3.75 (s, 3H), 2.89 (t, J = 7.6 Hz, 2H), 2.05–1.47 (m, 4H), 1.24 (br s, 26H), 0.97–0.81 (m, 6H); 13C NMR (50 MHz, CDCl3) δ 198.3, 170.9, 170.0, 160.0, 53.0, 52.4, 41.1, 36.8, 31.9, 31.8, 29.6, 29.5, 29.4, 29.3, 29.0, 27.4, 23.0, 22.6, 22.3, 14.1, 13.8; MS (ESI): m/z (%): 477 (77) [M+Na]+; Anal. Calcd for C25H46N2O5: C, 66.04; H, 10.20; N, 6.16. Found: C, 66.19; H, 10.13; N, 6.21.

4.2.2.2. (S)-Ethyl 2-(2-(2-oxohexadecanamido)hexanamido)acetate (4c)

Yield 76%; White solid; mp 63–65 °C; [α]D = −20.8 (c 0.5 CHCl3); 1H NMR (200 MHz, CDCl3) δ 7.51 (d, J = 8.4 Hz, 1H), 6.76 (t, J = 5.2, 1H), 4.50–4.36 (m, 1H), 4.19 (q, J = 7 Hz, 2H), 4.12–3.91 (m, 2H), 2.88 (t, J = 7.4 Hz, 2H), 2.05–1.45 (m, 4H), 1.26 (br s, 29H), 0.95–0.80 (m, 6H); 13C NMR (50 MHz, CDCl3) δ 198.3, 170.9, 169.5, 160.1, 61.6, 53.0, 41.3, 36.8, 32.0, 31.9, 29.6, 29.5, 29.4, 29.3, 29.0, 27.5, 23.0, 22.6, 22.3, 14.1, 13.8; MS (ESI): m/z (%): 491 (100) [M+Na]+, 469 (55) [M+]+; Anal. Calcd for C26H48N2O5: C, 66.63; H, 10.32; N, 5.98. Found: C, 66.58; H, 10.39; N, 5.91.

4.2.2.3. (S)-tert-Butyl 2-(2-(2-oxohexadecanamido)hexanamido)acetate (8a)

Yield 85%; White solid; mp 38–39 °C; [α]D = −17.2 (c 0.5 CHCl3); 1H NMR (200 MHz, CDCl3) δ 7.47 (d, J = 8.4 Hz, 1H), 6.57 (t, J = 5 Hz, 1H), 4.48–4.35 (m, 1H), 4.03–3.80 (m, 2H), 2.89 (t, J = 8 Hz, 2H), 2.02–1.52 (m, 4H), 1.45 (s, 9H), 1.24 (br s, 26H), 0.97–0.79 (m, 6H); 13C NMR (50 MHz, CDCl3) δ 198.3, 170.7, 168.5, 160.1, 82.4, 53.1, 42.0, 36.8, 32.0, 31.9, 29.6, 29.5, 29.4, 29.3, 29.0, 28.0, 27.5, 23.1, 22.6, 22.3, 14.1, 13.8; MS (ESI): m/z (%): 519 (100) [M+Na]+, 497 (32) [M+H]+; Anal. Calcd for C28H52N2O5: C, 67.70; H, 10.55; N, 5.64. Found: C, 67.58; H, 10.73; N, 5.58.

4.2.2.4. (S)-Ethyl 2-(2-(2-oxohexadecanamido)hexyloxy)acetate (13)

Yield 88%; White solid; mp 41–43 °C; [α]D = −8.4 (c 0.5 CHCl3); 1H NMR (200 MHz, CDCl3) δ 7.29 (d, J = 9.6 Hz, 1H), 4.21 (q, J = 7.4 Hz, 2H), 4.06 (s, 2H), 4.05–3.91 (m, 1H), 3.64 (dd, J1 = 9.6 Hz, J2 = 4.0 Hz, 1H), 3.52 (dd, J1 = 9.4 Hz, J2 = 3.8 Hz, 1H), 2.90 (t, J = 6.6 Hz, 2H), 1.71–1.48 (m, 4H), 1.24 (br s, 29H), 0.96–0.80 (m, 6H); 13C NMR (50 MHz, CDCl3) δ 199.2, 170.3, 159.9, 72.5, 68.2, 60.9, 49.4, 36.8, 31.9, 30.9, 29.6, 29.5, 29.4, 29.3, 29.0, 28.0, 23.1, 22.6, 22.4, 14.1, 13.9; MS (ESI): m/z (%): 478 (100) [M+Na]+; Anal. Calcd for C26H49NO5: C, 68.53; H, 10.84; N, 3.07. Found: C, 68.65; H, 10.71; N, 3.12.

4.2.2.5. (S)-tert-Butyl 2-(2-(2-oxohexadecanamido)hexyloxy)acetate (17)

Yield 93%; white solid; Low mp; [α]D = −10.8 (c 0.5 CHCl3); 1H NMR (200 MHz, CDCl3) δ 7.26 (d, J = 8.8 Hz, 1H), 4.03–3.85 (m, 3H), 3.61 (dd, J1 = 9.4 Hz, J2 = 4.0 Hz, 1H), 3.49 (dd, J1 = 9.4 Hz, J2 = 4.0 Hz, 1H), 2.89 (t, J = 7.2 Hz, 2H), 1.69–1.48 (m, 4H), 1.45 (s, 9H), 1.23 (br s, 26H), 0.96–0.79 (m, 6H); 13C NMR (50 MHz, CDCl3) δ 199.2, 169.4, 160.0, 81.7, 72.4, 68.7, 49.4, 36.7, 31.8, 31.0, 29.6, 29.5, 29.4, 29.3, 29.0, 28.0, 23.1, 22.6, 22.4, 14.0, 13.9; MS (ESI): m/z (%): 506 (57) [M+Na]+; Anal. Calcd for C28H53NO5: C, 69.52; H, 11.04; N, 2.90. Found: C, 69.57; H, 11.08; N, 2.83.

4.2.3. General method for the oxidation of 2-hydroxy-amides. Method B

To a solution of 2-hydroxy-amide (1.0 mmol) in a mixture of toluene (3 mL) and EtOAc (3 mL) a solution of NaBr (0.11 g, 1.1 mmol) in water (0.5 mL) was added followed by AcNH-TEMPO (2.2 mg, 0.01 mmol). To the resulting biphasic system, which was cooled at 0 °C, an aqueous solution of 0.35 M NaOCl (3.1 mL, 1.1 mmol) containing NaHCO3 (0.25 g, 3 mmol) was added dropwise under vigorous stirring, at 0 °C over a period of 1 h. After the mixture had been stirred for a further 15 min at 0 °C, EtOAc (10 mL) and H2O (10 mL) were added. The aqueous layer was separated and washed with EtOAc (20 mL). The combined organic layers were washed consecutively with 5% aqeous citric acid (10 mL) containing KI (0.04 g), 10% aqueous Na2S2O3 (10 mL), and brine and dried over Na2SO4. The solvents were evaporated under reduced pressure and the residue was purified by column-chromatography using CHCl3 as eluent.

4.2.3.1. (S)-Methyl 2-(3-methyl-2-(2-oxohexadecanamido)butanamido)acetate (4b)

Yield 73%; White solid; mp 118–119 °C; [α]D = −20.9 (c 1 CHCl3); 1H NMR (200 MHz, CDCl3) δ 7.46 (d, J = 9.2 Hz, 1H), 6.53–6.37 (m, 1H), 4.24 (dd, J1 = 6.6 Hz, J2 = 9.2 Hz, 1H), 4.05 (dd, J1 = 5.2, J2 = 9.6 Hz, 2H), 3.77 (s, 3H), 2.91 (t, J = 7.4 Hz, 2H), 2.22 (m, 1H), 1.61 (m, 2H), 1.26 (br s, 22H), 0.98 (t, J = 5.8 Hz, 6H), 0.88 (t, J = 6.6 Hz, 3H); 13C NMR (50 MHz, CDCl3) δ 198.3, 170.9, 170.0, 160.2, 58.6, 52.5, 41.1, 36.8, 31.9, 31.0, 29.6, 29.4, 29.3, 29.0, 23.1, 22.7, 19.2, 18.0, 14.1; Anal. Calcd for C24H44N2O5: C, 65.42; H, 10.07; N, 6.36. Found: C, 65.55; H, 9.97; N, 6.41.

4.2.3.2. (S)-tert-Butyl 2-(3-methyl-2-(2-oxohexadecanamido)butanamido)acetate (8b)

Yield 90%; White solid; mp 58–61 °C; [α]D = –17.2 (c 1 CHCl3); 1H NMR (200 MHz, CDCl3) δ 7.50 (d, J = 8.8 Hz, 1H), 6.59–6.45 (m, 1H), 4.34–3.18 (m, 1H), 4.01 (dd, J1 = 18.2 Hz, J2 = 5.4 Hz, 1H), 3.85 (dd, J1 = 18.2 Hz, J2 = 4.8 Hz, 1H), 2.89 (t, J = 6.8 Hz, 2H), 2.28–2.19 (m, 1H), 1.64–1.48 (m, 2H), 1.45 (s, 9H), 1.24 (br s, 22H), 0.96 (t, J = 5.8 Hz, 6H), 0.87 (t, J = 6.6 Hz, 3H); 13C NMR (50 MHz, CDCl3) δ 198.3, 170.1, 168.5, 160.2, 82.4, 58.5, 41.9, 36.8, 31.9, 31.1, 29.6, 29.5, 29.4, 29.3, 29.0, 28.0, 23.1, 22.6, 19.2, 18.0, 14.1; MS (FAB): m/z (%): 483 (24) [M+H]+; Anal. Calcd for C27H50N2O5: C, 67.18; H, 10.44; N, 5.80. Found: C, 67.31; H, 10.39; N, 5.68.

4.2.4. General method for the cleavage of tert-butyl protecting group

A solution of the tert-butyl ester derivative (1 mmol) in 50% TFA/CH2Cl2 (10 mL) was stirred for 1h at room temperature. The organic solvent was evaporated under reduced pressure. The residue was purified by recrystallization [EtOAc/petroleum ether (bp 40–60 °C)].

4.2.4.1. (S)-2-(2-(2-Oxohexadecanamido)hexanamido)acetic acid (9a)

Yield 61%; Colorless oil; 1H NMR (200 MHz, CDCl3) δ 7.66 (d, J = 8.8 Hz, 1H), 7.20-7.05 (m, 1H), 4.60-4.43 (m, 1H), 4.06 (d, J = 3.6 Hz, 2H), 2.88 (t, J = 3.8 Hz, 2H), 1.83–1.59 (m, 4H), 1.25 (br s, 26H), 0.89 (t, J = 8.0 Hz, 6H); 13C NMR (50 MHz, CDCl3) δ 198.1, 172.6, 171.4, 160.1, 53.0, 41.4, 36.9, 32.1, 31.9, 29.6, 29.4, 29.3, 29.0, 27.5, 23.0, 22.7, 22.3, 14.1, 13.8; MS (ESI): m/z (%): 442 (100) [M+H]+; Anal. Calcd for C24H44N2O5: C, 65.42; H, 10.07; N, 6.36. Found: C, 65.19; H, 10.32; N, 6.25.

4.2.4.2. (S)-2-(3-Methyl-2-(2-oxohexadecanamido)butanamido)acetic acid (9b)

Yield 68%; White solid; mp 87–89 °C; [α]D = −1.8 (c 0.5 CHCl3); 1H NMR (200 MHz, CDCl3) δ 7.71 (d, J = 9 Hz, 1H), 7.21–7.06 (m, 1H), 4.45–4.34 (m, 1H), 4.18–4.01 (m, 2H), 3.03–2.72 (m, 2H), 2.25–2.04 (m, 1H), 1.68–1.45 (m, 2H), 1.25 (br s, 22H), 0.97 (t, J = 5.8 Hz, 6H), 0.88 (t, J = 6.6 Hz, 3H); 13C NMR (50 MHz, CDCl3) δ 198.0, 172.3, 170.9, 160.4, 58.6, 41.3, 36.9, 31.9, 31.1, 29.6, 29.4, 29.3, 29.0, 23.0, 22.7, 19.1, 18.1, 14.1; MS (ESI): m/z (%): 425 (100) [M−H]; Anal. Calcd for C23H42N2O5: C, 64.76; H, 9.92; N, 6.57. Found: C, 64.65; H, 9.87; N, 6.63.

4.2.4.3. (S)-2-(2-(2-oxohexadecanamido)hexyloxy)acetic acid (18)

Yield 88%; White solid; mp 64–66 °C; [α]D = −5.2 (c 0.5 CHCl3); 1H NMR (200 MHz, CDCl3) δ 7.32 (d, J = 8.8 Hz, 1H), 4.14 (s, 2H), 4.07–3.95 (m, 1H), 3.66 (dd, J1 = 9.6 Hz, J2 = 4.8 Hz, 1H), 3.55 (dd, J1 = 9.6 Hz, J2 = 3.6 Hz, 1H), 2.90 (t, J = 7.4 Hz, 2H), 1.75–1.46 (m, 4H), 1.25 (br s, 26H), 0.98–0.80 (m, 6H); 13C NMR (50 MHz, CDCl3) δ 199.1, 174.6, 160.2, 72.9, 67.8, 49.5, 36.8, 31.9, 30.8, 29.6, 29.4, 29.3, 29.0, 28.0, 23.2, 22.6, 22.4, 14.1, 13.9; MS (ESI): m/z (%): 450 (100) [M+Na]+; Anal. Calcd for C24H45NO5: C, 67.41, H, 10.61, N, 3.28. Found: C, 67.59, H, 10.64, N, 3.14.

4..5. General method for the synthesis of dipeptides

The dipeptides were prepared following the general method 4.2.1.

4.2.5.1. (S)-Methyl 2-(2-(tert-butoxycarbonylamino)hexanamido)acetate (2a).38,39

Yield 65%; White solid; mp 95–96 °C; [α]D = −17.8 (c 1 CHCl3); 1H NMR (200 MHz, CDCl3) δ 6.85–6.73 (m, 1H), 5.12 (d, J = 8.2 Hz, 1H), 4.22–4.07 (m, 1H), 4.03 (d, J = 5.6 Hz, 2H), 3.74 (s, 3H), 1.93–1.72 (m, 1H), 1.69–1.48 (m, 1H), 1.43 (s, 9H), 1.39–1.21 (m, 4H), 0.88 (t, J = 6.6 Hz, 3H); 13C NMR (50 MHz, CDCl3) δ 172.6, 170.1, 155.7, 80.0, 54.4, 52.3, 41.0, 32.2, 28.2, 27.6, 22.3, 13.9.

4.2.5.2. (S)-Methyl 2-(2-(tert-butoxycarbonylamino)-3-methylbutanamido)acetate (2b)

Yield 73%; White solid; mp 104–106 °C; [α]D = −15.7 (c 1 CHCl3); 1H NMR (200 MHz, CDCl3) δ 6.64 (m, 1H), 5.09 (d, J = 7.6 Hz, 1H), 4.06–3.96 (m, 3H), 3.76 (s, 3H), 2.19 (m, 1H), 1.42 (s, 9H), 0.99 (d, J = 7.0 Hz, 3H), 0.94 (d, J = 7.0 Hz, 3H); 13C NMR (50 MHz, CDCl3) δ 171.9, 170.1, 155.7, 80.0, 59.7, 52.3, 41.0, 30.8, 28.2, 19.2, 18.0. Anal. Calcd for C13H24N2O5: C, 54.15; H, 8.39; N, 9.72. Found: C, 53.92; H, 8.52; N, 9.61.

4.2.5.3. (S)-Ethyl 2-(2-(tert-butoxycarbonylamino)hexanamido)acetate (2c).40

Yield 69%; Oil; [α]D = −14.8 (c 1 CHCl3); 1H NMR (200 MHz, CDCl3) δ 7.00–6.85 (m, 1H), 5.23 (d, J = 7.2 Hz, 1H), 4.22–4.05 (m, 3H), 4.02–3.93(m, 2H), 1.91–1.69 (m, 1H), 1.66–1.45 (m, 1H), 1.40 (s, 9H), 1.35–1.18 (m, 7H), 0.86 (t, J = 6.6 Hz, 3H); 13C NMR (50 MHz, CDCl3) δ 172.7, 169.6, 155.6, 79.8, 61.3, 54.3, 41.1, 32.3, 28.2, 27.5, 22.3, 14.0, 13.8.

4.2.5.4. (S)-tert-Butyl 2-(2-(benzyloxycarbonylamino)hexanamido)acetate (6a)

Yield 64%; White solid; mp 80–82 °C; [α]D = −8.4 (c 0.5 CHCl3); 1H NMR (200 MHz, CDCl3) δ 7.41–7.26 (m, 5H), 6.62–6.49 (m, 1H), 5.41 (d, J = 8.0 Hz, 1H), 5.12 (s, 2H), 4.29–4.11 (m, 1H), 3.92 (d, J = 4.6 Hz, 2H), 1.98–1.77 (m, 1H), 1.75–1.59 (m, 1H), 1.47 (s, 9H), 1.42–1.21 (m, 4H), 0.89 (t, J = 7 Hz, 3H); 13C NMR (50 MHz, CDCl3) δ 171.9, 168.7, 156.1, 136.2, 128.5, 128.1, 128.0, 82.3, 67.0, 54.9, 42.0, 32.4, 28.0, 27.5, 22.3, 13.8. Anal. Calcd for C20H30N2O5: C, 63.47; H, 7.99; N, 7.40. Found: C, 63.28; H, 8.15; H, 7.28.

4.2.5.5. (S)-tert-Butyl 2-(2-(benzyloxycarbonylamino)-3-methylbutanamido)acetate (6b)

Yield 81%; White solid; mp 139–141 °C; [α]D = −6.8 (c 0.5 CHCl3); 1H NMR (200 MHz, CDCl3) δ 7.42–7.28 (m, 5H), 6.47–6.35 (m, 1H), 5.39 (d, J = 8.6 Hz, 1H), 5.12 (s, 2H), 4.11–4.00 (m, 1H), 4.99–3.83 (m, 2H), 2.28–2.05 (m, 1H), 1.47 (s, 9H), 0.99 (d, J = 7.0 Hz, 3H), 0.94 (d, J = 7.0 Hz, 3H); 13C NMR (50 MHz, CDCl3) δ 171.2, 168.6, 156.0, 136.1, 128.5, 128.2, 128.0, 82.4, 67.1, 60.3, 42.0, 31.0, 28.0, 19.2, 17.6. Anal. Calcd for C19H28N2O5: C, 62.62; H, 7.74; N, 7.69. Found: C, 62.39; H, 7.91; N, 7.55.

4.2.6. (S)-Ethyl 2-(2-(tert-butoxycarbonylamino)hexyloxy)acetate (11)

To as stirred solution of Boc-L-Nle-ol (0.50 g, 1.9 mmol) in dry THF (15 mL), cooled at 0 °C under nitrogen, NaH (0.05 g, 2.1 mmol) was added. The reaction mixture was stirred for 45 min at 0 °C, followed by addition of 18-crown-6 (0.25 g, 0.9 mmol) and subsequently a solution of ethyl bromoacetate (0.47 g, 2.8 mmol) in dry THF (4 mL) was added dropwise at 0 °C. The reaction mixture was stirred for 1 h at 0 °C and overnight at room temperature. The organic solvent was evaporated under reduced pressure and the residue was purified by column-chromatography using CHCl3 as eluent. Yield 55%; Oil; [α]D = −13.9 (c 1 CHCl3); 1H NMR (200 MHz, CDCl3) δ 4.85–4.78 (m, 1H), 4.17 (q, J = 7.4 Hz, 2H), 4.03 (s, 2H), 3.72–3.58 (m, 1H), 3.56–3.41 (m, 2H), 1.62–1.21 (m, 18H), 0.85 (t, J = 6.6 Hz, 3H); 13C NMR (50 MHz, CDCl3) δ 170.3, 155.6, 78.9, 73.3, 68.4, 60.7, 50.3, 31.5, 28.3, 28.1, 22.5, 14.1, 13.9. Anal. Calcd for C15H29NO5: C, 59.38; H, 9.63; N, 4.62. Found: C, 59.16; H, 9.82; N, 4.51.

4.2.7 (S)-tert-Butyl 2-(2-(benzyloxycarbonylamino)hexyloxy)acetate (15)

To a stirred solution of Z-L-Nle-ol (0.20 g, 0.8 mmol) in benzene (0.8 mL), tert-butyl bromoacetate (0.47 g, 2.4 mmol) and subsequently aq. NaOH 50% (0.8 mL) and the phase transfer catalyst Bu4NHSO4 (0.07 g, 0.2 mmol) were added. The reaction mixture was vigorously stirred for 2 h. EtOAc (10 mL) and H2O (10 mL) were added and the aqueous layer was separated and extracted with EtOAc (2×10 mL). The combined organic layers were washed with brine and dried over Na2SO4. The organic solvent was evaporated under reduced pressure and the residue was purified by column-chromatography using petroleum ether (bp 40–60 °C)/EtOAc 8:2 as eluent. Yield 74%; Oil; [α]D = −12.2 (c 1 CHCl3); 1H NMR (200 MHz, CDCl3) δ 7.34–7.22 (m, 5H), 5.30 (d, J = 8 Hz, 1H), 5.06 (s, 2H), 3.91 (s, 2H), 3.80–3.62 (m, 1H), 3.55 (dd, J1 = 9.0 Hz, J2 = 4.0 Hz, 1H), 3.46 (dd, J1 = 9.2 Hz, J2 = 4.2 Hz, 1H), 1.62–1.45 (m, 2H), 1.43 (s, 9H), 1.37–1.21 (m, 4H), 0.85 (t, J = 6.8 Hz, 3H); 13C NMR (50 MHz, CDCl3) δ 169.5, 156.0, 136.6, 128.3, 128.0, 127.8, 81.5, 73.0, 68.7, 66.3, 50.9, 31.4, 28.0, 27.9, 22.4, 13.8. Anal. Calcd for C20H31NO5: C, 65.73; H, 8.55; N, 3.83. Found: C, 65.52; H, 8.71; H, 3.70.

4.3. In vitro PLA2 Assays

Phospholipase A2 activity was determined using the previously described modified Dole assay12 with buffer and substrate conditions optimized for each enzyme as described previously12,13,16,17: (i) GIVA cPLA2 substrate mixed-micelles were composed of 400 μM Triton X-100, 97 μM PAPC, 1.8 μM 14C-labeled PAPC, and 3 μM PIP2 in buffer containing 100 mM HEPES pH 7.5, 90 μM CaCl2, 2 mM DTT and 0.1 mg/ml BSA; (ii) GVI iPLA2 substrate mixed-micelles were composed of either (a) 400 μM Triton X-100, 99 μM DPPC, and 1.5 μM 14C-labeled DPPC in buffer containing 200 mM HEPES pH 7.0, 1 mM ATP, 2 mM DTT and 0.1 mg/ml BSA or (b) 400 μM Triton X-100, 98.3 μM PAPC, and 1.7 μM 14C-labeled PAPC in buffer containing 100 mM HEPES pH 7.5, 2 mM ATP and 4 mM DTT; and (iii) GV sPLA2 substrate mixed-micelles were composed of 400 μM Triton X-100, 99 μM DPPC, and 1.5 μM 14C-labeled DPPC in buffer containing 50 mM Tris pH 8.0 and 5 mM CaCl2.

4.2. In vitro PLA2 Inhibition Studies

Initial screening of compounds at 0.091 mole fraction inhibitor in mixed-micelles was carried out. We considered compounds displaying 25% or less inhibition to have no inhibitory affect (designated N.D.). We report average percent inhibition (and standard error, n = 3) for compounds displaying more than 25% and less than 90% enzyme inhibition. If the percent inhibition was greater than 90%, we determined its XI(50) by plotting percent inhibition vs. inhibitor molar fraction (7 points; typically 0.005 to 0.091 mole fraction). Inhibition curves were modeled in Graphpad Prism using either a linear (x, y intercept = 0) or non-linear regression (one-site binding model - hyperbola, BMAX = 100) to calculate the reported XI(50) and associated error values.

4.5. Inhibition of arachidonic acid release RAW 264.7 cell culture and AA quantitation

RAW 264.7 macrophages were maintained in a humidified atmosphere at 37 °C with 5% CO2, as described elsewhere.36 The cells were cultured in DMEM media that was supplemented with 10% fetal bovine serum, 2 mM glutamine, 100 units/mL penicillin, 100 μg/ml streptomycin, and non-essential amino acids. The cells were plated at a confluency of 2 × 106 cells per well in 6 well tissue culture plates and allowed to adhere overnight. Inhibitors were added to the medium 30 min before Kdo2-Lipid A was added and the supernatants were collected at 1 hour following stimulation. Subsequently, AA was extracted from the supernatants and quantitated by HPLC-MS, as described elsewhere.36,37 AA release levels were normalized to pmol AA release per million cells though DNA quantitation using the Broad Range DNA Quant-Kit (Invitrogen). IC 50 values were determined by estimating the concentration of inhibitor required for half maximal inhibition of the maximum inhibition observed with a given inhibitor.

Figure 2
Dose response curve for the inhibition of AA release from RAW 264.7 cells by inhibitor 18. RAW 264.7 cells were preincubated with the indicated concentrations of 18 prior to stimulation with Kdo2-Lipid A and AA quantitation. The data is graphically expressed ...
Figure 3
Dose-response curves for 2-oxoamide inhibitors of GVIA iPLA2. Inhibition of the activity of human GVIA iPLA2 by inhibitors (a) 8a and (b) 13 was tested on mixed-micelles containing 100 μM PAPC and 400 μM Triton X-100. Inhibition curves ...

Acknowledgments

The project is co-funded by the European Social Fund and National Resources (EPEAEK II) (G.K.) and by NIH Grants GM 20,501, GM 64,611, and U54 GM069338 (E.A.D).

Footnotes

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