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Synthesis of potent adenosine A2A and A3 receptor agonist from the modification of adenosine-5′-N-ethylcarboxamide (NECA) has been reported. Diastereoisomer possessing an (R) 3,4-dihydro-2H-pyranyl (DHP) moiety exhibited the highest affinity at the A2A and A3 receptors. The key steps involve the synthesis of (R)-3,4-dihydro-2H-pyran-2-carboxaldehyde (7), which was obtained through the enzyme catalyzed kinetic resolution of (±)-2-acetoxymethyl-3,4-dihydro-2H-pyran (5).
The metabolite adenosine plays a critical role in the regulation of cell function in many tissues. It mediates its actions via four subtypes of G protein-coupled receptors named A1, A2A, A2B and A3.1 Based on adenosine’s profound actions on the heart, kidney, brain, and the immune system, numerous potential therapeutic options have been discussed for adenosine receptor agonists and antagonists with selectivity for the known receptor subtypes.2
It has been demonstrated that a lipophilic linker in the N6 position is necessary to gain high affinity for the A3 receptor, and a polar linker possessing an additional lipophilic group in the C-2 position is necessary to gain high affinity for the A2A receptor.2 Thus, the major efforts in the discovery of new adenosine A2A and A3 agonists have been made with the introduction of appropriate substituents at the C-2 or N6 position in combination with the modification of the 5′ position of the nucleoside scaffold via various linkers.2,3
The stimulation of both A2A and A3 receptor subtypes have been shown to mediate antiinflammatory actions,4 generating interest in such compounds. Our interest in the synthesis of (R)-3,4-dihydro-2H-pyran-2-carboxaldehyde [(R)-acrolein dimer] arose from the investigation of one of our potent adenosine A2A and A3 receptor agonists 2, which possesses a (R)-3,4-dihydro-2H-pyran (DHP) moiety.5 During our structure-activity relationships study of 2-hydrazone-NECA derivatives, we discovered that compound 2 exhibits an interesting pharmacological profile. In a radioligand binding assay, it showed high affinity at both the A2A and A3 receptors. Compound 2 was initially prepared from racemic aldehyde 7 and 2-hydrazino-NECA 1 (Scheme 1). Due to the interesting properties of 2, diastereomeric separation of isomers 3 and 4 from 2 was then performed via preparative HPLC using reverse phase C-8 or C-18 columns. After the resolution of diastereomers, we found that the molecule possessing the (R)-DHP ring (3) is more potent than the (S)-DHP analogue (4). The R-isomer (3) was also the more selective of the two diastereomers (Table 1).
In order to make compound 3, we required the acrolein dimer (R)-7 on a large scale. However, the lack of a literature method for the synthesis of enantiomers of acrolein dimer led us to investigate a convenient synthetic route for acrolein dimer (R)-7. The racemic aldehyde has been synthesized from acrolein under high pressure6 or microwave assisted7 dimerisation (Figure 1). There is no report on the enantio-selective synthesis of acrolein dimer.
Pederson et al.8 have reported the parallel kinetic resolution of (±)-acrolein dimer by reacting with chiral phosphonates using asymmetric Horner-Wadsworth-Emmons reaction. Although resolution of the corresponding diastereoisomeric adducts with chiral auxiliaries has been completed, their conversion to pure enatiomers of (R) and (S)-aldehydes remains to be achieved. Herein, we report the convenient synthesis of (R)-7 and its application in the synthesis of potent adenosine agonist 3 (Scheme 1 and and22).
There are several syntheses9 and synthetic applications10,11 of the precursor 3,4-dihydro-2H-pyran-2-methanol (6) reported in literature. We decided to follow the method reported by Kang et al.12 and Ley et al.11a for the source of (R)-5 and (R)-6 (Scheme 2), which describes the enzymatic hydrolysis of (±)-5. Kang et al.12 performed enzymatic hydrolysis of (±)-5 using porcine pancreatic lipase (PPL), 0.01 M phosphate buffer and acetone that provided (S)-alcohol 3 in >99% e.e. However, the process reported by Kang et al.12 produced our desired product (R)-5 only in 48% e.e. Ley et al.11a modified the above method12 but it requires an additional enzymatic hydrolysis and two purification steps.
In an attempt to simplify the above reported methods,11a, 12 we performed the enzyme catalyzed kinetic resolution on (±)-5 by extending reaction time to 4 days as well as repeating the addition of PPL after the first 24 hours.13 These modifications pleasingly lead us to (R)-acetate 5 in >99% e.e. in one step instead of (S)-alcohol enriched product as reported by Kang et al.12 The (R)-acetate was then hydrolyzed to the corresponding (R)-alcohol 5 in the presence of 0.3 equivalent of 1.0 N potassium hydroxide.14
In order to make compound (±)-7, oxidation of commercially available racemic alcohol 6 was first carried out using various reagents. These results are summarized in Table 2. Our initial attempt of a Swern oxidation reaction15 of (±)-6 produced (±)-7 in poor yield. Several other oxidation reactions were attempted, which did not result in any trace of the desired product (±)-7: (1) Tetrapropylammonium perruthenate (TPAP), 4-methylmorpholine N-oxide (NMO), CH2Cl2; (2) MnO2, acetone; (3) Pyridinium chlorochromate (PCC), ether; (4) Pyridinium dichromate (PDC), CH2Cl2.
The reaction of (±)-6 in the presence of Dess-Martin periodinane16 in CH2Cl2 provided aldehyde 7 as the major product but the separation of 7 from impurities was complicated. Further attempts to oxidize (±)-6 using iodobenzene diacetate (BAIB) and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) in CH2Cl2 provided (±)-7 in better yield.17 Under similar conditions, the enantiomer (R)-7 was then prepared from (R)-6.18 (R)-Aldehyde was separated from the crude reaction mixture by column chromatography. Of the various oxidizing reagents studied, the method with BAIB and TEMPO was found to be efficient and clean.
Treatment of (R)-7 with 2-hydrazino-NECA (1) produced the diastereoisomer 3 enriched with (R)-3,4-dihydro-2H-pyranyl (DHP) side chain.19 The (R)-DHP analogue 3 showed highest affinity for both A2A and A3 receptors [Ki(A2A) = 3.76 and Ki(A3) = 4.51 nM]. Compound 3 is 7-fold better than reference compound CGS 21680, binds with a Ki(A2A) value of 3.76 nM, and is 12-fold selective versus A1. Compound 3 also showed 10-fold A3 selectivity versus the A1 subtype, which is slightly better than the selectivity of CGS21680 as one of the most potent and selective A2A agonists known (Table 1). The adenylyl cyclase functional assay1 has shown that it is a full agonist at A2A and A3 receptors (data not shown).
In summary, a synthesis of potent and selective adenosine A2A and A3 receptor agonist 3 has been accomplished from the coupling reaction of (R)-3,4-dihydro-2H-pyran-2-carboxaldehyde (7) and 2-hydrazino-NECA (1). The enantiopure (R)-2-acetoxymethyl-3,4-dihydro-2H-pyran (5) was prepared on multi-gram scale by modifying the earlier PPL catalyzed resolution methods.11a, 12 A method for the synthesis of aldehyde (R)-7 from alcohol (R)-6 has been discussed. The results from in vivo studies will be published elsewhere.
This work was supported by a grant from the National Institutes of Health (R44AI46167). We thank Sonja Kachler for her expert technical assistance.
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