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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
J Org Chem. Author manuscript; available in PMC 2010 September 7.
Published in final edited form as:
PMCID: PMC2935138
NIHMSID: NIHMS211886

A Simple Synthesis of Nitriles from Aldoximes1

Abstract

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Easily synthesized aldoximes have been converted to the corresponding nitriles under very mild conditions by a simple reaction with 1H-benzotriazol-1-yloxy-tris(dimethylamino)phosphonium hexafluorophosphate (BOP) and DBU in CH2Cl2, THF or DMF. As an alternative reagent that eliminates the formation of hexamethylphosphoramide as a byproduct, use of 1H-benzotriazol-1-yl-4-methylbenzenesulfonate (Bt-OTs) and DBU was investigated. Reactions with this reagent also proceeded smoothly and in good yields although in one case N-sulfonylation was observed. An attempt to gain mechanistic insight into the BOP-mediated reaction has been made using 31P{1H} NMR. However, no phosphorus-bearing intermediate could be readily observed. Finally, the method has been applied to the synthesis of an antiviral 4′-cyano adenosine analogue from a commercial precursor, using a single saccharide protecting group.

INTRODUCTION

The cyano moiety is a highly important one not only due to its synthetic value as precursor to other functionalities but also due to its presence in a variety of natural products, pharmaceuticals and novel materials. Although a plethora of methods are known for access to the cyano functionality,2, 3 dehydration of aldoximes remains a convenient route.4 Some recently reported methods for aldoxime dehydration involve NaICl2/aq. NH3,5 N-chlorosuccinimide and pyridine,6 W-Sn mixed hydroxide in o-xylene at 149 °C,7 thermal dehydration,8 reaction with ethyldichlorophosphate/DBU/3 Å MS,9 use of Silphos [PCl3-n(SiO2)n] in MeCN,10 ZnO/AcCl at 80 °C,11 reaction with chlorosulfonic acid in toluene at 90 °C,12 use of Ga(III)OAc/MeCN at 85–120 °C13 and reaction with dimethylacetylene dicarboxylate and Et3N.14

In connection with some on going studies on nucleoside modification, we had reason to examine mild methods for the conversion of aldoximes to nitriles. In this respect, use of PPh3/I2 in CH2Cl2 has been reported to yield nitriles in high yield and within short reaction times.15 However, in our hands, a test reaction of 2-naphthaldoxime under these conditions showed incomplete reaction in 5 h and upon prolonging the reaction time, formation of some 2-naphthaldehyde was also observed (resonance at δ 10.17 ppm in the 1H NMR) in addition to the nitrile. Switching from PPh3 to hexamethylphosphorus triamide [HMPT, (Me2N)3P] did not provide a significant improvement and aldehyde formation was again observed. This led us to question whether the formation of aldehyde could become a complicating problem in the dehydration of other oximes. On the basis of the foregoing, as well as the procedural aspects of several recently described methods and the belief that mild methods would be necessary for relatively fragile substrates, we decided to reinvestigate aldoxime dehydration. This paper reports our results on the development of a new method for the synthesis of nitriles from aldoximes.

RESULTS AND DISCUSSION

It has been reported that amides can be converted to nitriles via the use of PyBOP and (iso-Pr)2NEt in CH2Cl2 at 40 °C.16 This led us to consider whether aldoximes, that are generally more acidic than alcohols,17,18 could undergo dehydrative reactions with commercially available BOP (which is somewhat cheaper than PyBOP) and a base. Herein we report development of a simple dehydration of aldoximes using BOP. During the course of these studies, we have also evaluated the use of a sulfonate ester of HOBt (Bt-OTs) for this oxime to cyanide conversion. Finally, we have used this method as one of three steps in a short and efficient synthesis of adeninyl ribofuranonitrile, a compound that has demonstrated useful antiviral activity.

Our initial work commenced with screening of solvent and base so as to obtain optimal reaction conditions. These early reactions were performed using 2-naphthaldoxime as a representative, electronically unbiased substrate, and 2 molar equivalents of BOP. The results are shown in Table 1.

TABLE 1
Initial Experiments on the Dehydration of 2-Naphthaldoxime using BOP

From the results in Table 1 it is evident that use of BOP and DBU in CH2Cl2 led to fast conversion of the aldoxime and in good yield (entry 3). THF and DMF are also suitable solvents (entries 1 and 2) whereas CHCl3 was inferior in which the reaction did not proceed cleanly. The weaker base (iso-Pr)NEt2 also appears suitable although a much slower reaction was observed (entry 6). With this base, DMF proved to be an inferior solvent (entry 5). Presence of the base is important as demonstrated by absence of reaction without added base (entries 7 and 8).

At this point we wanted to assess the generality of this transformation and subjected a variety of aldoximes (prepared by conventional methods involving the use of NH2OH·HCl and aqueous Na2CO3, K2CO3 or NaOH) to the optimized reaction conditions. The results of these reactions are summarized in Table 2. During the course of these experiments we learned that use of CH2Cl2 at elevated temperature led to the formation of a product resulting from the reaction of hydroxybenzotriazole with CH2Cl2.19 Thus, THF is a preferred solvent for reactions at higher temperatures. Some of the nitrile syntheses were therefore performed in THF to assess its general suitability (entries 3, 4 and 7 in Table 2). In one case (entry 5) DMF was used for solubility reasons.

TABLE 2
Generality of the Dehydration Methodology Using BOP or Bt-OTs and DBUa

As can be seen from Table 2, reactions with BOP proceeded smoothly. However, we wanted to assess whether the formation of hexamethylphosphoramide [HMPA, (Me2N)3PO] as byproduct could be eliminated. This would make the reaction more useful for development of biologically important materials. For this we evaluated several options and settled on 1H-benzotriazol-1-yl-4-methylbenzenesulfonate (Bt-OTs) as a potential reagent. Bt-OTs is known in the literature20 and can be quite readily synthesized20a (Scheme 1).

SCHEME 1
Synthesis of 1H-Benzotriazol-1-yl-4-methylbenzene sulfonate (Bt-OTs)

Interestingly, Bt-OTs has not found much use in such dehydrative reactions. Application of Bt-OTs in the present cases also resulted in satisfactory conversions to the nitriles, and these results are shown in Table 2 as well. Reaction of the unprotected indole with BOP and DBU produced a very satisfactory return of the nitrile 7 (entry 7). However, reaction of this oxime with Bt-OTs and DBU produced an easily separable product mixture consisting of the carbonitrile 7 (42%) as well as the corresponding N-tosyl derivative21 8 (50%). Such N-sulfonylation has been observed during amide formation.20d Interestingly, the aldoxime derived from 3-phenyl-1-propanal also underwent conversion in good yield to the nitrile 9 (entry 8) despite the generally lower acidity of alkyl aldoximes.17 From the standpoint of functional group compatibility, reactions with BOP are tolerant of the nitro, organometallic and free amino entities (entries 2, 4 and 7) as well as ortho substituents on an aryl ring. Additionally, it can be reasoned that in cases such as indole carboxaldehyde, protection and dehydration can be effectuated in one step under appropriate conditions using Bt-OTs.

From a mechanistic consideration, we wanted to evaluate the course of the dehydration reaction of aldoximes with BOP and DBU. As shown in Scheme 2 there are two mechanistic possibilities. In Pathway 1, upon oxime deprotonation by DBU, initial reaction could occur at the phophorus atom of BOP with the formation of a new phosphonium species. In the alternative Pathway 2, an SN2′-like reaction at the nitrogen atom could result in a direct expulsion of HMPA. Since each pathway involves formation of new phosphorus-containing species, we felt that 31P{1H} NMR may prove useful in this assessment.

SCHEME 2
Two Possible Pathways for the Formation of Nitriles by Dehydration of Aldoximes Using BOP

Therefore, a series of experiments was undertaken. First, a reaction of 2-naphthaldoxime and DBU was conducted in CH2Cl2 in the absence of BOP. Consistent with our expectation, no reaction was observed over a 2-hour period and only starting material was present, clearly indicating that BOP is necessary for the reaction (which is complete within 1 hour in the presence of BOP). Next, a reaction between 2-naphthaldoxime and BOP was conducted in CD2Cl2 using DBU, and the reaction was monitored by 31P{1H} NMR (Figure 1). In CD2Cl2 the phosphonium resonance of BOP appears at δ 44.1 ppm relative to 85% H3PO4 as external standard (the PF6 septet appears at δ −143.9 ppm). As the reaction progressed, no new discernable signal for a second phosphonium species was observed even at a short reaction time of 5 min, but only formation of HMPA was observed at δ 25.8 ppm (pure HMPA appears at δ 25.4 ppm in CD2Cl2). Finally, after 100 min, the reaction mixture was spiked with 1 molar equiv of HMPA. Increase in the signal intensity δ 25.8 ppm was observed, confirming the HMPA resonance.

FIGURE 1
Monitoring the course of the reaction between 2-naphthaldoxime, 2 molar equiv BOP and 2.3 molar equiv DBU in CD2Cl2 using 31P{1H} NMR. (A) Napthaldoxime (0.2 M in CD2Cl2) + BOP; (B) 5 min after addition of DBU; (C) 45 min after addition of DBU; (D) 100 ...

A second 31P{1H} NMR experiment was conducted with o-bromobenzaldoxime (see Figure 1 in the Supporting Information). The result was identical to that obtained with 2-naphthaldoxime. Thus, if a phosphonium salt was indeed an intermediate in this reaction as in Pathway 1 shown in Scheme 2, it was perhaps a fleeting species, and not easily observed via 31P{1H} NMR. Carboxylic acids20c and amide functionalities of nucleosides22 have been shown to react with BOP via a phosphonium derivative. On the other hand, the NMR experiment raises the question of the alternate mechanism that causes rapid formation of HMPA without intermediacy of a new phosphonium species, such as Pathway 2 in Scheme 2. However, at the present time we have not been able to identify any other reactive intermediate and the question of the mechanism remains open.

In order to assess whether there is a substantial difference in the relative ease with which E- and Z-oximes undergo dehydration under the conditions described, we separated the isomeric oximes of cinnamaldehyde (silica gel eluted with 10% EtOAc in hexanes) and characterized each by NMR.23 Reaction of E-isomer with BOP and DBU under the optimized conditions (Scheme 3) led to complete reaction within 30 min, and an 83% isolated yield of nitrile 10. Similar reaction of the Z-isomer was also complete within 30 min with a 90% isolated yield of 10. Thus, it appears that oxime stereochemistry may not have a major influence on the ease of this conversion.

SCHEME 3
Independent Reactions of E- and Z-Cinnamaldoximes

As a final stage in the development of this method, we were interested in assessing its utility for the transformation of relatively sensitive molecules. Thus, we chose to synthesize adeninyl ribofuranonitrile from commercially available 2′,3′-acetonide of adenosine using minimal protecting groups. Adenyl ribofuranonitrile has shown promising acvitity agains vesicular stomatitis virus (VSV, EC50 1.2 μg/mL) and human cytomegalovirus (HCMV, EC50 1.4 μg/mL).24

Although the E/Z 5′-carbaldoximes derived from adenosine are known compounds,25 their synthesis posed some challenges. For instance, the exocyclic amino group needed protection and oxidation of the 5′-hydroxyl group had to be performed via a N,N′-diphenylethylenediamino derivative that yielded the aldehyde hydrate upon hydrolysis. This aldehyde hydrate had to be azeotroped with benzene prior to synthesis of the oxime mixture.26

On the basis of these considerations, we opted for a completely different route shown in Scheme 4. Relying on the recently reported O-(tert-butyldimethylsilyl)-N-tosylhydroxylamine as a convenient reagent for synthesis of oximes,23 the 5′-hydroxyl group of commercially available 11 was converted to the N-tosyl-O-silyl-hydroxylamine via a Mitsunobu reaction, to give 12 in 80% yield. This step can be readily accomplished without protection of the exocyclic amino group.27

SCHEME 4
A Short Synthesis of Adeninyl Ribofuranonitrile Using a Single Protecting Group

Next, fluoride-mediated desilylation of 12 led to the E/Z mixture of oximes 13 (85% yield), via the anticipated expulsion of p-toluene sulfinate and tautomerization of the incipient nitroso nucleoside. Comparison of the NMR data of this product mixture to reported data25 confirmed the formation of 13. Finally, exposure of 13 to 2 molar equiv BOP and 2.3 molar equiv of DBU in CH2Cl2 at room temperature led to the formation of the ribofuranonitrile 14 in 95% yield within 45 min. Alternatively, the use of 2.0 molar equiv Bt-OTs and 2.3 molar equiv of DBU in CH2Cl2 also led to the formation of 14 in 93% yield, within 35 min at room temperature. Both reactions proceeded smoothly and in the case of the nucleoside no N-sulfonylation was observed with Bt-OTs. Compound 14 has previously been synthesized from the adeninyl methyl ribofuranuronate via the amide in 46% yield.28

CONCLUSIONS

In summary, we have developed a mild and efficient conversion of aldoximes to nitriles via the use of BOP or Bt-OTs as dehydrating agent in the presence of a base such as DBU. These reactions proceed in CH2Cl2 at room temperature in almost all cases tested. THF and DMF are also suitable solvents when elevated temperature is necessary, or for solubility reasons. Using this dehydration reaction as one step in a 3-step sequence, we have developed a new approach to adeninyl ribofuranonitrile (as its 2′,3′-acetonide) requiring a minimal protecting group strategy. In principle, simple cleavage of the acetonide should yield the fully deprotected compound. The approach described should prove useful for modification of other nucleoside derivatives as well. Finally, we believe that the strategy utilizing the Mitsunobu reaction for oxime synthesis23 in combination with this new dehydration method will prove to be of use in the synthesis of a wide range of organic nitriles.

EXPERIMENTAL

Most of the cyano compounds listed in Table 2 are commercially available. Characterization of cyanoferrocene29 and 1-cyanopyrene30 have been reported in the literature.

General Procedure for Nitrile Synthesis Using BOP

In an oven-dried, two-necked, 50 mL round-bottomed flask, equipped with a stirring bar was placed a solution of the oxime (1.0 mmol) and BOP (2.0 mmol) in anhydrous CH2Cl2 (5.0 mL). The mixture was stirred at room temperature for 5 minutes and then DBU (2.3 mmol) was added dropwise to the stirring mixture over 2 minutes. The reaction mixture became a clear homogeneous solution after addition of DBU. The reaction was monitored by TLC, and upon complete consumption of the starting material the mixture was diluted with EtOAc and washed with water (2x) followed by brine. The organic layer was dried over MgSO4 and concentrated under reduced pressure. The crude mixture was purified by column chromatography. [Deviation from this procedure: (a) THF or DMF was appropriately substituted as reaction solvent when needed, e.g. in the reactions with o-bromobenzaldoxime, ferrocene carbaldoxime and pyrene-1-carbaldoxime.]

General Procedure for Nitrile Synthesis Using Bt-OTs

Into an oven-dried, two-necked, 50 mL round-bottomed flask, equipped with a stirring bar was placed a solution of the oxime (1.0 mmol) and Bt-OTs (2.0 mmol) in anhydrous CH2Cl2 (5.0 mL). The mixture was stirred at room temperature for 5 minutes and then DBU (2.3 mmol) was added dropwise to the stirring mixture over 2 minutes. The reaction was monitored by TLC, and upon complete consumption of the starting material the reaction mixture was diluted with EtOAc and washed with water (2x) followed by brine. The organic layer was dried over MgSO4 and concentrated under reduced pressure. The crude mixture was purified by column chromatography. [Deviation from this procedure: (a) the reaction was conducted with 0.75 mmol of pyrene-1-carbaldoxime, (b) THF or DMF was appropriately substituted as reaction solvent when needed, e.g. in the reactions with o-bromobenzaldoxime, ferrocene carbaldoxime and pyrene-1-carbaldoxime.]

5′-Deoxy-5′-[N-(tert-butyldimethylsilyloxy)-N-(p-toluenesulfonyl)]amino-2′,3′-O-(isopropylidene)adenosine (12)

In a 100 mL oven-dried, round-bottomed flask equipped with a stirring bar were placed 2′,3′-O-(isopropylidene)adenosine 11 (0.75 g, 2.44 mmol), O-(tert-butyldimethylsilyl)-N-tosylhydroxylamine (TsNHOTBDMS, 1.10 g, 3.66 mmol) and PPh3 (1.28 g, 4.88 mmol). Anhydrous toluene (12 mL) and THF (4 mL) were added and the reaction mixture was cooled to 0 °C with stirring. DEAD (576 μL, 3.65 mmol) was added dropwise. The reaction mixture was stirred at 0 °C for 1 h and then allowed to warm to room temperature. After 5 h TLC showed complete consumption of the starting material. The reaction mixture was diluted with EtOAc, and washed with saturated aq NaHCO3 followed by water and brine. The organic layer was dried over Na2SO4 and concentrated under reduced pressure. The product was loaded onto a silica gel column using CH2Cl2 and eluted with 15% EtOAc in CH2Cl2 followed by 40% EtOAc in CH2Cl2. Compound 12 was obtained as white, foamy solid (1.163 g, 81% yield). Rf (50% EtOAc in CH2Cl2) = 0.16. 1H NMR (CDCl3): δ 8.37 (s, 1H, Ar-H), 7.86 (s, 1H, Ar-H), 7.58 (d, 2H, Ar-H, J = 8.2), 7.27 (d, 2H, Ar-H, J = 8.2), 6.03 (d, 1H, H-1′, J = 1.7), 5.94 (br s, 2H, NH2), 5.56 (dd, 1H, H-2′, J = 6.3, 1.7), 5.15 (dd, 1H, H-3′, J = 6.3, 2.7), 4.50 (dt, 1H, H-4′, J = 6.9, 2.7), 3.43 (m, 1H, H-5′), 2.91 (m, 1H, H-5′), 2.41 (s, 3H, p-toluyl CH3), 1.58 and 1.38 (2s, 6H, isopropylidine CH3), 0.89 (s, 9H, tert-Bu), 0.34, 0.24 (2s, 6H, SiCH3). 13C NMR (CDCl3): δ 155.8, 153.1, 149.3, 145.0, 140.5, 129.9, 129.5, 120.5, 114.3, 91.8, 84.3, 84.1, 83.6, 58.0, 27.2, 26.2, 25.6, 21.8, 18.5, −4.1, −4.3. HRMS (ESI) calculated for C26H39N6O6SSi [M + H]+ 591.2421, found 591.2427.

1′-Adenin-9-yl-2′,3′-O-(isopropylidene)-β-D-ribofuranurononitrile (14)

Step 1

Compound 12 (1.32 g, 2.22 mmol) was dissolved in anhydrous CH3CN (22 mL) and CsF (0.674 g, 4.44 mmol) was added. The reaction mixture was stirred at 60 °C for 1.5 h at which time TLC showed complete consumption of the starting material. Saturated aq. NH4Cl was added to the cooled reaction mixture and the mixture was extracted with EtOAc. The organic layer was washed with water followed by brine. The organic layer was dried over Na2SO4 and concentrated under reduced pressure. The crude material was loaded onto a dry-packed silica gel column and eluted using 30% acetone in hexanes. Compound 13 (E/Z mixture) was obtained as white powder (0.569 g, 80% yield). Rf (4% MeOH in CH2Cl2) = 0.03. HRMS calculated for C13H17N6O4 [M + H]+ 321.1311, found 321.1311. The NMR data for this E/Z mixture has been reported.25 The 1H NMR spectrum of this mixture is furnished in this Supporting Information section.

Step 2 using BOP

In an oven-dried, 50 mL two-necked round-bottomed flask equipped with a stirring bar was placed a solution of 13 (0.320 g, 1.0 mmol) and BOP (0.885 g, 2.0 mmol) in anhydrous CH2Cl2 (5.0 mL). The mixture was stirred at room temperature for 5 min and then DBU (344 μL, 2.30 mmol) was added dropwise over 2–3 minutes to the stirring solution. The reaction mixture became clear after addition of DBU. After 45 min TLC showed complete consumption of the starting material. The reaction mixture was diluted with EtOAc and washed with water (2x) followed by brine. The organic layer was dried over MgSO4 and concentrated under reduced pressure. The crude product was chromatographed on a silica gel column using 2% EtOH in CH2Cl2 as eluting solvent (the chromatography was repeated a second time). Compound 14 obtained as white solid (0.288 g, 95% yield).

Step 2 using Bt-OTs

In an oven-dried, 50 mL two-necked round-bottomed flask equipped with a stirring bar was placed a solution of 13 (0.277 g, 0.864 mmol) and Bt-OTs (0.500 g, 1.73 mmol) in anhydrous CH2Cl2 (5.0 mL). The mixture was stirred at room temperature for 5 min and then DBU (297 μL, 1.99 mmol) was added dropwise over 2 minutes to the stirring solution. The reaction mixture became clear after addition of DBU. After 35 min TLC showed complete consumption of the starting material. The reaction mixture was diluted with EtOAc and washed with water (2x) followed by brine. The organic layer was dried over MgSO4 and concentrated under reduced pressure. The crude product was chromatographed on a silica gel column using 2% EtOH in CH2Cl2 as eluting solvent (the chromatography was repeated a second time). Compound 14 was obtained as a white solid (0.246 g, 93% yield). Rf (4% MeOH in CH2Cl2) = 0.27. 1H NMR (CDCl3): δ 8.39 (s, 1H, Ar-H), 7.89 (s, 1H, Ar-H), 6.20 (s, 1H, H-1′), 5.83 (d, 1H, H-2′, J = 5.7), 5.79 (dd, 1H, H-3′, J = 5.7, 1.4), 5.64 (br s, 2H, NH2), 4.98 (d, 1H, H-4′, J = 1.4), 1.58 and 1.43 (2s, 6H, isopropylidine CH3). 13C NMR (CDCl3): δ 155.8, 153.5, 149.7, 140.1, 120.2, 116.2, 115.0, 92.0, 84.9, 84.1, 75.5, 26.7, 25.2. HRMS (ESI) calculated for C13H15N6O3 [M + H]+ 303.1206, found 303.1207.

Supplementary Material

SI

Acknowledgments

We are grateful to Dr. Natalya Gutner (CCNY) for help in obtaining IR data for the products and to Dr. Cliff Soll (Hunter College) for the high-resolution mass spectral data for the new compounds described. Partial support via PSC-CUNY awards is acknowledged and MKS was supported via NSF Grant CHE-0640417. Infrastructural support at CCNY was provided by NIH RCMI Grant G12 RR03060.

Footnotes

SUPPORTING INFORMATION PARAGRAPH

1H NMR spectra of carbonitriles 1–10 shown in Table 2 as well as those of 12, 13, and 14, COSY spectra of 12 and 14. This data is available free of charge via the Internet at http://pubs.acs.org.

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