Syntheses of New Phosphorus-Containing Azabicycloalkanes and Their Microbial Hydroxylation Using Beauveria bassiana

Abstract

Representative novel phosphorus-containing azabicyclic substrates have been synthesized and subsequently microbially hydroxylated in fair to good yields using the microorganism Beauveria bassiana. (7-Azabicyclo[2.2.1]hept-7-yl)phosphonic acid diethyl ester was hydroxylated at the unactivated methylene carbon to give (2-endo-hydroxy-7-azabicyclo[2.2.1]hept-7-yl)phosphonic acid diethyl ester in 43% yield and 64% ee, while N-(diphenylphosphinoyl)-7-azabicyclo[2.2.1]heptane was similarly hydroxylated to give 2-endo-hydroxy-7-(diphenylphosphinoyl)-7-azabicyclo[2.2.1]-heptane in 35% yield and 20% ee. (7-Azabicyclo[2.2.1]hept-7-yl)phosphonic acid diphenyl ester yielded two distinct hydroxylated products: monohydroxylated (2-endo-hydroxy-7-azabicyclo[2.2.1]-hept-7-yl)phosphonic acid diphenyl ester in 7% yield and 7% ee and dihydroxylated (2-endo-hydroxy-7-azabicyclo[2.2.1]hept-7-yl)phosphonic acid phenyl, p-hydroxyphenyl ester in 37% yield and 77% ee. HPLC studies indicated that the monohydroxylated metabolite is formed first during fermentation, and becomes a substrate for a second enzymatic hydroxylation at one of the aromatic rings with induced enantioselection, to give the dihydroxylated metabolite. All microbially hydroxylated metabolites were easily N-deprotected using TFA-CH 2 Cl 2 (1:1). Thus, N-phosphinyl groups are good facilitators of hydroxylation reactions with B. bassiana and offer a new choice for an N-substituent when substrates are hydroxylated with this microorganism. By offering a new N-substituent, this work extends the general utility of B. bassiana as a preparatively useful unactivated methylene hydroxylator.

Total Synthesis of (±)-Epibatidine Using a Biocatalytic Approach

“…7 We have also studied the effect of several phosphorus-containing N-substituents on the microbial hydroxylation of 7-azanorbornanes. 6 We and Johnson's group have found that B. bassiana is able to accept a variety of functionalities on the heteroatom of the substrate. 5, 6 We found that good reproducible yields are obtained when the N-substituent is a benzoyl group and therefore selected the corresponding metabolite as the intermediate to complete a synthesis of epibatidine.…”

“…6 We and Johnson's group have found that B. bassiana is able to accept a variety of functionalities on the heteroatom of the substrate. 5, 6 We found that good reproducible yields are obtained when the N-substituent is a benzoyl group and therefore selected the corresponding metabolite as the intermediate to complete a synthesis of epibatidine. We might attribute the low enantioselectivity of the microbial oxidation to the hindered rotation of the N-benzoyl group, which was improved when utilizing phosphorus-containing N-substituents albeit in slightly lower isolated yields.…”

“…Oxidation of N-substituted 7-azanorbornanes with the fungus Beauveria bassiana has been studied independently by Johnson's group and ourselves . We recently reported that easily prepared 7-azanorbornanes carrying an appropriate N-substituent are microbially oxidized stereoselectively on an unfunctionalized methylene carbon. , In this note, we communicate our total synthesis of epibatidine using a selected metabolite generated from this biotransformation.

1 Preparation of Substrate and Microbial Transformation

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“…5 We recently reported that easily prepared 7-azanorbornanes carrying an appropriate N-substituent are microbially oxidized stereoselectively on an unfunctionalized methylene carbon. 5,6 In this note, we communicate our total synthesis of epibatidine using a selected metabolite generated from this biotransformation. …”

Preparative biotransformations

“…199 7-Azabicyclo[2.2.1]heptanoylphosphonic acid diesters are transformed by the same organism with the same regioselectivity (37-43% yield; 64-77% ee) but the corresponding diphenylphosphinoyl-7-azabicycle gave a lower yield and ee. 200 Griengl and co-workers have developed their idea of temporarily introducing a stereodirecting group into a molecule to enhance the predictability of biohydroxylations using Beauvaria bassiana and other microorganisms. Thus, while cyclopentanone is not readily hydroxylated using B. bassiana, simple derivatization of the ketone group and biohydroxylation furnishes the bicyclic compound 96 (90% de).…”