Stereoselective microbial/enzymatic oxidation of (exo, exo)-7-oxabicyclo [2.2.1] heptane-2,3-dimethanol to the corresponding chiral lactol and lactone

Abstract

A key chiral intermediate, lactol 2[3aS(3aα, 4α, 7α, 7aα)]-hexahydro-4,7-epoxy-isobenzofuran-1 (3H)-one, and the corresponding chiral lactone 3 were made in high optical purity by stereoselective enzymatic and microbial oxidation of the parent diol 1. Horse liver alcohol dehydrogenase (HLADH) in the presence of nicotinamide adenine dinucleotide (NAD +) and riboflavin oxidized diol 1 (exo, exo)-7-oxabicyclo [2.2.1] heptane-2,3-dimethanol to the corresponding lactol 2 and lactone 3. Regeneration of NAD + required for the oxidation of diol 1 was carried out by the NADH-dependent alanine dehydrogenase and leucine dehydrogenase in the presence of alanine and leucine, respectively. Since expensive enzyme (HLADH) and cofactors (NAD +) were required for the oxidation reaction, various microorganisms were screened for the ability to catalyze the stereoselective oxidation of diol 1. Two organisms, Nocardia globerula ATCC 21505 and Rhodococcus sp. ATCC 15592, catalyzed the efficient oxidation of diol 1 to the corresponding chiral lactol 2 and lactone 3. The reaction yield of 70% and optical purity of 96% was obtained for lactone 3 prepared from the oxidation of diol 1 by cell suspensions (10% w/v, wet cells) of N. globerula ATCC 21505. Substrate was used at 5 gl −1 concentration. A 10% (w/v, wet cells) cell suspension of Rhodococcus sp. ATCC 15592, after 120 h reaction period, produced lactol 2 in 12% yield and lactone 3 in 34% yield. An overall 46% reaction yield was obtained based on diol 1 oxidation. Substrate was used at 5 gl −1 concentration. Optical purity of 96.7% and 98.4% was obtained for lactol 2 and lactone 3, respectively. Lactone 3 purified by slilica gel chromatography gave a specific rotation of +117° and enantiomeric excess of >99%.