The reduction of tertiary N-styrylenamides

Abstract

Magnesium and methanol reduction of N,9-dibenzyl-8-azabicyclo[4.3.0]non-1(6)-en-7-one (6) gave all four racemates of N,9-dibenzyl-8-azabicyclo[4.3.0]nonan-7-one (1)–(4), with a 13:6 ratio of cis-fused to trans-fused products. Selective reduction of N,9-dibenzyl-8-azabicyclo[4.3.O]non-1(6),3-dien-7-one (5) gave almost exclusively the two cis-fused racemates of N,9-dibenzyl-8-azabicyclo-[4.3.0]non-3-en-7-one (9) and (10). Magnesium and methanol did not reduce (E)-N-benzyl-9-benzylidene-cis-bicyclo[4.3.0]nonan-7-one (7) and (E)-N-benzyl-N-(1-methyl-2-phenylvinyl) acet-amide (16a), but did reduce (E)-N-benzyl-N-styrylacetamide (16b) and (E)- and (Z)-N-styryl-pyrrolidin-2-one (13) and (14); incomplete reduction of (Z)-N-benzyl-N-(1-methyl-2-phenylvinyI)-acetamide (15a) and (Z)-N-benzyl-N-styrylacetamide (15b) was observed. Reduction does not occur when the styryl phenyl group is twisted out of conjugation.

Sodium and liquid ammonia reduction of (E)-N-benzyl-9-benzylidene-cis-8-azabicyclo-[4.3.0]nonan-7-one (7) gave (1RS,6SR,9RS)-9-benzyl-8-azabicyclo[4.3.0]nonan-7-one and N-benzyl-2-(2-phenylethyl)cyclohexane-1-carboxamide (20). A similar cleavage of the β-styryl-nitrogen bond was observed in the reduction of (Z)-N-benzyl-N-(1-methyl-2-phenylvinyl)acetamide (15a), but not with (E)-N-styrylpyrrolidin-2-one (13).

Several tertiary N-styrylenamides were not reduced by sodium cyanoborohydride in acetic acid but N-benzyl-9-benzylidene-cis-8-azabicyclo[4.3.0]non-3-en-7-one (8)and N-benzyl-9-benzylidene cis-8-azabicyclo[4.3.0]nonan-7-one (7) gave (1RS,6SR,9RS)-N,9-dibenzyl-8-azabicyclo[4.3.0]non-3-en-7-one (9) and (1RS,6SR,9RS)-N,9-dibenzyl-8-azabicyclo[4.3.0]nonan-7-one (2) respectively.