Diastereoselective Synthesis of gamma-Amino Acids and Their Derivatives from Nitroethane via Intermediacy of 5,6-Dihydro-4H-l,2-oxazines Bearing the CH_2CH(CO_2Me)_2 Substituent at C3

摘要

Stereoselective two-step reduction of available 2-[(5,6-dihydro-4H-l,2-oxazin-3-yl)methyl]malonates provides an efficient route to derivatives of different y-amino acids. The mechanism and stereochemistry of the first step, reduction of the C=N bond with sodium cyanoborohydride, is discussed.Catalytic hydrogenation of six-membered cyclic oxime ethers, 5,6-dihydro-4H-l,2-oxazines 1, is an efficient method for the synthesis of various types of products such as pyrrolidines,1 pyrroles,2 1,4-amino alcohols,3 1,4-dike-tones,4 and furan derivatives.5 Usually, the hydrogenation process starts with cleavage of the relatively weak en-docyclic N-O bond and all the above-mentioned products result from transformations of the initially formed y-hy-droxy imines B (Scheme 1). The type of final product primarily depends on the presence (or absence) of an alkoxy substituent at C6 in the dihydrooxazines 1.