Stereoselective Organocatalytic Approach to α,β‐Disubstituted‐β‐amino Acids: A Short Enantioselective Synthesis of Cispentacin

Abstract

Abstractα‐Branched α,β‐unsaturated aldehydes have been tested in the organocatalytic tandem Michael addition/cyclization with N‐(benzyloxycarbonyl)hydroxylamine, a reaction which until now has been restricted to α‐unsubstituted enals. Starting from cyclopentene‐2‐carbaldehyde, and using diphenylprolinol trimethylsilyl ether as a chiral amine catalyst, this approach has led to the development of a practical, high yielding (93–98 % overall yield, three steps), and highly enantioselective (up to 98:2 er) route to the cyclic β‐amino acid cispentacin, which compares favourably with previously described asymmetric syntheses of this biologically active natural product. When using acyclic α‐branched α,β‐unsaturated aldehydes as substrates, the reaction yields depend on the substitution pattern of the aldehydes, and mixtures of cis‐ and trans‐isomers are obtained. Nevertheless, this strategy has proved to be successful in some instances, and (3R,4R)‐benzyl 3‐ethyl‐4‐methyl‐5‐oxoisoxazolidin‐2‐carboxylate could be obtained in 70 % overall yield (two steps) from the reaction of 2‐ethylcrotonaldehyde and N‐(benzyloxycarbonyl)hydroxylamine under catalysis with diphenylprolinol trimethylsilyl ether, and with high enantiomeric purity (99:1 er).