Enantioselective synthesis and determination of the configuration of stenusine, the spreading agent of the beetle Stenus comma

Abstract

The enantioselective total synthesis of two of the four possible stereoisomers of stenusine (1), the spreading agent of the beetle Stenus comma, is described. The silyl ether-substituted aldehyde SAMP hydrazone 2 was alkylated with (S)-1-bromo-2-methylbutane (3) yielding the hydrazone 4 in high diastereomeric purity (de >95 %). By several steps including the reduction of the hydrazone functionality and the cleavage of the N-N bond of the intermediates, 4 was converted into the BOC-protected amino alcohol 6. Subsequent cyclization of 6 afforded the (S,S) diastereomer of stenusine with 96.6 % de, >99.9 % ee, and in 11.3 % overall yield. Repetition of this synthesis using the aldehyde RAMP hydrazone (R)-2 as the starting material produced (SR)-1 with 95.0 % de, >99 % ee, and in 8.2 % overall yield. The synthetic samples of 1 were employed to investigate the stereochemistry of natural stenusine by means of GC analysis on both a chiral and an achiral, stationary phase. As a result of these studies natural stenusine was found to be a mixture of all four stereoisomers in a ratio of (S,S)/(SR)/(RR)/(R:S) = 43:40:13:4.