SYNTHESIS OF THE ALIPHATIC DEPSIDE (+)-BOURGEANIC ACID

The lichen metabolite (+)-bourgeanic acid (1) was synthesized in 12 steps and 3.4% overall yield from (R)-2-methyl-1-iodobutane (53) by a sequence which confirmed that this aliphatic depside is the self-esterification product of (2S,3S,4R,6R)-2,4,6-trimethyl-3-hydroxyoctanoic acid. Alkylation of the enolate of (S)-N-propionylprolinol (48) with 53 gave the amide 60 which was transformed to (2R,4R)-2,4-dimethylhexanal (4). The latter was reacted with the crotylboronate 68, prepared from (S,S)-(-)-diisopropyltartrate, to afford (3R,4S,5R,7R)-3,5,7-trimethyl-1-nonen-4-ol (65) as the major diastereomer. Protection followed by ozonolysis and oxidation furnished (-)-hemibourgeanic acid (2). The beta-lactone 73 derived from 2 was used to acylate 65, and the resulting ester 74 was subjected to oxidative ozonolysis to yield (+)-1.