Absolute stereochemistry of exogonic acid

Exogonic acid (2-(carboxymethyl)-7-methyl-1,6-dioxaspiro[4.4]nonane), a resin constituent of the Brazilian tree Ipomoea operculata (Martin) is demonstrated to be predominantly the E,E and Z,Z diastereomers, with the 2S,5S,7R and 2S,5R,7R configurations, respectively. Minor amounts of the 2R,5S,7R E,Z and 2R,5R,7R Z,E isomers are also present. These conclusions are based on chiral gas chromatographic analyses of suitable derivatives and enantioselective syntheses employing (S)-1,2-epoxypropane and (2S)-4-[[(1,1-dimethylethyl)dimethylsilyl]oxy]-1,2-epoxybutane as alkylating agents for anions of acetone N,N-dimethylhydrazone.