Investigations of the biosynthesis of the naturally occurring 1,2-dithiolane asparagusic acid (1) in Asparagus officinalis have shown that the substance is derived from isobutyric acid via the intermediacy of methacrylic acid, 2-methyl-3-mercaptopropionic acid, and S-(2-carboxy-n-propyl)cysteine. The conversion of isobutyric acid to methacrylic acid in Asparagus has also been shown to proceed by oxidation of the 2-pro-S methyl group of isobutyrate. Finally, the absolute configuration of naturally occurring S-(2-carboxy-n-propyl)cysteine has been determined.