AlliumChemistry: Microwave Spectroscopic Identification, Mechanism of Formation, Synthesis, and Reactions of (E,Z)-PropanethialS-Oxide, the Lachrymatory Factor of the Onion (Allium cepa)

Abstract

Flow pyrolysis of 2-methyl-2-propyl 1‘-propenyl sulfoxide (9b) affords a 98:2 mixture of (Z)- and (E)-propanethial S-oxide ((Z)- and (E)-5b), both characterized by Fourier transform microwave (FT-MW) spectroscopy. Sulfines (Z)- and (E)-5b are also identified by FT-MW in chopped onion volatiles and by NMR spectroscopy in onion extracts. Similarly, flow pyrolysis of 2-methyl-2-propyl vinyl sulfoxide (9c) affords (Z)- and (E)-isomers of ethanethial S-oxide (5a), identified by FT-MW methods. Pyrolysis in the presence of D2O affords (Z)-5a-d1 and (Z)-5a-d2 from 9c and (Z)-5b-d1 from 9b; (Z)-5b-d1 is also produced when an onion is homogenized in D2O. Pyrolysis of 9c with ethyl propiolate gives ethyl (E)-3-(vinylsulfinyl)acrylate (10). Neat 5a at 100 °C gives acetaldehyde. On standing, 5b dimerizes to trans-3,4-diethyl-1,2-dithietane 1,1-dioxide (12a); Me3SiCHS+−O- (5f) undergoes an analogous dimerization. Compound 5b shows moderate potency as an anticarcinogen in inducing the enzyme quinone reductase.