Allium chemistry: Microwave spectroscopic identification, mechanism of formation, synthesis, and reactions of (E,Z)-Propanethial S-oxide, the lachrymatory factor of the onion (Allium cepa)

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-d(1) and (Z)-5a-d(2) from 99 and (Z)-5b-d(1) from 9b; (Z)-5b-d(1) is also produced when an onion is homogenized in D2O. Pyrolysis of 9c with ethyl propiolate gives ethyl (E)-3-(vinylsulfinyl)acrylate (10). Neat 59 at 100 degrees C gives acetaldehyde. On standing, 5b dimerizes to trans-3,3-diethyl-1,2-dithietane 1,1-dioxide (12a); Me(3)SiCH=S+-O- (5f) undergoes an analogous dimerization. Compound 5b shows moderate potency as an anticarcinogen in inducing the enzyme quinone reductase.