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 D 2 O affords ( Z )- 5a - d 1 and ( Z )- 5a - d 2 from 9c and ( Z )- 5b - d 1 from 9b; ( Z )- 5b - d 1 is also produced when an onion is homogenized in D 2 O. 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 ); Me 3 SiCH S + −O - ( 5f ) undergoes an analogous dimerization. Compound 5b shows moderate potency as an anticarcinogen in inducing the enzyme quinone reductase.