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-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 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); Me3SiCHS+−O- (5f) undergoes an analogous dimerization. Compound 5b shows moderate potency as an anticarcinogen in inducing the enzyme quinone reductase.

Rotational Spectra, Molecular Structure, and Electric Dipole Moment of Propanethial S-Oxide

“…3 A sample of 2 was transferred to a steel trap and pressurized to about 20 psi above atmospheric pressure with spark gas (∼80% neon/∼20% helium). The gas output from the trap flowed through a 1 mm i.d.…”

“…3 Since the microwave spectra of the skew rotatomers of 1a and 1b are observed, other conformers must be higher in energy and are not populated in the cold gas pulses. In addition, there are no resolvable splittings of the observed rotational transitions of 1a and 1b.…”

“…2 Recently, propanethial S-oxide was detected in the vapor above chopped onions by a direct in situ spectroscopic technique. 3 Freshly chopped onions were placed in a tube and the volatile gases carried in Ne/He to a modified solenoid gas valve which sampled the gas mixture for analysis in the Fabry-Perot cavity of a Fourier transform microwave spectrometer. 3 The high resolution and molecular isotopic specificity of the Fourier transform microwave technique provided an unambiguous way to identify the site of deuterium incorporation in propanethial S-oxide obtained from an onion-D 2 O homogenate.…”

Identification of Amino Acid Residues Essential for Onion Lachrymatory Factor Synthase Activity

“…Musah et al have reported an enzyme that catalyzes the formation of the sulfine LF, (Z)-phenylmethanethial S-oxide, from the roots of Petiveria alliacea, 11) but the reaction catalyzed by this enzyme differed considerably from that catalyzed by onion LFS. While onion LF is produced through an intra-molecular proton (H þ ) substitution reaction, 12) Petiveria LF is produced by a dehydrogenation reaction. Additionally, a database survey failed to identify any known functional domains exhibiting high sequence homology with onion LFS.…”

Article

“…Specifically, 1propenesulfenic acid (3) has been shown to be the precursor of (Z)-and (E)-propanethial S-oxides (4), the familiar lachrymatory factor of onions (9). Unsaturated sulfenic acid amides have demonstrated usefulness in synthesis (10)(11)(12).…”