Isoalliin-Derived Thiolanes Formed in Homogenized Onion

Several families of 3,4-dimethylthiolane-based compounds spontaneously formed upon cutting of onion (Allium cepa) were studied. We report the isolation of the first known example of a naturally occurring dithiolactone, 5-hydroxy-3,4-dimethylthiolane-2-thione (cepadithiolactone A, C₆H₁₀OS₂). Furthermore, on the basis of conceivable spectroscopic evidence (MS, NMR, IR), we could disprove the structure previously proposed for onionin A (C₉H₁₆O₂S₂), which is shown to be in fact (E)-3,4-dimethyl-5-(1-propenylsulfinyl)thiolane-2-ol. The identification of hitherto unknown methyl and propyl homologues of onionin A (dubbed onionins B and C, respectively) is also reported. Furthermore, the existence of the methyl and propyl homologues of cepathiolanes A (C₉H₁₆O₂S₃), trivially named cepathiolanes B and C, respectively, has been newly revealed. The organoleptic properties of these 3,4-dimethylthiolanes and their role in the formation of the pink discoloration of processed onion were also evaluated.

Ajothiolanes: 3,4-Dimethylthiolane Natural Products from Garlic ( Allium sativum)

Stereoisomers of 5-(2-allylsulfinyl)-3,4-dimethylthiolane-2-ol, a family of 3,4-dimethylthiolanes of formula C₉H₁₆O₂S₂ we name ajothiolanes, were isolated from garlic ( Allium sativum) macerates and characterized by a variety of analytical and spectroscopic techniques, including ultraperformance liquid chromatography (UPLC), direct analysis in real time-mass spectrometry (DART-MS), and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Ajothiolanes were found to be spectroscopically identical to a family of previously described compounds named garlicnins B_1-4 (C₉H₁₆O₂S₂), whose structures we demonstrate have been misassigned. 2D ¹³C-¹³C NMR incredible natural abundance double quantum transfer experiments (INADEQUATE) were used to disprove the claim of nine contiguous carbons in these compounds, while X-ray absorption spectroscopy (XAS) along with computational modeling was used to disprove the claim that these compounds were thiolanesulfenic acids. On the basis of the similarity of their NMR spectra to those of the ajothiolanes, we propose that the structures of previously described, biologically active onionins A_1-3 (C₉H₁₆O₂S₂), from extracts of onion ( Allium cepa) and Allium fistulosum, and garlicnin A (C₁₂H₂₀O₂S₄), from garlic extracts, should also be reassigned, in each case as isomeric mixtures of 5-substituted-3,4-dimethylthiolane-2-ols. We conclude that 3,4-dimethylthiolanes may be a common motif in Allium chemistry. Finally, we show that another garlic extract component, garlicnin D (C₇H₁₂O₂S₃), claimed to have an unprecedented structure, is in fact a known compound from garlic with a structure different from that proposed, namely, 2( E)-3-(methylsulfinyl)-2-propenyl 2-propenyl disulfide.