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.

Allithiolanes: Nine Groups of a Newly Discovered Family of Sulfur Compounds Responsible for the Bitter Off-Taste of Processed Onion

The compounds responsible for the bitter off-taste of processed onion ( Allium cepa) were studied. Using a series of sensory-guided HPLC fractionations, the existence of nine groups of hitherto unknown sulfur compounds has been revealed. On the basis of spectroscopic data (MS, NMR, and IR), it was found that these compounds, trivially named allithiolanes A-I, are members of a large family of structurally closely related derivatives of 3,4-dimethylthiolane S-oxide, with the general formulas of C _xH _yO₂S₄, C _xH _yO₃S₅, and C _xH _yO₄S₆ ( x = 10-18, y = 18-30). The presence of multiple stereoisomers was observed for each group of allithiolanes. Allithiolanes possess an unpleasantly bitter taste with detection thresholds in the range of 15-30 ppm. Formation pathways of these newly discovered sulfur compounds were proposed.