The Organosulfur Chemistry of the Genus Allium – Implications for the Organic Chemistry of Sulfur

Abstract

A Cook's tour is presented of the organosulfur chemistry of the genus Allium, as represented, inter alia, by garlic (Allium sativum L.) and onion (Allium cepa L.). We report on the biosynthesis of the S‐alk(en)yl‐L‐cysteine S‐oxides (aroma and flavor precursors) in intact plants and on how upon cutting or crushing the plants these precursors are cleaved by allinase enzymes, giving sulfenic acids—highly reactive organosulfur intermediates. In garlic, 2‐propenesulfenic acid gives allicin, a thiosulfinate with antibiotic properties, while in onion 1‐propenesulfenic acid rearranges to the sulfine (Z)‐propanethial S‐oxide, the lachrymatory factor (LF) of onion. Highlights of onion chemistry include the assignment of stereochemistry to the LF and determination of the mechanism of its dimerization; the isolation, characterization, and synthesis of thiosulfinates which most closely duplicate the taste and aroma of the freshly cut bulb, and additional unusual compounds such as zwiebelanes (dithiabicyclo[2.1.1]hexanes), a bis‐sulfine (a 1,4‐butanedithial S,S′‐dioxide), antithrombotic and antiasthmatic cepaenes (α‐sulfinyl disulfides), and vic‐disulfoxides. Especially noteworthy in the chemistry of garlic are the discovery of ajoene, a potent antithrombotic agent from garlic, and the elucidation of the unique sequence of reactions that occur when diallyl disulfide, which is present in steam‐distilled garlic oil, is heated. Reaction mechanisms under discussion include [3, 3]‐ and [2, 3]‐sigma‐tropic rearrangements involving sulfur (e.g. sulfoxide‐accelerated thio‐ and dithio‐Claisen rearrangements) and cycloadditions involving thiocarbonyl systems. In view of the culinary importance of alliaceous plants as well as the unique history of their use in folk medicine, this survey concludes with a discussion of the physiological activity of the components of these plants: cancer prevention, antimicrobial activity, insect and animal attractive/repulsive activity, olfactory–gustatory–lachrymatory properties, effect on lipid metabolism, platelet aggregation inhibitory activity and properties associated with ajoene. And naturally, comments about onion and garlic induced bad breath and heartburn may not be overlooked.