Synthesis of Diastereomers of Alliin, Selenoalliin, and Isoalliin

Alliin (S-allyl-L-cysteine sulfoxide) and isoalliin (S-1-propenyl-L-cysteine sulfoxide) are major compounds in Allium species that contain sulfur and carbon stereocenters. Alliin is the precursor of allicin, which is one of the most biologically active molecules in garlic. Isoalliin is the precursor of onion's lachrymatory factor, and is responsible for the pink discoloration of onion and the green/blue discoloration of garlic. Many synthesis schemes of diastereomers of alliin and isoalliin are discussed in this review. Alliin can be synthesized by not only the classical method of sulfur oxidation using hydrogen peroxide, but also asymmetric sulfur oxidation using tetraisopropyl ortho-titannate or oxygenase. Besides typical method of using allyl bromide and then isomerization by a base catalyzed reaction, isoalliin can also be synthesized by Pd-catalyzed carbon sulfur coupling of (E)-1-bromoprop-1-ene. Fractional crystallization and HPLC have proven to be effective tools in isolation of diasteriomers. Synthesis of the selenium analogue of alliin is also presented in this review.