Allium Chemistry: Structure, Synthesis, Natural Occurrence in Onion (Allium cepa), and Reactions of 2,3-Dimethyl-5,6-dithiabicyclo[2.1.1]hexane S-Oxides

Peracetic acid oxidation of di-1-propenyl disulfide ( 8 ) gives (±)-(1α,2α,3β,4α,5β)-2,3-dimethyl-5,6-dithiabicyclo[2.1.1]hexane 5-oxide ( 10a; 10%) and (1α,2α,3α,4α,5β)-2,3-dimethyl-5,6-dithiabicyclo[2.1.1]hexane 5-oxide ( 11a; 11%), both also isolated from extracts of homogenized onion. Compound 10a could be converted into bissulfoxides (±)-(1α,2α,3β,4α,5α,6α)-2,3-dimethyl-5,6-dithiabicyclo[2.1.1]hexane 5,6-dioxide ( 16 ) and (±)-(1α,2α,3β,4α,5β,6α)-2,3-dimethyl-5,6-dithiabicyclo[2.1.1]hexane 5,6-dioxide ( 17a ); these could be oxidized further to (±)-(1α,2α,3β,4α,5α,5β,6α)-2,3-dimethyl-5,6-dithiabicyclo[2.1.1]hexane 5,5,6-trioxide ( 18a ) and (±)-(1α,2α,3β,4α,5α,5β,6β)-2,3-dimethyl-5,6-dithiabicyclo[2.1.1]hexane 5,5,6-trioxide ( 18b ) from 16 and 18a from 17a . Extended oxidation of 10a gave (±)- trans -2,3-dimethyl-5,6-dithiabicyclo[2.1.1]hexane 5,5,6,6-tetraoxide ( 19 ). Oxidation of 11a gave (1α,2α,3α,4α,5β,6β)-2,3-dimethyl-5,6-dithiabicyclo[2.1.1]hexane 5,6-dioxide ( 21a ) which was further oxidized to trioxides (1α,2α,3α,4α,5α,5β,6β)-2,3-dimethyl-5,6-dithiabicyclo[2.1.1]hexane 5,5,6-trioxide ( 23a ) and (1α,2β,3β,4α,5α,5β,6β)-2,3-dimethyl-5,6-dithiabicyclo[2.1.1]hexane 5,5,6-trioxide ( 23b ) and a bissulfone ( cis -2,3-dimethyl-5,6-dithiabicyclo[2.1.1]hexane 5,5,6,6-tetraoxide ( 24 )). The structures of 18a, 19, 21a, and 23a were determined by X-ray crystallography. With the proof of the structure of 18a, structures for 17a and 10a are thereby unequivocally established.