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 (+/-)-(1 alpha,2 alpha,3 beta,4 alpha,5 beta)-2,3-dimethyl-5,6-dithiabicyclo[2.1.1]hexane 5-oxide (10a; 10%) and (1 alpha,2 alpha,3 alpha,4 alpha,5 beta)-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 (+/-)-(1 alpha,2 alpha,3 beta,4 alpha,5 alpha,6 alpha)-2,3-dimethyl-5,6-dithiabicylo[2.1.1]hexane 5,6-dioxide (16) and (+/-)-(1 alpha,2 alpha,3 beta,4 alpha,5 beta,6 alpha)-2,3-dimethyl-5,6-ditiabicyclo[2.1.1]hexane 5,6-dioxide (17a); these could be oxidized further to (+/-)-(1 alpha,2 alpha,3 beta,3 alpha,5 alpha,5 beta,6 alpha)-2,3-dimethyl-5,6-dithiabicyclo[2.1.1]hexane 5,5,6-trioxide (18a) and (+/-)-(1 alpha,2 alpha,3 beta,4 alpha,5 alpha,5 beta,6 beta)-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 (1 alpha,2 alpha,3 alpha,3 alpha,5 beta,6 beta)-2,3-dimethyl-5,6-dithiabicyclo[2.1.1]hexane 5,6-dioxide (21a) which was further oxidized to trioxides (1 alpha,2 alpha,3 alpha,4 alpha,5 alpha,5 beta,6 beta)-2,3-dimethyl-5,6-dithiabicyclo[2.1.1]hexane 5,5,6-trioxide (23a) and (1 alpha,2 beta,3 beta,4 alpha,5 alpha,5 beta,6 beta)-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.