Studies on the rearrangement of 1,3-oxathiolane sulfoxides

Abstract

A mechanistic study of rearrangement of 2-methyl-1,3-oxathiolane-2-acetic methyl ester and amide sulfoxides 1 is described. Oxidation of 2-methyl-N-phenyl-1,3-oxathiolane-2-acetamide 3a and 2-methyl-1,3-oxathiolane-2-acetic acid methyl ester 3b gave a mixture of cis and trans sulfoxide 5 major and 6 minor. In $C_6H_6$-DMF at 50$^\circ$C cis sulfoxides 5 were transformed via sulfenic acids 9 to thiolsulfinates 16. In DMF at 100$^\circ$C the trans sulfoxides 6 rearranged to sulfenic acids 10 involving the unactivated 2-methyl group, followed by eyclization to isomeric dihydrooxathiin 12. The ring opening of the cis and trans sulfoxide to form corresponding sulfenic acid 9 and 10 occurs probably by [2,3] sigmatropic process. In refluxing toluene thiolsulfinate 16a was transformed via intermediates thioaldehyde 25 and sulfenic acid 9a to a 1:1 mixture of epi-sulfide 24 and cis sulfoixde 5a. With PTSA as acid catalyst in $C_6H_6$-DMF both the cis and trans sulfoxides 5a and 6a were transformed via sulfenic acids 9a and thiolsulfinate to a 5:4:1 mixture of -hydroxy sulfide 29a, dihydro-1,4-oxathiin 2a, and acetoacetanilide 4a in quantitative yields. The mechanism of formation of 28a and 2a was confirmed by isotope labelling experiment using $H_^{18}O$. With triethylamine as base catalyst the cis sulfoxides 5 were transformed to thiolsulfinate 16 in high yield while the trans isomer 6 were recovered unchanged. In the presence of potassium hydroxide in aqueous dioxane, both the cis and trans sulfoxides 5 and 6 were converted to a mixture of disulfide 14, sulfinic acid 42, and a small amount of isomeric disulfide 43, of which structure was identified by various spectral data, elemental analysis, and X-ray crystallography. The mechanism of formation of 14, 42, 43 are also discussed.