Ring-expansion of thioacetal ring via bicyclosulfonium ylide. Effect Of protic nucleophile on ylide intermediate

The reaction of 2-(3-diazo-2-oxopropane-1-yl)-2-methyldithiolane 9a, 2-(4-diazo-3-oxobutane-2-yl)-2-methyldithiolane 9c, and 2-(3-diazo-2-oxopropane-1-yl)-2-methyl-1,3-dithiane 9b with Rh(2)(OAc)(4) gave three-carbon ring-expansion products dithiocan-2-en-1-ones 13a, 13c and dithionan-2-en-1-one 13b, respectively. 2-(5-Diazo-4-oxopentyl)-2-methyldithiolane 9e also gave the five-carbon ring-expansion products dithionan-3-en-1-one 13e and 5-methylenedithionane-1-one 13'e. On the other hand, reaction of 2-(4-diazo-3-oxobutyl)-2-methyldithiolane 9d in the presence of AcOH gave the four-carbon ring-expansion product 16d substituted by an acetoxyl group. In addition, the reaction of 2-(3-diazo-2-oxopropyl)-2-methyl-3-oxathiolane 9f in the absence of AcOH gave 4-oxa-7-thiocan-2-en-1-one 19 via a sulfonium ylide intermediate, whereas, in the presence of AcOH, an alternative regioisomer 20 was also formed competitively with 19 via an oxonium ylide intermediate.