Studies on the lewis acid mediated ring expansion of cyclic ketones and functional group conversion

Abstract

Two new methods for the ring expansion of cyclic ketones via an alkylidenesulfonium and an episulfonium ion intermediate have been developed. The α,β-doubly deprotonated dianion of phenylthionitromethane is readily added to cyclic ketones and subsequent rearrangement of the resulting adducts proceeds well in the presence of aluminum chloride to afford the ring expanded α-phenylthio ketones via aluminum chloride mediated denitration. Tert-Butyldimethylsilyl ethers of 1-vinyl-1-cyclo-alkanols have been also found to be rearranged, upon successive treatment with benzenesulfenyl chloride and silver tetrafluoroborate, to produce the ring expanded α-(1-phenylthioalkyl) ketones via an episulfonium ion intermediate. The synthetic utilities of magnesium bromide and boron tribromide have been investigated. Magnesium bromide can be utilized as a mild and efficient reagent for the selective cleavage of tetrahydropyranyl ethers in the presence of tertbutyldimethylsilyl ethers. Selective conversion of acetals into thioacetals, monothioacetals, α-alkoxyazides, and α-alkoxyalkyl thioacetates have been performed successfully with magnesium bromide in the presence of appropriate nucleophiles such as thiols, hydrozoic acid, and thiolacetic acid. Boron tribromide has been found to be effective for the direct conversion of tert-butyldimethylsilyl and tert-butyldiphenylsilyl ethers into alkyl bromides. This reaction is believed to proceed via a dual mechanism involving complex formation from TBDMS ethers and boron tribromide followed by internal nucleophilic displacement ($S_Ni$) along with $S_N1$.