エーテル型オキソニウムイリド形成を仮説とする合成反応

Abstract

Ethereal oxonium ylides are highly reactive and short-lived intermediates compared with other onium ylides. While they possess appreciable potential for synthetic use, e.g. in substituting one of the two ethereal ligands for a carbene-derived group, systematic use of this type of ylides for synthesis has been limited. Recently, we have found novel methods of controlling the reactivity of the ylides on the basis of designing intramolecular ketocarbenoid or carbene reactions with cyclic ethereal moieties. The findings are (1) use of protic nucleophiles for trapping the ylides in the form of oxonium ions to suppress both undesired side-reactions and dissociation to carbenes. Thus, with cyclic ethers, acetals and orthoesters, ring-enlargement reactions take place predominantly over sigmatropic pathways of oxonium ylides, thus enabling the synthesis of macrocycles bearing ketopolyether functionality. (2) Strained bicyclo [3.n.O] oxonium ylides derived from cyclic ethers undergo sigmatropic ring-cleavage reaction to give alkenyloxyketenes efficiently. (3) Silyl reagents can trap the ylides to form ring-enlarged enol silyl ethers, which are versatile intermediates for both the ring-enlargement and novel C-C bond formation reactions with carbon electrophiles. (4) Not only ketocarbenoids but also free alkylidenecarbenes bearing a cyclic ether or acetal group undergo the similar ring-enlargement. Similar reaction of sulfonium analogues is also described.