Ring-enlargement reaction of alkylidenecarbenes bearing a cyclic ether or acetal group. Formation of medium-sized cyclic enol ethers or dienol ethers via bicycloalkenyloxonium ylides
The reaction of 2-acetonyltetrahydrofuran 1a and 2-acetonyltetrahydropyran 1b with the potassium salt of dimethyl diazomethylphosphonate (DAMP) in the presence of MeOH produced ring enlargement product 1-methyl-3-oxacyclooctene 6a (22%) and 1-methyl-3-oxacyclononene 6b (28%), respectively, in addition to nonrearranged products. When the side-chain was elongated by one carbon unit (1c), ring enlargement did not take place. Analogous reactions of 2-acetonyl-substituted 1,3-dioxolane 9a, 1,3-dioxane 9b and 1,3-dioxepane 9c also produced, respectively, 3,5-dimethyl-1,6-dioxacycloocta-2,4-diene (16a, 58%), a mixture (combined yield 83%) of 3,5-dimethyl-1,6-dioxacyclonona-2,4-diene (16b) and 5-exo-methylene-3-methyl-1,6-dioxacyclonon-2-ene (17b), and a mixture (combined yield 54%) of 3,5-dimethyl-1,6-dioxacyclodeca-2,4-diene (16e) and 5-exo-methylene-3-methyl-1,6-dioxacyclodec-2-ene (17e). Side-chain-elongated dioxolane 9d did not undergo enlargement and, instead, ring-switched product 3,6-dimethyl-6-[2-(tert-butyloxy)ethoxy]-5,6-dihydropyran 22 was formed. The formation of products 6, 16, 17 and 22 can be explained in terms of the intermediacy of bicyclooxonium ylides, which are formed in an intramolecular manner between the alkylidenecarbene and a cyclic ether or cyclic acetal unit. In most reactions of acetonyl-substituted cyclic acetals, the major products were di(enol ether)s 16 and 17 even in the presence of a protic nucleophile such as MeOH. A reversible intramolecular process between alkylidenecarbenes and ylides is also proposed.