Gold-catalysed generation of diol equivalents from epoxides and their intramolecular reaction with C≡C bonds to generate bicyclicketals is presented. This reaction essentially involves the formation of an acetonide, which subsequently cyclises on the alkyne intramolecularly under gold catalysis conditions. This method could be extended to make optically pure bicyclicketals. Deuterium incorporation
A quantitative study on the effects of methyl and gem-dimethyl groups in the intramolecular ring opening of epoxides by bis-sulfonyl carbanions is reported for the formation of cyclopropanes. Reaction rates are increased by methyl groups on the chain connecting the nucleophile with the oxirane and depressed by methyl substitution in the epoxide ring. When gem-dimethyl groups are present on the epoxide, ring opening is apparently inhibited. It will be shown that in this case the reaction is reversible and the apparent stabilization of the gem-dimethyl-substituted oxirane is actually due to a combination of effects on both the forward and the reverse reactions. On the basis of current theories on intramolecular reactions, it is suggested that release of ground-state strain and van der Waals repulsions in the transition state can account for the observed reactivity.
Intramolecular Ring Opening of Epoxides by Bis-Activated Carbanions. The Influence of Ring Size on Reactivity and Selectivity
A quantitative study on the effect of ring size in the intramolecular ring opening of epoxides by carbanions is described. Two series of substrates were examined: alpha,alpha-bis-sulfonyl omega-epoxides 1 and alpha-cyano-alpha-sulfonyl omega-epoxides 2; in each series the carbanion is tethered to the epoxide by a chain of variable length from one to four methylene groups. The nucleophile can attack either electrophilic position of the oxirane ring, or both; exo ring opening of cyano sulfonyl epoxides 2 is followed by a second cyclization leading eventually to bicyclic, fused gamma-lactones. Both series of epoxides show the same trend of reactivity as a function of ring size, in the formation of three- to seven-membered rings, with reactivity maxima corresponding to the formation of cyclopropane and cyclopentane derivatives. Unlike S(N)2 ring closure of omega-halogeno carbanions, cyclization to a five-membered ring is the fastest process in this case. The ratio k3/k5 between formation of three- and five-membered rings drops from over 100, in the S(N)2 cyclization of omega-iodo bis-sulfones, to less than 0.5, in the cyclization of omega-epoxy bis-sulfones 1. The difference is discussed in terms of trajectory of approach of the carbanion to the nucleophilic center. Cyclization of cyano sulfonyl epoxide 2a, in which the nucleophilic center and the epoxide are spaced by a single methylene group, is diastereoselective and leads to a bicyclic product with a cis fusion between the gamma-lactone and the cyclopropane ring.
BENEDETTI, FABIO;BERTI, FEDERICO;FABRISSIN, SILVIO;GIANFERRARA, TERESA;RI+, J. ORG. CHEM., 56,(1991) N1, C. 3530-3537