Medium-ring .gamma.-epoxy sulfones. Regio- and stereochemistry of the butyllithium-promoted transannular epoxide ring opening
摘要:
Cyclic 8- and 9-membered E and Z gamma-epoxy sulfones yield bicyclic hydroxy sulfones stereospecifically upon treatment with BuLi in THF. The distribution of regioisomeric products markedly depends on the metalation mode, portionwise or at once. This suggests that the metal/proton exchange between the two positions alpha to the sulfone function is slow, relative to transannular cyclization, under conditions of complete metalation, but becomes fast in the presence of unmetalated sulfone (probably acting as a H+-transfer agent). The product distribution also depends, though to a lesser extent, on temperature, perhaps in relation to changes in the aggregation of the organolithium intermediate. An unexpected phenomenon was observed with the 8-membered Z epoxy sulfone at -70-degrees-C, where the time dependence of the transannular cyclization is consistent with an equilibrium being reached at 2/3 conversion. However, at -40-degrees-C or higher the reaction proceeds to completion without difficulty. A low-temperature metastable equilibrium is suggested involving some kind of aggregate of the lithio sulfone with the lithioalkoxide product.
Ru2O . nH(2)O and [Ru3O(AcO)(6)(H2O)(3)](AcO-)-Ac-+ were examined for catalytic activity in the hydrogenation of a series of unsaturated sulphides under heterogeneous and homogeneous conditions, respectively. By the appropriate combination of these two methodologies, a number of saturated sulphides could be synthesized in satisfactory to good yields, thus minimizing side reactions.
Cere, Vanda; Peri, Francesca; Pollicino, Salvatore, Journal of the Chemical Society. Perkin Transactions 2 (2001), 1998, # 4, p. 977 - 979
Anionic desilylation of alpha-silylated (E)-4,5-epoxythiacyclononane 1,1-dioxides results in formation of transannular cyclization products whose distribution is independent of the stereochemistry of the starting material. Unlike the species formed by direct BuLi metalation,1 the intermediate arising from the 9-sily regioisomer rapidly and indiscriminately attacks either epoxidic carbon forming both cis- and trans-joined bicyclic products. This lack of selectivity suggests the reactive intermediate is not a hypervalent silicon species, but is more likely a carbanion which, being associated to a large cation (K+ or Bu4N+) is less discriminating than that arising from BuLi metalation. The intermediate formed by desilylation of the 2-silyl regioisomer appears to cyclize relatively slowly, such that transfer of the carbanionic functionality from C-2 to C-9 becomes competitive with cyclization. With this substrate an unexpected phenomenon has emerged, namely that t-BuOK may act as a base or a desilylating agent depending on the temperature.
CERE, V.;PAOLUCCI, C.;POLLICINO, S.;SANDRI, E.;FAVA, A., J. ORG. CHEM., 1982, 47, N 15, 2861-2867
作者:CERE, V.、PAOLUCCI, C.、POLLICINO, S.、SANDRI, E.、FAVA, A.