Abstract
Silylcupration of allene followed by conjugate addition to enones and carbonyl epoxidation provides a simple and high yielding route to epoxyallylsilanes carrying the useful phenyldimethylsilyl group. Lewis acid catalyzed intramolecular cyclization of epoxyallylsilanes is a powerful strategy for carbocyclic annulation of much potential in synthesis. In this article we show A general procedure for the synthesis of 3-methylene-1-cyclohexanols fused to five, six and seven-membered carbocycles by intramolecular cyclization of epoxyallylsilanes, which might be of great interest as an approach to bicyclic systems contained in many naturally occurring products, is described.
Highlights
Contrary to the normal 5-exo9 or 6endo10,11 attack, we have recently reported that the acid-catalyzed reaction of epoxyallylsilanes containing the phenyldimethylsilyl12 or the tert-butyldiphenylsilyl13,14 group follows an unusual rearrangement-cyclization process which leads to methylenecyclohexanols12 or to allyl- and vinylsilane-containing cyclohexanols13 respectively (Scheme 2)
Epoxyallylsilanes have been previously prepared by Wittig reaction of an aldehyde with Ph3P=CHCH2SiMe3 followed by epoxidation15 and by cross metathesis of alkenyl epoxides and allylsilanes
The intermediate allylsilane-vinylcuprate 1 can be captured by a great variety of electrophiles giving functionalized allylsilanes.2a For example, the use of α,β-unsaturated oxocompounds leads to oxoallylsilanes of type 2 which have been used as powerful building blocks for cyclopentane annelations.2a,6a,18 Epoxidation of 2 via dimethylsulfonium-ylides gives epoxyallylsilanes 3 in high yield (Scheme 3)
Summary
Organosilicon compounds and in particular allylsilane chemistry have attracted a great deal of attention due to the increasing number of new methodologies that allow useful synthetic transformations.1 Over the last decade allenes have emerged as one of the best sources for the synthesis of allylsilanes.2 They are readily attacked by silylcuprates giving rise to a great variety of allylsilanes with different substitution patterns.3 The stoichiometry of the silylcuprate (higher or lower order) is responsible for the final regioselectivity of the reaction, leading selectively to allylsilanes when a lower order cyanosilylcuprate (R3SiCuCNLi) is used.4Scheme 1 α,β-Unsaturated oxocompounds have been successfully used in this reaction, leading to a wide range of oxo-functionalized allylsilanes, which are valuable intermediates for carbocyclic annulations.4,5 In recent reviews, we have shown a general summary of the advances in allylsilane chemistry and their significance as precursors for the synthesis of three to sevenmembered rings.2b,6. In this paper we describe the synthesis and cyclization of epoxyallylsilanes containing five-, six- and seven-membered carbocycles as an efficient route for the construction of bicyclic 3methylene-1-cyclohexanols systems which are present in the skeleton of many terpenoid products. Lewis acid-catalyzed intramolecular cyclization seems to occur with concomitant rearrangement of the epoxy group and trapping of the intermediate aldehyde (Table 2).19
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