Cycloadditions of Ene-Vinylcyclopropanes

Although the [5+2] cycloadditions of alkynes and vinylcyclo-propanes service a number of objectives in synthesis, the [5+2] cycloaddition with alkenes also has enormous potential value and in addition presents a remarkable opportunity to address the intrinsic stereoselectivity of the process because two diastereoisomeric products can form. We began our investigation with substrate 38 (Table 4). Treating ene-vinylcyclopropane 38 with a catalyst derived from 0.1 mol% RhClfPPhjjj and 0.1 mol% AgOTf after 17 h at 110 °C gives cycloadduct 39 in 86% isolated yield as a single diastereomer. The 

Entry RhCHPPhih Additiv Concn (m) Time(h) Yield (%) 

Notes mol% AgOTf = mol% RhCl(PPh3)3-Reaction run on 1 g scale. 

The generality of this methodology was established with a variety of ene-vinylcyclopropane substrates as illustrated (Eqs. 58-62). As was expected on the basis of the results of the cycloaddition of substrate 38, ene-vinylcyclopropane 44 produces exclusively cis-fused cycloadduct 45 in 94% yield by GC and in 70% isolated yield (Eq. 58). The isolated yield of this product is reduced because of its 

Substrates 46 (E Z =5.5 1) and 48 were selected for study because of the number of naturally occurring bicyclo[5.3.0]decanes bearing an angular methyl group and because of the general difficulties associated with quaternary center formation. These methyl-substituted substrates (46 and 48) react rapidly (reaction times 1 h) and with high efficiency ( 90%) to afford exclusively the cw-fused products 47 and 49, respectively (Eqs. 59 and 60). In these cases, silver triflate is required for clean conversion. In its absence, decomposition occurs more rapidly than cycloaddition. At higher substrate concentration and 

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