Cationic intermediates ring contraction rearrangements

The limitation of the pinacol is the need for symmetry. This section and the next suggest ways of avoiding this problem. Unsymmetrical epoxides are easily made from alkenes and open with Lewis acid catalysis to give the more substituted of the two possible cations.9 Even such a weak Lewis acid as LiBr opens the epoxide 51 to give the tertiary cation 52 which rearranges by ring contraction to the aldehyde 53. The authors prefer to have the bromocompound 54 as an intermediate.10... 

Here the substrates are 3-silyloxy-l,5-enynes the concept was to combine a metal-induced 6-endo-trig cyclization with a ring contraction by a sigmatropic rearrangement (Scheme 12.12).27 From the cationic intermediate, elimination to the biaryl product can be observed in addition to the aldehyde. 

First, we will take up cyclopropyl group formation by the rearrangement of carbon skeletons via cationic intermediates encountered in various mono- and sesquiterpenes, and also examine the illudin biosynthesis where contraction of a cyclobutyl cation to a cyclopropane has been invoked. We will then discuss the head-to-head condensation of isoprenoid alcohols at the C15 or C20 level to generate the cyclopropyl intermediates, presqualene pyrophosphate and prephytoene pyrophosphate, on the way to the C30 and C40 polyene hydrocarbons, squalene and phytoene respectively. Conversion of 2,3-oxidosqualene via common intermediate protosterol cation to cycloartenol or lanosterol represents an important pathway in which the angular methyl group participates in the three-membered ring formation. The cyclopropanation outcome of this process has been carefully studied. 

The symmetrical mechanism necessitates an acidic hydrogen at the a -center, and the intermediacy of a cyclopropanone or zwitterion/oxyallyl cation (Scheme 4). The likely existence of a symmetrical intermediate was first suggested by McPhee and Klingsberg when they showed that the two ketones (1) and (3) rearranged to yield the same ester (2 Scheme 6). More cogent experimental support was provided by Loftfield who reacted [l,2- C2]-2-chlorocyclohexanone with sodium pentylate and obtained two ring-contracted products. These had an identical isotope distribution to the starting ketone (Scheme 7). 

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