Stereoselective pinacol-type rearrangement

Intramolecular reaction of an allylsilane and an aldehyde was performed to prepare cyclic compounds as illustrated in Eq. (90) [233], which shows a high diastereoselec-tivity. The cyclization of optically active allylsilane proceeds stereoselectively in a manner consistent with the antiS l mechanism (Eq. 91) [234]. Development of a new reagent with a bis-allylsilane moiety effected tandem inter- and intramolecular cyclizations to give cyclic compounds as exemplified in Eqs (92) [235] and (93) [236-238]. In Eq. (92), the double addition product initially formed underwent a pinacol-type rearrangement under the influence of TiCU to give, eventually, the methyl cyclopentyl ketone. Further examples of intramolecular cyclization of allylsilanes are summarized in Table 8. 

Epoxide opening accompanied by the pinacol-type rearrangement of a carbon framework in the presence of a titanium Lewis acid has been used for the stereoselective synthesis of acyclic compounds [642,643], Equations (278) and (279) illustrate this transformation and exemplify the dependence of stereospecificity on the geometry of the epoxide... 

Pinacol-type rearrangement of cis-diol 26.137 by esterification with TsCl in pyridine at rt readily proceeded with high stereoselectivity to afford directly the azulenofuran 26.138 (Scheme 40). However, direct... 

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