Cyclizations tandem semipinacol rearrangements

A number of tandem cyclization-semipinacol rearrangements have been reported, whereby cycliza-tion is initiated by an acetal and terminated by a semipinacol rearrangement (equation 8). 

A semipinacol rearrangement in tandem with a cyclization has been shown to be a powerful method for the construction of carbocyclic rings. Here the cyclization is initiated by an acetal function and terminated by a semipinacol rearrangement (Scheme 25). In some cases, the cyclization has been found to be stereospecific at the acetal center (equation 48), and up to four chiral centers can be generated in a single step (equation 49). Highly substituted tetrahydrofurans are obtained from cyclic acetals (Scheme 26), and stereoselective tetrahydrofuran annelations are possible using bicyclic acetals (Scheme 27). ... 

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