Opening hydroxy neighboring group

When relais substance 431 was selectively dehydrated via the triflate of the secondary alcohol the path toward coiianin (21) was opened (Scheme 50). The authors used the fact that neighboring hydroxy groups enhance the reducing power of borohydrides to selectively reduce one of the lactones to the hemiacetal 434. Reduction to tetrahydrofuran 435 was achieved by thioacetal formation with... 

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