Hydroxy group from hydroxylation reaction

Stereospecificity of reaction of diols with equatorial tertiary hydroxyl, (140) and (141), is in accord with an SN- -type mechanism. When the tertiary OH group is axial approach of the acid is hindered and reaction only occurs at all when the p-hydroxyl is axial (142). The inertia of (143) is considered to arise from the pronounced rate-retarding inductive effect of the P-hydroxy-group which prevents reaction in the absence of a favourable conformational situation. 

With ring G in place, the construction of key intermediate 105 requires only a few functional group manipulations. To this end, benzylation of the free secondary hydroxyl group in 136, followed sequentially by hydroboration/oxidation and benzylation reactions, affords compound 137 in 75% overall yield. Acid-induced solvolysis of the benzylidene acetal in 137 in methanol furnishes a diol (138) the hydroxy groups of which can be easily differentiated. Although the action of 2.5 equivalents of tert-butyldimethylsilyl chloride on compound 138 produces a bis(silyl ether), it was found that the primary TBS ether can be cleaved selectively on treatment with a catalytic amount of CSA in MeOH at 0 °C. Finally, oxidation of the resulting primary alcohol using the Swem procedure furnishes key intermediate 105 (81 % yield from 138). 

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