Ozonides, reduction with sulfur dioxide

The starting material is ergocalciferol, the triene system and alcohol group of which are protected as a sulfur dioxide adduct and a silyl ether, respectively. The side-chain is degraded by ozonolysis at -10 °C in dichloromethane. Direct reduction with sodium borohydride destroys the ozonide and reduces the aldehyde to the alcohol. The yield amounts to 87 %. 

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