Preparation of the Diquinane Building Block

The carbonyl group in 24 could be temporarily protected as the dioxolane,16 or somewhat more efficiently as the dimethyl acetal 25.17 Subsequent conventional hydroboration led stereoselectively to the P alcohol 26. Although our earliest considerations called for ultimate base-promoted elimination of the OR substituent in intermediates more advanced than 16, we later came to recognize that a methoxy derivative was not going to fulfill this role to our satisfaction. Alternatively, the tactic of forming the benzyl ether at this point ultimately proved well suited to the eventual return to an unmasked hydroxyl at this site.17,18 

Fortunately, the reactivity levels of the two ketone carbonyls in 32 were sufficiently disparate to permit selective ketalization of that located on the pendant chain. The conversion to 33 was achieved in good yield by means of trimethyl orthoformate in excess methanol with p-toluenesulfonic acid as catalyst. Subsequent exposure of this 

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