Blocking groups acid-sensitive

Removal of blocking groups. Regeneration of aldehydes from their acetals can be accomplished by refluxing in formic acid for 10-60 min.17 The reaction is applicable to sensitive aldehydes, such as a-acetylenic aldehydes. 

The crucial step in this synthesis, a base-catalyzed aldol condensation, furnished the hydroxyketone 214 in quantitative yield. Thus, three of the five rings of target compound 3 were constructed by a simple reaction sequence. The hydroxyketone was converted to the ketal 215 by a standard method. The benzyl blocking group was then transferred from the primary to the secondary alcoholic function in 65% overall yield by conventional reaction sequences, as shown above. At this point, the authors suggested that a much more direct conversion of 215 to 221 may be possible, using the acid-sensitive p-methoxybenzyl group as R1 in compound 215. This operation was not carried out, however, for the overall yield from 215 to 221 was satisfactory. 

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