Selectivity in the cycloadditions of carbonyl ylides with glyoxylates: an approach to the zaragozic acids—squalestatins

Reaction of diazodiketoester 8 with glyoxylates in the presence of catalytic rhodium(II) acetate generates 6,8-dioxabicyclo[3.2.1]octanes 9 and 11 in good yield. Elaboration of 9 provides a suitable alcohol 25 for acid-catalysed rearrangement to give the 2,8-dioxabicyclo[3.2.1]octane skeleton 26 of the zaragozic acids—squalestatins. More substituted diazodiketoesters 36 and 40 also undergo highly regio- and diastereoselective cycloaddition with glyoxylates to give the cycloadducts 41, 43 and 44.