Tri-substituted cyclooctene synthesis at the limits of relay ring-closing metathesis: a racemic difluorinated analog of fucose

A telescoped sequence based on metalated enol acetal chem. allowed the efficient delivery of a ring-closing metathesis (RCM) precursor, which was used to form a cyclooctenone product with a tri-substituted alkenyl group in moderate yield, though a high loading of second generation Grubbs' pre-catalyst was required. A relay RCM precursor was synthesized to deliver the key alkylidene at a higher rate; the catalyst loading required was high, and increasing the reaction temp. simply resulted in the loss of the cyclizing alkylidene by a non-productive cross-metathesis pathway. We were forced to use high diln. conditions to suppress the unwanted CM and secure the cyclooctenone product. The cyclooctenone product could be progressed to analogs of fucose and 6-deoxyidose, e.g. I, by UpJohn dihydroxylation.