Removal of the benzyl group was accomplished by catalytic
hydrogenation in trifluoroethanol,12 followed by O-methylation
and N-benzoylation; Weinreb amide formation and LiAlH4 reduction
yielded the aldehyde 10a. To our delight, relatively mild conditions
led to cyclization,13 for which reduction by NaBH4 gave a
single diastereomeric product14 with three new chiral centres.