Stereoselective Syntheses of Protected Amino-hydroxy-cyclopentane-carbaldehyde Derivatives from 7-Oxabicyclo[2.2.1]hept-5-en-2-yl Systems

Cycloaddition of azido formates to the double bond of 7-oxabicyclo[2.2.1]hept-5-en-2-yl derivatives gave mixtures of triazolines that could be separated by chromatography. Their acid-catalyzed dediazoniations generate 7-oxabicyclo[2.2.1]hept-2-yl cation intermediates that rearrange into the corresponding more stable 2-oxabicyclo[2.2.1]hept-3-yl cation intermediates which are quenched externally or internally to give products that are protected amino-hydroxy-cyclopentane-carbaldehyde derivatives, with high stereoselectivity. The migratory aptitude of an alkyl group (e.g. σ bond C(1)–C(6) in 5-oxo-7-oxabicyclo[2.2.1]hept-2-yl cation intermediates) is significantly lower than that of the acyl group (e.g. σ bond C(1)–C(6) in 6-oxo-7-oxabicyclo[2.2.1]hept-2-yl cation intermediates) in pinacolic rearrangements.