Glucal carbamates 1 can form rhodium-stabilized nitrenoid intermediates that intramolecularly insert into the glucal C(1)-C(2) double bond. Subsequent addition by a glycosyl acceptor alcohol completes the alkene amidation and glycosylation sequence— together known as amidoglycosylation. Amidoglycosylation results in the formation of α- and β-oxazolidinone products 2, as well as a C(3)-oxidation byproduct, dihydropyranone 3 (Scheme 1). The observed diastereoselectivity is a function of the 4O,6O protection. Glucals with acyclic 4O,6O protection, such as dibenzyl, are conformationally flexible and exhibit high diastereoselectivity, especially in nonpolar solvents such as benzene. We believe that the observed chemoselectivity depends on the relative amounts of carbamate in the “normal” half-chair ⁴H₅ conformation versus the conformationally inverted ⁵H₄ conformation. Placing increasingly electronegative 4O and 6O protecting groups on the carbamate increased the proportion of ⁵H₄ conformation as assessed by ¹H NMR J_4,5 analysis and reduced byproduct formation while maintaining excellent α-anomeric stereocontrol.  

Scheme 1. Rhodium-nitrenoid amidoglycosylation.