Carbamates enantio-enriched

Altman et al. have used the Bamberger imidazole cleavage reaction of substituted imidazoles in producing enantio-enriched vicinal di-acylamines. Specifically ring cleavage of 25 with (-)-menthyl chloroformate introduces chiral carbamate substituents on the double bond in menthyl carbamates 28 and 29. Hydrogenation of the hydrolyzed product 30 leads to preferential formation of vicinal diamide 32 over 31 in a diastereomeric ratio of 5 1. 

The mechanism proposed by the authors involved the rapid formation of a transient acyl pyridinium species (ion-pair) followed by a slow transfer of the methoxycarbonyl group to the amine (stereo-determining step) to afford the corresponding enantio-enriched carbamate (Scheme 41.44). 

The organolithium intermediate can also react intramolecularly [340] as exemplified by the intramolecular anti-selective 5-exo-dig carbolithiation of a-lithiated cy-carbamoyloxy-l-alkynyl carbamates for the synthesis of highly enantio-enriched protected 2-alkylidene-cycloalkane-l,3-diols [341] (Scheme 6.40). 

---