Anomalous Face-Selectivity in Sharpless Asymmetric Dihydroxylation of o-Allylbenzamides

摘要

The osmium tetraoxide-catalyzed asymmetric dihy-droxylation (AD) of olefins has proved to be one of the most useful and reliable procedures, in both the homogeneous and the heterogeneous phase, for the synthesis of chiral diols. An important feature of this reaction is the possibility of achieving both enantiomers using, as chiral ligands, derivatives of the pseudoenantiomeric alkaloids dihydroquinine (DHQ) and dihydroquinidine (DHQD). Moreover, an empirical face-selectivity rule (Figure 1) proposed by Sharpless et al. allows predictions upon the absolute stereochemistry of diols obtained in the AD process with the two classes of alkaloid ligands. This rule, based on a very large amount of examples and supported by molecular mechanics studies, is commonly used as a guideline for the choice of the right ligand. To date, only a few exceptions have been found to the Sharpless rule, mainly concerning the AD of vinylidenic substrates, and of chiral olefins, where double diaste-reoselection was involved. A sole case of reversal of π-facial selectivity in an achiral monosubstituted olefin has been recently reported. Herein, we report several exceptions to the Sharpless face-selectivity rule in the AD of differently substituted o-allylbenzamides.