Stereoselective Rh-catalyzed hydrogenation of cyclobutyl chiral enamides: double stereodifferentiation vs catalyst-controlled diastereoselection.

The hydrogenation reactions of several cyclobutyl enamides derived from (-)-alpha-pinene or (-)-verbenone have been investigated by using different catalysts. The chiralities of both the substrate and the catalyst as well as the Z/E stereochemistry of the double bond have been considered, and the observed diastereoselectivity has been rationalized. For enamides with the double bond separated from the cyclobutane by a methylene, the Wilkinson catalyst did not induce any diastereoselection, but excellent diastereoselectivity was observed when using Et-DuPHOS-Rh and ChiraPHOS-Rh. The configuration of the new stereogenic center was catalyst-dependent and can be rationalized according to the Halpern mechanism. For (Z)-enamides with the double bond directly linked to the cyclobutane ring, the chirality of the substrate governed the diastereoselection and the Halpern mechanism seemed not to be operative in the hydrogenation with ChiraPHOS, with the configuration of the new stereogenic center being determined by steric effects. On the contrary, the chirality of the catalyst was the factor determining the stereochemistry of the major products with alkyl-DuPHOS-Rh. Z/E stereochemistry influenced the stereodifferentiation, and a different behavior for each Z or E stereoisomer was found. For both (Z)- and (E)-enamides, some instances of match/mismatch between the chirality of the substrate and that of the catalyst were observed. As a result of all of these studies, a series of new cyclobutyl alpha-amino acids has been synthesized. These products are interesting to incorporate into conformationally constrained peptides.