Enantioselective synthetic approaches to cyclopropane and cyclobutane β-amino acids:: synthesis and structural study of a conformationally constrained β-dipeptide

Synthetic approaches to carbocyclic compounds, namely cyclopropane and cyclobutane beta -amino acids, are presented. One of them is based on enzymatic desymmetrization of meso diesters, leading to the enantioselective production of cis-hemiesters, which afforded beta -amino acids through Curtius rearrangements. The enantiomeric excess for the cyclobutane derivatives was 91% whereas the cyclopropanes were obtained in 63% ee. According to another strategy, an enantiomerically pure cyclopropane trans-beta -amino acid, bearing a quaternary center, has been synthesized from a homochiral precursor easily available from D-glyceraldehyde. The preparation and structural investigation of the first synthesized cyclobutane containing dipeptide is also described. A hairpin-like conformation of this molecule in the solid state has been demonstrated by X-ray structural analysis, showing crystal packing induced by the presence of the rigid cyclobutane moiety and the formation of intermolecular hydrogen bonds. NMR experiments confirmed that these molecules also tend to produce aggregates in solution. On the contrary, theoretical calculations suggest that intramolecular interactions are important in the gas phase, as expected. (C) 2000 Elsevier Science Ltd. All rights reserved.