Autoassembly of cage structures. 4. Stereochemistry of 2,5-di-tert-butyl-?-butyrolactone-4-carboxy-s-(?)-phenylethylamide

Abstract

The diastereoisomeric lactonamides (2a, b), obtained from the dilactone (1) and S-phenylethylamine, have been separated. X-ray crystallography shows that the high-melting isomer (2a) has theR-configuration at the chiral centers C2 and C4, the enantiomeric conformation of the γ-lactone ring being of theS-type in thetwist form, intermediate between envelope 2 E and semi-chair 3 2 T. It is shown by molecular mechanics that the minimum steric energy of2a corresponds to a conformation of the heterocycle close to the envelope form3E. Examination of van der Waals interactions shows that the calculated structure for2a is preferred. The reasons for the nonidentity of the forms of the γ-lactone ring of2a in the crystal and the free state are discussed. The crystal structure of2a is composed of two geometrically similar independent molecules associated along the axis by weak hydrogen bonds of two types, the energies of which have been estimated from the vXH values, which are related by the expression vXH=f(RX...O), where X=N, O.