Chiroptical properties of N-(2-pyrazinoyl)-α-amino-esters, -aziridines, and related compounds

Abstract

A series of pyrazinoyl-amino-esters, -amino-alcohols, and -amines, as well as pyrazinoyl- and quinoxaloylaziridinds, all with the (S)-configuration, were synthesized and their spectral data analysed. The c.d. Spectra of the aliphatic and aromatic amino-ester derivatives show sign reversal in the medium bands at ca. 270 and 240 nm. Comparable c.d. Trends were also observed in the corresponding amino-alcohol and amine derivatives. This was rationalized as due to differences in conformational isomerism. Such c.d. Behaviour was not observed in the pyrazinoylaziridines nor in the quinoxaloylaziridines, a fact that suggests similar conformational equilibria in each of these two series. These equilibria are associated with rapid nitrogen inversion and rapid rotation around the CO–N bond. Support for these conclusions comes from ¹H n.m.r. and chiral shift reagent n.m.r. Studies.