Crystal structure and preferred conformation of beta-lactams derived from (S)-1-arylethyl isocyanates and vinyl ethers.

The configurational analysis of beta-lactams prepared from [2 + 2] cycloaddition of vinyl ethers to pure enantiomers of 1-arylethyl isocyanates was carried out by high resolution 1H NMR. The addition of a chiral shift reagent revealed that the most important conformation of the studied beta-lactams in solution is that in which the methine proton, of the exocyclic stereogenic carbon, points towards the carbonyl oxygen atom. Since the configuration of the stereogenic exocyclic carbon is known, the orientation of the aromatic ring allows the correlation of the chemical shifts with the absolute configuration of the new stereogenic centers. This method is particularly useful to establish the stereochemistry of oily beta-lactams having the N-(1-arylethyl) group. The X-ray crystallographic analysis carried out with (1R,5S)-7-[(1S)-1-(1-naphthyl) ethyl]-2-oxa-7-azabicyclo[3.2.0]heptan-6-one, is consistent with the proposed model for beta-lactams in solution.