Stereochemistry and Chiroptical Properties of 1,3-Dialkylaziridinones ( α-Lactams). Chiral Rules for the Nonplanar Amide Chromophore

Structural features, configurational stability, and chiroptical properties of the nonplanar amide group in α-lactams were investigated by means of ab initio (6-31 + G*) molecular orbital calculations on (1R)-aziridinone 1, (1R)-1-methylaziridinone 2, (1R,3R)-3-methylaziridinone 3, (1R,3R)-1,3-dimethylaziridinone 4a, its cis diastereomer (1S,3R)-1,3-dimethylaziridinone 4b, and (1R,3R)-3-tert-butyl-1-methylaziridinone 5, and by experimental CD spectra of 1-tert-butyl- and 1-(1′-adamantyl)-substituted 3(R)-3-tert-butylaziridinones 6 and 7. The nitrogen inversion barriers of 4a and 4b are 2.8 and 1.6 kcal mol−1, respectively. The lowest excited singlet state of all of the aziridinones is a valence state (the nO–πCO* transition); the second is a Rydberg state (the nN–3s transition). The signs of the first and second Cotton effects in the CD spectra of the compounds 6 and 7 coincide with the calculated ones for 1 and 2 and the trans isomers 3, 4a, and 5. According to the calculated and experimental data for azi...