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 4 a, its cis diastereomer (1S,3R)-1,3-dimethylaziridinone 4 b, 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 4 a and 4 b 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 n O -π CO * transition); the second is a Rydberg state (the n N -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 , 4 a, and 5 . According to the calculated and experimental data for aziridinones 1 - 7 as well as to the well-known data for other nonplanar amides, the sign of the first Cotton effects is determined by the intrinsic chirality of the nonplanar amide chromophore and obeys a spiral rule. For cases where the chromophore has the conformation around the NC(O) bond, which is close to the antiperiplanar, a reverse octant rule is proposed.