STEREOCHEMISTRY AND CHIROPTICAL PROPERTIES OF 1,3-DIALKYLAZIRIDINONES(A-LACTAMS) - CHIRAL RULES FOR THE NONPLANAR AMIDE CHROMOPHORE

Structural features, configurational stability, and chiroptical proper ties of the nonplanar amide group in alpha-lactams were investigated b y 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-methylaziridinon e 5, and by experimental CD spectra of 1-tert-butyl- and 1-(1'-adamant yl)-substituted 3(R)-3-tert-butylaziridinones 6 and 7. The nitrogen in version 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 val ence state (the n(O)-pi(CO) transition); the second is a Rydberg stat e (the n(N)-3s transition). The signs of the first and second Cotton e ffects in the CD spectra of the compounds 6 and 7 coincide with the ca lculated ones for 1 and 2 and the trans isomers 3, 4a, and 5. Accordin g 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 t he chromophore has the conformation around the N-C(O) bond, which is c lose to the antiperiplanar, a reverse octant rule is proposed.