Lactams. XXV. Reassignments of the C=C and C=O Stretching Vibrations in Six-Membered α, β-Unsaturated Lactams

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A new method for differentiating the lactam CO infrared (IR) absorption from the C=C absorption in the six-membered, α, β-unsaturated lactam system (type 2) has been developed by utilizing association of dichloroacetic acid or a similar carboxylic acid with the lactam CO, which decreases the frequency of the lactam CO absorption by 20-40 cm^<-1>. On the basis of such an association shift as well as the results obtained from solvent shift experiments with the lactams 5,6,15,and 17-19,the lactam CO and the conjugated C=C absorptions observed for each of the α, β-unsaturated lactams 14-19 in the 1590-1670 cm^<-1> region were differentiated from each other. As a result, it has now become clear that in six-membered lactams the carbonyl frequency is slightly (10-20 cm^<-1>) lowered by conjugation with a double bond, in line with the usual lowering. This is the opposite of what has been proposed in the literature. In addition, the deuterated α, β-unsaturated lactam 24 was prepared from the exocyclic methylene lactam 23 by treatment with NaOD in a mixture of D_2O and EtOD. This deuterium labeling was found to decrease the frequency of the 1666 cm^<-1> absorption, assigned to the C=C stretching vibration, by 17 cm^<-1>, supporting the correctness of the above differentiation.
公益社団法人日本薬学会の論文
1985-07-25