13- Lactam ultraviolet absorption

Ultraviolet absorptions ofvinylogous lactams were found by MOLCAO calculations and compared with experimental values (663). Infrared spectroscopic studies of vinylogous amides (664) and some fifty vinylogous urethanes (665) allowed configurational and structural assignments. The effect of enamine-imine equilibrium in a series of benzophenone derivatives was established (666) and the effect of structure on enamine basicity studied (667). 

O-H bond. Among such properties a prominent one is the ultraviolet absorption spectrum and the theory may therefore be used for the examination of some of the spectroscopic shifts which accompany the lactam-lactim tautomerization. Much caution must, however, be exercised in this respect. Thus, in a recent paper Kwiatkowski135,137 performed Pariser-Parr-Pople-type calculations on the electronic structure of hydroxypurines, essentially to interpret their ultraviolet spectra. In these calculations he assumed that these compounds exist predominantly in their lactim form, and the results of his calculations, at least for 6- and 8-hydroxypurine, did not seem to contradict this assumption. It is only in the case of the 2-hydroxy isomer that a particularly striking disagreement between theory and experiment led him to admit that this last compound may exist in the lactam form. Calculations carried out for this form gave, in fact, a more satisfactory agreement with experiment.138 As we have seen, unambiguous infrared spectroscopy evidence clearly show s that all three isomers exist essentially in the lactam form. This shows that ultraviolet absorption may provide only very uncertain evidence about the lactam-lactim tautomerism in hydroxypurines and related compounds. 

An interesting series of degradation products has been reported recently by Muller et al. (356). In addition to allocolchicine and some recovered starting material, a desacetylthiocolchicine, CjoH23N04S, has been obtained from the action of sodium methyl mercaptide on methylthiocolchicide. This substance was optically active and showed an ultraviolet absorption spectrum that was different from that of methylthiocolchicide. The infrared spectrum showed carbonyl absorption at 5.93 fi. This was interpreted by the authors as indicative of a y-lactam, and desacetylthiocolchicine was assigned the tentative structure CXII. 

This section deals with the ultraviolet absorption and circular dichroism (CD) spectra of the (3-lactam antibiotics. Semiempirical MO calculations have helped assign the bands associated with the 3-cephem and penam nuclei. 

Carbonyl group frequencies for quinuclidin-2-ones are on the average 80 cm-1 higher than for common lactams, and integral intensities of the same absorptions are nearly half those of quinuclidin-2-ones. Ultraviolet (UV) absorption maxima for quinuclidin-2-ones are midway between those for amides and ketones. On account of the lack of amide mesomerism quinuclidin-2-ones gain in reactivity. This is shown by the rather high rates of hydrolysis of such compounds... 

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