endo-Adducts of cyclopenta-or cyclohexa-1, 3-dienes with cyclohexa-1,4-dienes, especially p-benzoquinones (Ia) , cyclise on ultraviolet irradiation to cage-like isomers (Ib) by formation of a cyclobutane ring from the two double bonds. The effects of interaction between the two carbonyl groups in the photoisomers (Ib) appear in their infrared and ultraviolet spectra. * After the appearance of our preliminary note 3 we learnt €hat the photoisomerisation of benzoquinone adducts had been independently observed in the laboratory of Professor C. H. De Puy (Iowa State College, Ames, U.S.A.), who very generously offered to leave the field to us.

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A study of the orbitals of a-aminoacetone and a-aminoacetaldehyde as a function of the two important torsions in these molecules is reported. The individual level shifts are much greater than the variation of the total energy. We identify four significant interactions affecting the level ordering. Each interaction has its distinctive conformational dependence. In order of decreasing spectroscopic importance the interactions are as follows.(1) Through bond coupling of the nitrogen and oxygen lone-pairs. (2) Through space interaction of these lonepairs. (3) Hyperconjugation between the C-N G* level and the carbonyl x-system. (4) Direct overlap of the nitrogen lone-pair with the carbonyl x-system.OUR continuing interest in the interaction of orbitals through space and through bonds prompted us to examine the electronic structure of a series of a-substituted carbonyl compounds (1). This particular R = H , Me(1) carbonyl grouping was examined for the following reasons. (1) Previous investigations have focused on the interaction between equivalent lone-pairs ; the series (1) serves as a useful prototype for the study of hetero-

The circular dichroisrn of a range of 1,3-dithiolans, mainly in the steroid series, generally consists of two maxima of opposite sign a t about 270 and 245 nm., with the latter more intense.spectra of 1,3-dithiolans exhibit a weak band at about 245 nm. (E -350) corresponding to the n+a* transitions of the sulphur atoms. In the circular dichroism spectra (in methanol,, dioxan,, or, in the present work, cyclohexane 3, two oppositely signed absorptions are generally seen, one between 260 and 280 nm., and the other, which is always more intense, between 240 and 250 nm. To understand the spectra it is necessary to examine carefully the shape and symmetry of the dithiolan chromophore. N.m.r. spectra of 2-substituted 1,3-dioxolans show that the ring undergoes pseudorotation between the envelope and the two enantiomeric half-chair conformations, but even with very bulky groups, e.g., t-butyl, no preference for the envelope form (which would tend to minimise 1,3interactions) could be detected. Therefore, although a " time-average conformation " of a molecule exhibiting pseudorotation is not necessarily the same as the " average conformation," the 1,3-dioxolan ring (and, by extension, the 1,3-dithiolan ring) appears to be planar. The planar dithiolan chromophore (Figure 1) c\ I /D HzC--!-CHI 4 F. Alderweireldt and M. Anteunis, Bztll. SOC. chim. belges,