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Prior to 1962 few examples of the photochemical addition of an enone to an ethylenic linkage were to be found in the literature, and these were concerned either with dimerization or with intramolecular photoaddition. In 1962 the mixed intermolecular addition of a quasi-,/3-unsaturated ketone, enolized acetylacetone, to an alkene (eq 1) was reported;2 3•8 within months this was followed by the report of the related reaction of cyclopentenone (eq 2).4 5It was then pointed out2•4 that intermolecular photoannelation had considerable potential in synthesis. This estimate was correct, and within recent years more and more use has been made of this technique for the construction of cyclobutanecontaining products. More interestingly, it has also been utilized for the construction of systems derived by subsequent transformations of such rings. Some examples were cited by Eaton.2 Others, taken from the field of natural products, are steps in the synthesis of 7-tropolone (eq 3),6 ß-himach alene (eq 4),6 atisine (eq 5),6 loganin (eq 6),7 methyl isomarasmate (eq 7),8 and the prostanoic acid skeleton (eq 8).9(1) Photochemical Synthesis.

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A detailed study of the emission, excitation spectra and fluorescence decay of pyrene adsorbed on silica gel, porous Vycor, alumina, and calcium fluoride is reported. Monomer emission and a longer-wavelength broad-band emission (neither decaying with a single exponential) was observed on silica. The excitation spectrum of the broad-band emitter was red shifted in comparison with the absorption responsible for the monomer emission.These, and other, results are interpreted as indicating the existence of bimolecular ground-state pyrene association.The effect of coadsorbates (glycerol, 1-decanol) was examined. At high coverage the monomer emission decay approaches a single exponential and the long-wavelength emission can be deconvoluted with a double exponential with a negative preexponential term indicating true excimer formation. Adamantanol did not have this effect. A model for the pyrene-silica interaction is proposed to rationalize these and other observations.

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