Photochemistry four-membered

Photochemistry offers the possibility of simple syntheses of some materials that would be very difficult to synthesize by other means. Stereoselective syntheses of four-membered rings are an excellent example of this (Scheme 7.3). As discussed previously, pericyclic reactions, such as... 

The ladder polysilanes also show interesting photochemistry on irradiation with a high-pressure mercury lamp, the tricyclic ladder extruded the transient intermediate four-membered cyclic disilene which could be trapped using methanol, 1,3-butadiene, and anthracene, as shown in Scheme 38. 

Saturated three- and four-membered heterocyclics absorb little in the readily accessible regions of the UV spectrum. Sulfur-containing rings are an exception, as can be seen in Table 9, Despite the lack of absorption of most parent compounds, there is a wealth of photochemistry of small heterocyclics. Light absorption by substituents, and energy transfer from photoexcited molecules present in the photoreactive system make photoconversion of the heterocycles practical. On the other hand, the lack of substantial absorption of their own can be exploited in the preparation of small heterocycles, by designing the system to be unsuitable for destructive energy transfer. 

The photochemistry of a-diketones has been a subject of interest for about a century. Since the appearance of comprehensive review articles u in 1969 and 1971, activity in this area has continued with investigation of a number of new systems, particularly unsaturated diketones and diketones incorporated in a four-membered ring. New types of chemistry of synthetic and mechanistic interest have been revealed. The purpose of this review is to summarize these newer developments with briefer reference to some significant developments in the older reactions. 

The quantum yield 0 is 0.03. The mechanism of this [l,2,3,4]-rearrangement [53] is unknown. Formally, the reaction can be considered an H migration from the OH group to the end of a four-membered chain with a concomitant acyl group migration from position 2 to 3 within that chain. According to the current canon of photochemistry, an oxa-di- r-methane rearrangement with subsequent... 

The photochemistry of the cyclobutenyl ketone (485) has been studied as one of a series of cycloalkenyl ketones. The cyclobutenyl ketone is unusual in that the four-membered ring does not allow the double bond to twist in its excited state. With acetone sensitization, irradiation of (485) gave a mixture of the bicyclo[2,l,0]pentane (486) and the exomethylene ketone (487). Direct irradiation of (485) also gave (487) and the products of a-cleavage. 

The aromaticity of three- and four-membered cyclic sulphur compounds has been discussed as part of an overall review of aromaticity in heterocyclic compounds. The photochemistry of organic sulphur compounds, including reactions involving small rings, has been reviewed. ... 

Matrix Photochemistry of Four-Membered Ring Compounds... 

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