Cyclo photochemical, cyclobutane

In attempts to isolate the aforementioned irradiated products of thymine derivatives at lower temperature, the photochemical reactions were carried out in frozen aqueous solutions containing either thymine or 1,3-dimethylthymine. The resulting products were not hydrates, but had elementary analyses corresponding to the starting material. Molecular weight determination indicated that the products were dimers, and infrared and ultraviolet spectral data suggested cyclo addition across the 5,6-double bond to form a cyclobutane system... 

Cyclobutane formation, occasionally in fairly low yield due to ring-opening reactions, also occurred by photochemical [2 - - 2] cycloaddition of vinylcyclopropane derivatives to an a, -unsaturated ketone moiety, " a cyclobuta-1,3-diene, a 1,4-benzoquinone, 2-acylthiophenes, and on irradiation of e t/o,enr/o-2,4-bis[( )-2-(methoxycarbonyl)vinyl]bi-cyclo[1.1.0]butane. A formal [2-1-2] cycloaddition also occurred on reaction of tricyclo[3.1.0.0 ]hex-3-ene with methyl 6-oxo-5-phenyl-l,3,4-oxadiazine-2-carboxylate to give methyl 9-oxo-10-phenyl-8-oxapentacyclo[4.4.0.0 ". 0 .0 °]decane-7-carboxylate, albeit in very low yield, after nitrogen extrusion. ... 

The 5,6-double bond in uracils is involved in several [2 + 2] photochemical reactions. The cyclo-octapyrimidine-2,4-dione (338) is formed by photocycloaddition of the A-substituted 6-chlorouracil (336) to benzenes in the presence of an acid, preferably TEA (Scheme 57). The reaction is assumed to proceed by orr//o-addition, with a cyclobutane (337) as intermediate. Without acid, 6-phenyluracil is formed in low yield. In toluene as solvent and participating reagent, a regioisomeric mixture with the methyl group in the 6-, 7-, 8-, or 10-position was formed. In fluorobenzene, all of the regioisomers... 

The impact of (2 + 2)-cycloaddition and (2 + 2)-cycloreversion reactions of heterocyclic compounds on organic chemistry over the last 10 years is clearly illustrated by several examples. Various members of the important /Hactam antibiotics, penicillin and cephalosporin C, as well as structurally related heterobicyclic compounds have been obtained by (2+ 2)-cycloaddition of heterocycles with ketenes (Section II,D,l).n Intramolecular photochemical (2 + 2)-cycloadditions of 2-pyrones yield 2-oxabicyclo 2.2.01hex-5-en-3-ones, which upon further irradiation afford cyclobutadienes (Section III,D,2).12 Intermolecular (2 + 2)-cyclo-additions of vinylene carbonates with olefins and with acetylenes offer a simple route to cyclobutanes and cyclobutenes, respectively (Sections III,B,3 and 5).13 (2 + 2)-Cycloaddition and (2 + 2)-cycloreversion reactions have contributed substantially to the development of the chemistry... 

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