Cyclobutene derivatives, electrocyclic ring opening

For the synthesis of estradiol methyl ether 4-319, the cyclobutene derivative 4-317 was heated to give the orthoquinonedimethane 4-318 which cydized in an intramolecular Diels-Alder reaction [109]. The thermally permitted, conrotatory electrocyclic ring-opening of benzocyclobutenes [110] with subsequent intramolecular cycloaddition also allowed the formation of numerous complex frameworks (Scheme 4.70). 

Spiro-cyclobutene derivatives prepared from the intramolecular Wittig reaction of DMAD with ethyl oxo-(2-oxocycloalkyl)ethanoates yield cycloalka[6]pyrans via a thermal electrocyclic ring-opening - ring-closing sequence (Scheme 1) <03X2001 >. 

In refluxing xylene (160 °C), fused Dewar s thiophene 40 mainly desulfurized to give cyclobutene 41 but under flash vacuum pyrolysis conditions, a mixture of isomeric dihydrothiophenes 42, 43, and 5H-benzocycloheptatriene 44 was formed (Scheme 9). Evidently the latter product derived from a formal electrocyclic ring opening of the cyclobutene intermediate 41 <1990H(31)1769>. 

The Electrocyclic Ring Opening of Huonnaled Cyclobutene Derivatives Dolbier, W R, Jr, Koroniak, H Mol Struct Energ S, 65-81 51 5... 

The areas in which work on alkyne photochemistry has been most prolific are those involving cycloaddition to carbon-carbon double bonds in alkenes, aromatics and related compounds. The simplest type of reaction involves formation of a cyclobutene from an alkene and an alkyne (equation 41) . The cyclobutene product may itself be photolabile, and if radiation is used which is absorbed more strongly by the cyclobutene, the product isolated may be the 1,3-diene derived from it by electrocyclic ring opening (equation 42) . 

One report has appeared of a system in which the first-formed cyclobutene adds a second molecule of alkyne photochemically, leading eventually to a benzene derivative as a result of electrocyclic ring-opening and a retro-Diels-Alder process (equation 55). 

The electrocyclic ring opening of heavily substituted cyclobutene derivatives which was investigated by Plieninger et al. [113], shows negative volumes of activation of... 

In addition, Swenton and cowm kers have reported that the dianion derivatives of certain pyrimidine-acetylene photoadducts undergo an electrocyclic reaction which may involve an anion-accelerated cyclobutene ring opening see R. N. Comber, J. S. Swenton and A. J. Wexler, /. Am. Chem. Soc., 1979,101, 5411. 

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