Oxepins cyclobutene

The single crystal X-ray structure of the enantiopure tetrahydro oxepin-2-one 61, a mimic of steroidal androgens, has been reported <06ZN(B)111>. A new procedure has been described for the synthesis of substituted furano-fused oxepines based on expoxidation of strained fused cyclobutenes followed by thermal rearrangement <06OL5183>. 

Leyhane and Snapper have epoxidized a series of fused cyclobutenes, and the adducts were subject to thermal rearrangement leading to oxepine-containing bicyclo[5.3.0] ring systems (e.g., Scheme 13) <20060L5183>. 

An electrocyclic photochemical rearrangement of oxepins to cyclobutene ring-containing valence isomers has also been reported. Thus 2-oxabicyclo-[3.2.0]hepta-3,6-diene has been isolated upon photoisomerization of 1. Similarly, when 1-benzoxepin 100 was irradiated, the valence tautomer3,4-benz-2-oxabicyclo[3.2.0]-hepta-3,6-diene was formed. ... 

A useful reaction for the synthesis of unsaturated seven-membered heterocycles is the (2 + 2)-cycloaddition of heteroaromatic compounds, e.g., 1 //-pyrrole, furan, or thiophene derivatives, with acetylenes. In combination with a subsequent intramolecular (2 + 2)-cycloreversion (Section IV,B,2) of the annulated cyclobutene moiety, ring enlargement with two carbon atoms can be achieved. 1-Heterocycloheptatrienes, such as benzol6]azepines,26,27,65,66 benzo[fc]oxepins,67,68 benzo[6]-thiepins,69,70 and thiepins,18,71 have been successfully prepared in this way other routes are either nonexistent or laborious.72 In these compounds the reacting carbon-carbon double bond constitutes part of a (4n + 2)7r-electron system and in the (2 + 2)-cycloaddition the resonance energy of the aromatic nucleus is lost. Just like the nonaromatic heterocycles, heteroaromatic compounds have been reported to undergo (2 + 2)-cycloaddition reactions both with electron-deficient and with electron-rich acetylenes. 

X ]-antara,antara-Copc rearrangement Dimethyl acetylenedicarboxylate has been used in [2 + 2]cycloadditions with enamines to give cyclobutenes, which in turn have been used as intermediates in preparations of dihydrothiepins and benzo[h]oxepins. Stable bicyclic 1,2-dimethoxycarbonylcyclobutenes have also been obtained from dimethyl acetylenedicarboxylate with 1-alkyl-1,4-dihydropyridines and 1-alkyl-1,4-dihydroquinolines. The former products showed no tendency to isomerize whereas benzazocine derivatives could be obtained from the latter compounds simply by refluxing them in benzene. 

ElectrocycHc ring opening of the cyclobutene rings in adducts 422 and 424 produces the bridged oxepine products 423 and 425 in high yields (Scheme 89 2000CEJ3706). 

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