Benzocyclobutenones, synthesis

Cava, M.P. and Spangler, R.J., 2-(Carbomethoxy)Benzocyclobutenone. Synthesis of a Photochem-icaUy Sensitive Small-Ring System by a Pyrolytic Wolff Rearrangement, /. Am. Chem. Soc., 89,4550, 1967. 

The next three procedures provide useful synthetic intermediates. A stereospecific synthesis of ETHYL (Z)-3-BROMO-2-PROPENOATE affords an alternative vinyl bromide partner for the coupling chemistry in the preceding procedure. A very simple but elegant illustration of the flash vacuum pyrolysis technique is used to prepare BENZOCYCLOBUTENONE from o-toluoyl chloride. Another member of the functionalized indole family of synthetic intermediates is presented in a four-step procedure for 5-METHOXYINDOLE-2-ACETIC ACID METHYL ESTER. 

The characteristic pyrolytic process for acid chlorides is loss of HC1 and this occurs for the bicyclic example 235 on FVP at 800 °C to give cyclohexa-1,2-diene 238 by way of the ketene 236 and carbene 237 as shown113. The product can be directly observed by low-temperature IR and forms a [2 + 2] dimer on warming up. FVP of >-toluoyl chloride 239 at 630 °C also results in loss of HC1 to provide a dependable large-scale synthesis of benzocyclobutenone 240 in excellent yield114. Pyrolysis of trichloroacetyl chloride, CI3C—COC1, over a bed of zinc at 420 °C results in dechlorination... 

In 1,2-disubstituted series such as cis- and frans-2-alkyl-l-alkoxybenzocyclobutenok, the traits isomer is more likely to lead to a high yield of cycloaddition product than the cis since the 1,5-hydrogen shift is precluded, llie forced inward rotation of an aryl substituent in 1-methoxy-l-phenylbenzocyclobutene is potentially advantageous and leads to anthracene derivatives." See also the anthraquinone synthesis fiom the corresponding benzocyclobutenones (Section 6.1.5). 

Synthesis and appHcation of benzocyclobutenones and related compounds 13S563. 

A simple synthesis of fulveneallene (292) is the pyrolysis of the commerically available homophthalic anhydride (293). Fulvenallene is obtained in 71 % yield the pyrolysis temperature can be lowered to give benzocyclobutenone as the major product. The solid adduct (294) with tetracyanoethylene, formed via unstable (292), can be used as a source of the latter on heating. 29... 

Alkene insertion reactions also occurred with benzocyclobutenones [55]. Benzocyclobutenone 85 containing an alkene chain at the 6-position yielded the fused product 86 (Scheme 3.47). Use of a chiral ligand enabled enantioselective synthesis of benzotricyclic ketones in good enantiomeric excesses [56]. 

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