Diels-Alder adducts from acetylenic esters reactions

N-Methyl- and N-phenyl-2-vinylpyrroles 20a,b react with DMAD at reflux temperature in chloroform to give, in moderate yields, the dihydroindoles 22 via a 1,3-H shift from the Diels-Alder intermediate 21 (55-75%) (80JOC4515). These adducts were readily converted into the corresponding indoles 23 with Dichlorodicyanoquinone (DDQ). 2-Vinyl-pyrrole failed to give [4 + 2]-cycloadducts with acetylenic esters (80JOC4515). Spectroscopic analysis of the product mixtures indicated the presence of polymeric compounds resulting from Michael addition reactions. 

The reverse reactions of Diels-Alder reactions for thermal dissociations of cycloadducts in to dienes and dienophiles at higher temperatures or in the presence of Lewis acid or base are known as the retro-Diels-Alder (rDA) reactions. These reactions in most cases proceed in a concerted process. These reactions are often used for separation of diene or dienophile from their mixture with other compounds. Proper selection of conditions of these reactions provides new dienes and dienophiles, which are important synthons for synthesis of several bioactive natural products and organic molecules of complex structures. For example, the D-A adduct of 4-phenyl oxazole 110 with methyl acetylene dicarboxylate, on retro-D-A reaction gives new compounds, benzonitrile, and furan 3,4-dicarboxylic acid methyl ester 111 [65]. 

Pentalenes.—The X-ray structure of the tri-t-butylpentalene (60) has been determined the compound is planar and the double bonds are localised. Pentalenes (62), together with dimers, are produced by the action of acetylenes RC=CH (R = C02Me, CHO, or CN) on l,3-di-t-butyl-6-dimethylamino-fulvene (61). The aldehyde (62 R = CHO) dimerized to yield mainly compound (63), whereas the head-to-tail dimer (64) was isolated from the cyano-derivative (62 R = CN). The latter adds water under acidic conditions to give the alcohol (65), while dimethylamine affords the rearranged adduct (66). Cycloaddition reactions of the pentalene di-ester (67) proceed differently with dicyanoacetylene and cyclopentadiene the former yields the cyclobutene derivative (68), the latter the Diels-Alder product (69). ... 

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