Diels-Alder adducts from acetylenic esters

A further type of degradation occurs during the photolysis or thermolysis of derivatives of the 2,3-oxaphosphabicyclo[2.2.2]octene system, formally derivatives of cyclic phos-phonic or phosphinic acids. The compounds 42 are obtained by Baeyer-Villiger oxidations, with 3-chloroperoxybenzoic acid, of the 1 1 adducts from monomeric l//-phospho-les and iV-phenylmaleimide, and 43 by the identical oxidation of the products of dimerization of monomeric but unstable li/-phospholes. The simpler substrates 44, similarly obtained by the oxidation of the Diels-Alder adducts prepared from acetylene dicarboxylic ester, have also been examined. The thermolysis (at 80-110 °C in toluene) or... 

Pyridine, and its monomethyl and 3,5-dimethyl derivatives " combine exothermically with dimethyl acetylenedicarboxylate in ether yielding some ether soluble materials, including trimethyl pyrrocoline-1,2,3-tricarboxylate (Section III,F,3) and deep red ether-insoluble gums. A number of crystalline compounds have been isolated from these gums by fractional crystallizations and will now be considered in detail. In the case of pyridine, Diels et al. ° isolated a red labile 1 2 molar adduct, which they formulated as (75), which isomerized rapidly on standing to a yellow stable adduct (76). These formulations are no longer accepted. Diels and Alder also suggested that the acetylenic ester first dimerized to the diradical (74) which then combined with the pyridine. 

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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