Ethyl propiolate, radical addition

From the Fischer rate study, it appears that primaiy ester-substituted radicals are not electrophilic but ambiphilic and the borderline between ambiphilic and electrophilic radicals is not at all clear. Consider our results68 (Scheme 16) on the atom transfer additions of ester-substituted radicals to alkynes (with the caution that it may be dangerous to compare yields in place of rate constants). The primary ester-substituted radical adds more efficiently to 1-heptyne but the tertiary ester-substituted radical prefers ethyl propiolate. 

An interesting three-component reaction occurred when a solution of diphenyl diselenide (SI), an electron-deficient alkyne (e.g., ethyl propiolate), and an excess of an isocyanide (SO) was irradiated through Pyrex with a tungsten lamp (hv > 300 nm). The reaction is based on a selective sequentially radical addition, as illustrated in Scheme 3.33, and the coupling products were formed in 58-85% yield [83], As for the example reported in Scheme 3.33, the elaboration of compound 52 yielded a precursor for the construction of the carbapenem framework. 

A highly efficient two-step synthesis of benzofurans uses o-acylphenols as readily available starting materials. Addition to ethyl propiolate and subsequent radical cylization/dehydration both proceed in excellent yields (Scheme 85) <2005S387>. 

Coupling reaction. Photoinduced three-component coupling combines an alkene (enol ether, silyl enol ether, 1,3-diene, etc.) and ethyl propiolate to form a carbon chain in which the two PhSe groups from PhSeSePh are separately located. The reaction starts from p-addition of PhSe radical to the ester. 

The direct combination of selenium and acetylene provides the most convenient source of selenophene (76JHC1319). Lesser amounts of many other compounds are formed concurrently and include 2- and 3-alkylselenophenes, benzo[6]selenophene and isomeric selenoloselenophenes (76CS(10)159). The commercial availability of thiophene makes comparable reactions of little interest for the obtention of the parent heterocycle in the laboratory. However, the reaction of substituted acetylenes with morpholinyl disulfide is of some synthetic value. The process, which appears to entail the initial formation of thionitroxyl radicals, converts phenylacetylene into a 3 1 mixture of 2,4- and 2,5-diphenylthiophene, methyl propiolate into dimethyl thiophene-2,5-dicarboxylate, and ethyl phenylpropiolate into diethyl 3,4-diphenylthiophene-2,5-dicarboxylate (Scheme 83a) (77TL3413). Dimethyl thiophene-2,4-dicarboxylate is obtained from methyl propiolate by treatment with dimethyl sulfoxide and thionyl chloride (Scheme 83b) (66CB1558). The rhodium carbonyl catalyzed carbonylation of alkynes in alcohols provides 5-alkoxy-2(5//)-furanones (Scheme 83c) (81CL993). The inclusion of ethylene provides 5-ethyl-2(5//)-furanones instead (82NKK242). The nickel acetate catalyzed addition of r-butyl isocyanide to alkynes provides access to 2-aminopyrroles (Scheme 83d) (70S593). 

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