Heterocyclic compounds formation from alkynes

Cycloisomerizations are one of the most popular methods for the formation of carbo- and heterocyclic compounds [1, 2], Although in most cases alkenes and/or alkynes are still involved, there now exist a significant number of similar applications of allenes which benefit from the higher reactivity of the allene-unit. 

In Chapter 11 of Part A, the mechanistic classification of 1,3-dipolar cycloadditions as a type of concerted cycloadditions was developed. Dipolar cycloaddition reactions are useful both for the synthesis of heterocyclic compounds and for carbon-carbon bond formation. Table 6.2 lists some of the types of molecules that are capable of dipolar cycloaddition. These molecules, which are called 1,3-dipoles, have r-electron systems that are isoelectronic with allyl anion, consisting of two filled and one empty orbital. Each molecule has at least one charge-separated resonance structure with opposite charges in a 1,3-relationship. It is this structural feature that leads to the name 1,3-dipolar cycloadditions for this class of reactions. The other reactant in a dipolar cycloaddition, usually an alkene or alkyne, is referred to as the dipolarophile. Other multiply bonded functional groups such as imine, azo, and nitroso groups can also act as dipolarophiles. The transition states for 1,3-dipolar cycloadditions involve four tc electrons fi om the 1,3-dipole and two from the dipolarophile. As in the Diels-Alder reaction, the reactants approach one another in parallel planes. 

Heterocycle Formation. Nitric acid has found an increasing role in the preparation of heterocyclic compounds. Two preparations of isoxazoles using nitric acid have been reported. In one procedure, the heterocycle is prepared from 1,1-dihalocyclopropanes (eq 39). In the other procedure, the isoxazole is prepared from terminal alkynes in the presence of a gold catalyst (eq 40). ... 

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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