Cycloaddition reactions heterocycles

The publications that appeared up to 1975 on the transformations of oxiranes into other heterocyclic compounds are reviewed in detail in the monograph by van der Plas and the relevant chapters of other reviews. Accordingly, we merely supplement the earlier data with more recently reported results and, at the same time, present some of the varied transformations of the oxiranes. The possibility of stereoselective synthesis of heterocyclic systems was broadened considerably by 1,3-dipolar cycloaddition reactions. Heterocycles are formed if the ylide intermediates produced from oxiranes in photolytically or thermally induced reactions are made to react intramolecularly or with external dipolarophiles. Reactions of these types will be treated in Sections IV.8 and IV.9. 

Shishido, K., Umimoto, K., and Shibuya, M. (1990) An alternative total synthesis of (-F)-pallescensin A based on the intramolecular [3-F2] cycloaddition reaction. Heterocycles, 31, 597-598. 

The success of the cycloaddition reaction of maleic anhydride varies gready depending on which heterocyclic diene is used. The cycloaddition of maleic anhydride to furan [110-00-9] occurs ia a few seconds under ambient conditions (42,43). Although the endo adduct (14) is favored kiaeticaHy, the exo adduct (13) is isolated. 

Dipolar cycloaddition reactions with azides, imines, and nitrile oxides afford synthetic routes to nitrogen-containing heterocycles (25—30). 

Furan has the greater reactivity in cycloaddition reactions compared with pyrrole and thiophene the latter is the least reactive diene. However, A -substituted pyrroles show enhanced dienic character compared with the parent heterocycle. 

Benzo[Z)]furans and indoles do not take part in Diels-Alder reactions but 2-vinyl-benzo[Z)]furan and 2- and 3-vinylindoles give adducts involving the exocyclic double bond. In contrast, the benzo[c]-fused heterocycles function as highly reactive dienes in [4 + 2] cycloaddition reactions. Thus benzo[c]furan, isoindole (benzo[c]pyrrole) and benzo[c]thiophene all yield Diels-Alder adducts (137) with maleic anhydride. Adducts of this type are used to characterize these unstable molecules and in a similar way benzo[c]selenophene, which polymerizes on attempted isolation, was characterized by formation of an adduct with tetracyanoethylene (76JA867). 

The chalcogene heterocycles have been used as stable precursors for sulfur-said selenium-cantaining hetero-l,3-dienes in cycloaddition reactions 3//-l,2,4-Thiaselenazoles are a convenient source of 4,4-bis(trifluoromethyl)-l-thia-3-aza-buta-1,3-dienes, and 3//-diselenazoles are a convenient source of 4,4-bis(trifluoromethyl)-l-selena-3-azabuta-l,3-dienes as well as bis(tnfluoro-methyl)-substrtuted nitrile ylides [137]... 

Intramolecular cycloaddition reactions of allylic cations with participation and/ or formation of heterocycles, mainly [4+3]-cycloaddition to furan system 97T6235. 

Formation of 4- to 6-membered heterocycles in cycloaddition reactions 98JCS(P1)3873. 

Al-Heterocycles, formation from olefins and acetylenes in a metallocomplex-catalyzed cycloaddition reaction and further transformations 98IZV816. 

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