Five-membered ring heterocycles organic reactions

An organic reaction of interest is the Diels-Alder reaction that sulfur dioxide undergoes with butadiene and other acyclic dienes. With butadiene, the product is suhblene, C4H6S, a five-membered S-heterocyclic ring compound which is hydrogenated to form sulfolane, C4H8S. 

Addition of a 1,3-dipole to an alkene to give a five-membered ring is a classical organic reaction. Indeed, 1,3-dipolar cycloaddition reactions are useful for formation of carbon-carbon bonds and for preparation of heterocyclic compounds. 

The application of thermolytic reactions (e.g. Diels-Alder and reverse Diels-Alder processes) for the synthesis of fluoro-organic compounds has been reviewed. The cycloaddition of the meso-ionic dithiolone or thiazolone compounds (209 X = S or NPh) with the Diels-Alder adduct of cyclo-octatetraene and dimethyl acetylenedicarboxylate occurs at the cyclobutene ring to give the exo-adducts (210). Pyrolysis of compounds (210) yields dimethyl phthalate and 7,9-diheteratricyclo[4,2, 1,0 ]non-3-en-8-ones (211 X = S or NPh) by retro-[4-t- 2] addition. The similar additions of (209) and related compounds to dimethyl 7-oxabicyclo[2,2,l]-hepta-2,5-diene-2,3-dicarboxylate afforded [4 -1-2] adducts which on pyrolysis suffered double thermal fragmentation e.g. loss of XCO and 3,4-dimethoxycarbonyl-furan) to afford five-membered aromatic heterocyclics. ... 

The dehalogenation reaction has also been used for preparing unusual polymers that contain the Si-Si o-bonds in conjugation with an organic n-system. Thus, reaction of dichlorosiloles with lithium metal results in the formation of l,r-polysiloles (see Eq. 1.7) [20, 21]. Every silicon in these polymers is also a part of a five-membered unsaturated heterocyclic ring. 

Sulfolane is the most common commercially available sulfone solvent. The solvent, also known as tetrahydrothiophene-1,1-dioxide, is a colorless, highly polar liquid consisting of a fully hydrogenated five-member sulfur-carbon heterocyclic thiophene ring. The solvent is available as both anhydrous sulfolane and as sulfolane containing 3 wt% deionized water. Sulfolane is used as a reaction medium, as a solvent for a wide variety of organic chemicals and polymers, and as an extraction solvent. 

Many of the reactions to sulfonamides involve the skin and mucous membranes. The more severe reactions that occur include potentially lethal toxidermias and a delayed hypersensi-tivity-type syndrome characterized by fever, skin rash, and multi-organ toxicity, hnmediate type I reactions are the most well-defined sulfonamide-induced hypersensitivity reactions with the best defined allergenic drug structmes. Sulfonamides with one methyl substituent on a five- or six-membered aromatic heterocyclic ring on the carbon p to the sulfon-amido substituent are the structmes most complanentary to anti-sulfamethoxazole IgE antibody combining sites. 

Reactions of carbenes other than cyclopropanation can also be performed, and recent examples include the ring expansion of five-ring heterocycles, such as the indoles (18), to their six-membered counterparts (19), and the production of formamides from secondary amines (Scheme 7), both with dichlorocarbene. The latter method is of interest because of its relation to the catalysis of dichlorocarbene generation by tertiary amines in two-phase systems. Recent work indicates that such catalysis is possible because the carbene, after generation at the phase boundary, is transferred into the organic phase (to undergo reaction) in the form of the N-ylid adduct (20). 

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