Cycloaddition reactions fused ring compounds

Azirine, trans-2-methyl-3-phenyl-racemization, 7, 33, 34 1-Azirine, 2-phenyl-reactions, 7, 69 with carbon disulfide, S, 153 1-Azirine, 3-vinyl-rearrangements, 7, 67 Azirines, 7, 47-93 cycloaddition reactions, 7, 26 fused ring derivatives, 7, 47-93 imidazole synthesis from, 5, 487-488 photochemical addition reactions to carbonyl compounds, 7, 56 photolysis, 5, 780, 7, 28 protonated... 

Fused aziridines are interesting compounds owing to the fact that the strained three-membered ring can easily open and cause dipolar cycloaddition reactions as well as their photochromic properties. Therefore, most of this chapter covers the chemical and photochemical properties of bi- and tricyclic aziridines. Some properties of aziridinyl ketones are also reviewed, in particular, reactions leading to aziridinyl anils. 

During the present decade, a wide variety of polycyclic carbacephem derivatives have been reported starting from readily available monocyclic /3-lactams, which after transformation in more functionalized compounds and further cyclization yielded different fused carbacephems. Several approaches for the preparation of fused carbacephem derivatives including cycloaddition reactions such as the [2+2], 1,3-dipolar, and Diels-Alder reactions, as well as transition metal-catalyzed reactions such as the Pauson-Khand and ring-closing metathesis (RCM) reactions have been reported in the literature. 

Thermal intramolecular cycloaddition reactions of unsaturated nitrones 1341 derived from a series of N- 2-alkenyl)-2-pyrrolecarbaldehydes 1340 and benzylhydroxylamine lead to competitive formation of two kinds of intramolecular cycloadducts, namely the fused- and the bridged-ring regioisomers 1342 and 1343, respectively (Scheme 255) <2001T8323>. Further elaboration of compounds 1342 and 1343 has given pyrrolizidine and indolizidine derivatives, respectively. A similar regiochemical trend was observed when aldehydes 1340 were reacted with (/ )-a-methylbenzylhydroxylamine in order to synthesize optically active compounds. 

Several fused-ring thiiranes have been synthesized and characterized by H, NMR, and MS. For instances, thiiranes 23-25 are products from the reaction of dimethoxycarbene with 2,2,4,4-tetramethyl-3-thioxocyclobutanone <2002CEJ2184>. Compound 24 was also identified as a product from the cycloaddition of a thiocarbonyl ylide <2001HCA1805>. 

Several ring-fused tetrazole compounds have been reported. 2-Methyl-3-cyanopyridines were converted into their corresponding 2-azidomethyl derivatives, which underwent intramolecular cycloaddition reactions to give 3-(tetrazol-5-yl)pyridines 186 <04TL9127>. Fused tetrazole derivatives 187 were obtained via tandem cycloaddition and yV-allylation reactions <04JOC1346>. Expeditive synthesis of homochiral fused tetrazole piperazines 188 from 3-amino alcohols has been reported <04TL3725>. A novel Ugi-five-center-four-component reaction (U-5C-4CR) of aldehydes, primary amines, trimethylsilyl azide and 2-isocyanoethyl tosylate afforded tetrazolopiperazine type compounds <04TL6421>. 

Pyrolysis of the compounds 250 yields nitrene intermediates, which undergo a wide range of insertion reactions.502 529 W-Cyanopyrroles, which are obtained from the pyrolysis of tetrazolopyridines, also arise from nitrene intermediates via ring contraction.512,530 Mass spectral studies have indicated that the first step in the gas-phase pyrolysis of fused tetrazolopyridines and pyrimidines involves tautomerism to the azide with subsequent nitrene formation.523 Copper compounds have been found to catalyze the generation of nitrene intermediates from the compounds 250 and 244.531 The compounds 250 also undergo cycloaddition reactions through the azido form.511... 

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