From three-membered rings

The few examples in which azirines or az.iridines ring expand in the presence of a suitable CN. synthon do not constitute an important synthetic route (o imidazoles, but the high yields frequently reported merit a brief summary here. 

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Another interesting variation of the 1,3-dipolar cycloaddition involves generation of 1,3-dipoles from three-membered rings. As an example, aziridines 7 and 8 give adducts derived from apparent formation of 1,3-dipoles 9 and 10, respectively.148... 

O ) By formation of seven-from three-membered rings This is the most widely used synthetic route to monocyclic oxepins. The key step in the synthesis of oxepin-benzene oxide (7) is the dehydrohalogenation of a dibromoepoxide precursor (64AG(E)S10). Since the benzene oxide valence tautomer is formed initially the valence tautomerization of the latter to oxepin (equation 51) may be considered as a ring expansion reaction. 

By formation of seven- from three-membered rings... 

Various cyclic compounds from three-membered rings to macrocycles have been prepared by intramolecular allylation. A typical example of this cyclization is the reaction of the monoacetate of 1,4-butenediol derivative 91 with the active methylene compound 92, which afforded the allylic alcohol 93. The three-membered chrysantemic acid derivatives 94 and 95 were then prepared after acetylation of 93, followed by Pd-catalysed intramolecular allylation [53],... 

For the period 1995-2006, the formation of fused-ring thietanes from three-membered rings is exclusively confined to the transformation of oxiranes fused to complex ring systems. It was reported <1997PS389> that the 5- y-acetyl-2,3-epoxide derived from D-xylose 116 was easily converted to 2-oxa-6-thia-bicyclo[3.2.0]heptane 117 by the action of sodium acetate (Equation 34). 

From three-membered rings In pro tic media, l-acyl-2-cyclopropene derivatives undergo a ring expansion reaction to cyclobutenols [7]. - Ring expansion of cyclopropylmethanols to fluorinated cyclobutans [8]. 

Another possibility for the preparation of bicyclic systems such as III/47 from three-membered rings can be realized by a [2+2] cycloaddition of the cyclopropene, III/41, and an unsaturated molecule, III/46, such as alkene, alkyne, ketene, ketenimine, ketone, isocyanate, etc. A large number of examples of this reaction type have been reviewed recently [55]x). 

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