“…In general, there are three types of species that can participate in (3 + n )-cyclodimerizations: (1) 1,3-dipoles, (2) 1,3-biradicals, and (3) three-membered rings. Thus, (3 + 2)-cyclodimerization of nitrile oxides is a preparative method of furoxans synthesis. , There are also scarce examples of other (3 + 2)-cyclodimerizations, − among which the most important is a formation of cyclopentanes from the corresponding cyclopropane derivatives. , Alternatively, (3 + 3)-cyclodimerizations have been reported for various 1,3-dipoles including nitrile oxides, , carbonyl oxides, − carbonyl imides, nitrile imides, , carbonyl ylides, thiocarbonyl ylides, − azomethine ylides, etc. , (3 + 3)-Cyclodimerization has also been reported for 1,3-biradicals, such as trimethylenemethanes; however, this reaction usually proceeds with low yields and is accompanied by formation of various side products. − Additionally, there are limited reports of (3 + 3)-cyclodimerization for three-membered heterocycles (oxiranes, − dioxiranes, thiiranes, , thiirenes, aziridines, , and azirines − ), which can be considered as precursors of the corresponding 1,3-dipoles.…”