Heterocyclic compounds 2 - cycloreversion reactions

The heteroatom in a three-membered heterocycle can be eliminated via various cycloreversion reactions, for example by nitrosation of aziridines, or by the reaction of thiiranes with trivalent phosphorus compounds. 

The impact of (2 + 2)-cycloaddition and (2 + 2)-cycloreversion reactions of heterocyclic compounds on organic chemistry over the last 10 years is clearly illustrated by several examples. Various members of the important /Hactam antibiotics, penicillin and cephalosporin C, as well as structurally related heterobicyclic compounds have been obtained by (2+ 2)-cycloaddition of heterocycles with ketenes (Section II,D,l).n Intramolecular photochemical (2 + 2)-cycloadditions of 2-pyrones yield 2-oxabicyclo 2.2.01hex-5-en-3-ones, which upon further irradiation afford cyclobutadienes (Section III,D,2).12 Intermolecular (2 + 2)-cyclo-additions of vinylene carbonates with olefins and with acetylenes offer a simple route to cyclobutanes and cyclobutenes, respectively (Sections III,B,3 and 5).13 (2 + 2)-Cycloaddition and (2 + 2)-cycloreversion reactions have contributed substantially to the development of the chemistry... 

Four different types of (2 + 2)-cycloaddition and -cycloreversion reactions of heterocyclic compounds are known intermolecular and intramolecular reactions, both thermal and photochemical. Three of these have already been discussed in the previous sections on (2 + 2)-cycloadditions, and as far as the mechanism is concerned both the forward and the reverse reaction suffer from the same ambiguity Do they proceed via a concerted or a nonconcerted mechanism do they involve an ionic or a diradical intermediate are they symmetry-allowed or forbidden So far only one reaction type is known to be limited to the reverse reaction, viz., the thermal intramolecular reaction [Eq. (10)], in which one o-bond is broken and a conjugated -electron system is... 

Members of the previously unknown class of heterocycles, 1,2,4-selenadiphospholes 33 and 69, have been prepared (Scheme 44). The thermal reaction of 1,2,3-selenadiazole 1 with phosphaalkynes 209 gave products 33 and 69 in 17% and 16% yields, respectively <1996PS99>. These compounds are proposed to form by a sequence of [3+2] cycloreversion and cycloaddition reactions (see also Section 6.12.5.9). 

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