Three-membered rings fragmentation

The interaction between a three-membered ring fragment and a n part is found to be comparable to that between two rr-moieties. The interaction between a four-membered ring fragment and a rr-system is found to resemble that of an unstrained ring and a rr-moiety. 

Expansions of cyclic sulfones to cyclic sulfinates are known124, and a similar mechanistic pathway of the expansion of the three-membered ring to a four-membered ring has been suggested for the photolytic fragmentation of the 2,3-diphenylthiirene oxide 18a22. 

The main result of the thermolysis of the three-membered ring sulfoxides and sulfones is the extrusion of the sulfur monoxide and the sulfur dioxide moieties (Section III.C. I)99 10 5. Only in the presence of a suitably disposed /J-hydrogen does the ordinary sulfoxide-sulfenic acid fragmentation take place in the thiirane oxide series (equation 9). 

The fragment of 1,4-PD is also present in 1,1-divinylcyclopropane (DVC), where the central methylene group is replaced by a three-membered ring. For this strained molecule a strong interaction between cyclopropane Walsh and vinyl 7r-orbitals was expected. The photoelectron spectra of DVC5 could be best understood with the assumption of optimal... 

In contrast to "illogical disconnections", in "plausible disconnections" a reasonable or, at least, plausible mechanism exists which involves the motion of electron pairs and leads to stable fragments, either ions or molecules. In any case, however, the "principle of maximum simplicity" does not hold here, since one of the resulting fragments is always a dissonant three-membered ring. 

Interactions between three-membered rings in SPC which are observed in their PE spectra could be used to account for some unusual properties of these compounds. For example, tetracyanoethylene (TCNE) was found to react with the cyclopropyl fragment of hydrocarbon 25, giving a formal insertion product 26 (equation 22)12. For another... 

The chain of three-membered rings in linear triangulanes has a unique helix-like geometry (e.g. in all tra .v-[ ]triangulane 61). This allows cyclopropyl fragments which are separated by several three-membered units to potentially reside very close in space. Thus, triangulanes are very useful models for studies of proximity-induced reactions. 

The presence of an aroyl fragment in azomethine ylides obtained from opening of three-membered rings in the case of dipolarophiles with high LUMO (lowest unoccupied molecular orbital) energy or in the absence of an external dipolarophile can lead to the possibility of such unusual reactions as intramolecular 1,3-dipolar cycloaddition [80]. Examples of such reactions are the thermal isomerization of aroyl aziridines 63 into a pyrrole derivative 64 [81, 82] or into 2,5-diphenyloxazole 65 (in the presence of diphenyliodonium iodide) [83] (Scheme 1.16). 

Some explanation is needed Treatment of 9 with base and bromine must produce the potassium enolate of the bromoketone that cyclises 12 to form the three-membered ring. Reduction presumably gives the exo— alcohol4 13 whose tosylate can fragment with hydroxide 14 to give 11. 

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