Ring contraction fragmentation

Kassou, M. Castillon, S., Ring Contraction Vs Fragmentation in the Intramolecular Reactions of 3-0-(Trifluoromethanesulfonyl) Pyranosides - Efficient Synthesis of Branched-Chain Furanosides. J. Org. Chem. 1995,60, 4353-4358. 

The mass spectral behavior of 2,3-dihydro-4//-l,3-oxazin-4-ones under electrospray ionization could be characterized by a predominant RDA fragmentation pathway, producing a pair of complementary ions 18 and 19. However, the decomposition of 17 also followed another pathway, resulting in ring contraction to form oxete ions 20 by the loss of isocyanate (Scheme 3) <2004RCM1116>. 

The photolysis of cyclic polysilanes results in ring contraction with concomitant extrusion of a silylene fragment. Although the formation of two reactive intermediates potentially complicates mechanisms for product formation, it has provided a useful method for the synthesis of both unstable and stable disilenes... 

The importance of ring contractions, fragmentations, and rearrangements in these types of reactions was again emphasised, and inferences were drawn from this work as to the mechanism of the reaction, based on steric modelling of the compounds. (See Sect. 4.3). 

The mechanism of the photochemical degradation of catenated silicon derivatives has received considerable attention (25). Substituted cyclic derivatives photochemically extrude a silylene fragment which can be intercepted by appropriate trapping reagents (e.g., trialkylsilanes or 2,3-dimethyl butadiene). This extrusion results in the formation of the corresponding ring contracted cyclopolysilane. The process continues upon additional irradiation until a cyclotetrasilane results which then undergoes... 

On the other hand, in bi- or polycyclic molecules a decarbonylation reaction such as a [5 -> 4 + 1] fragmentation of a five-membered ring is quite common. However, in these cases the reaction is not accompanied by ring contraction. As an example, norbornen-7-one (16) affords cyclohexadiene (86CP307 90JA5089). 

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