Photolytic reactions ring contraction

The photolytic cleavage of cyclic ketones 14 leads to formation of a diradical species, that can undergo analogously the various reactions outlined above. The decarbonylation followed by intramolecular recombination yields a ring-contracted cycloalkane 15 ... 

Scheme 10.14 gives some other examples of Wolff rearrangement reactions. Entries 1 and 2 are reactions carried out under the classical silver ion catalysis conditions. Entry 3 is an example of a thermolysis. Entries 4 to 7 are ring contractions done under photolytic conditions. Entry 8, done using a silver catalyst, was a step in the synthesis of macbecin, an antitumor antibiotic. Entry 9, a step in the synthesis of a drug candidate, illustrates direct formation of an amide by trapping the ketene intermediate with an amine. 

The preparative application of photolytic ring contraction reactions of cyclotetrasi-lanes, which also lead to the formation of cyclotrisilanes, is largely impeded by the limited stability of the cyclotrisilane moiety. The photolysis of octaisopropylcyclote-trasilane, for instance, affords the corresponding cyclotrisilane only as an intermediate, which can be detected by UV spectroscopy. Further silylene extrusion gives rise to the... 

Related carbenes, i.e. 3//-benzocycloheptatrien-3-ylidene and 5/7-dibenzo[a,c]cycloheptatrien-5-ylidene generated from the corresponding tosylhydrazones, thermally or photolytically, isomerize to benzo-12 or dibenzobicyclo[4.1.0]hepta-2,4,6-trienes, respectively. In contrast to cycloheptatrienylidene, 5/7-dibenzo[a,c]cycloheptatrien-5-ylidene underwent ring contraction to form a cyclopropene derivative 13 before reaction with alkenes. 

Photolytic or thermolytic cleavage of the N-0 bond of tetrahydro-1,2-oxazine-3,6-diones (407), followed by decarboxylation, gives intermediate 1,4-diradicals (408) which close to form 3-lactams (409), albeit in poor yields. Diradical intermediates analogous to (408) may be formed in the biosynthesis of isopenicillin N from trlpeptide precursors. In a similar vein, Easton has published full details of the synthesis of 3-lactams by ring contraction of isothiazolidinones, a reaction which may also be relevant to... 

In other cases the tetrachlorobicyclo[4.2.0]octadiene cleaves off tetrachlorobenzene when irradiated through quartz glass with a low pressure mercury lamp. In this reaction the cyclobutene ring of the educt becomes degraded which is of synthetic value if the cyclobutene ring results from a photolytic ring contraction (eq 19) or from a Diels-Alder reaction with 7,8-dichlorobicyclo-[4.2.0]octadiene. 

Photolysis of certain quinoline N-oxides gives rise to benzoxazepines, e.g. (59), whose further photolytic and other reactions have been studied in aprotic solvents, indoles may be obtained.Photolysis of benzothiepins (60 H = CN, H, OAc, or OMe) caused ring-contraction as the primary reaction [to (61) ], while in similar reactions on 1-benzoxepins (62) the tendency for ring-contraction has been related to the 7r-donor and -acceptor properties of various substituents (R ... 

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