Ring opening cycloelimination

Phenyl- (24a) and 2,5-diphenyl-l,3,4-oxadiazole (24b), in their electronic excited states, undergo nucleophilic attack by lower MW alcohols to give adducts which ring-open (path a) or undergo cycloelimination (path b) with subsequent formation of a triazole (Scheme 4) (77CL1207). 

Biochemical reactions of l-amino-2-ethylcyclopropane-l-carboxylic acid showed strong stereoselectivity. Thus, ring-opening to 2-ketohexanoate was observed only with the (IS, 2S)-isomer (505) . On the other hand, cycloelimination to give butene by the ethylene forming enzyme in apples or pea epicotyls was performed preferentially with allocoronamic acid (649), the (IR, 2S)-isomer Pea epicotyl enzyme (cell-free system) catalyzed the formation of 1-butene for all stereoisomers (i.e. 505 and 649)" ... 

The Norrish Type I reaction usually leads to decarbonylation. This is the case with dicyclopropyl ketone on irradiation at 193 nm. Decarbonylation, however, is a second step and this is preceded by ring opening of the cyclopropyl moieties to diallyl ketone. Calculations have shown that decarbonylation of cyclobutanone occurs from the nji triplet state. The resultant triplet trimethylene biradical undergoes ISC to the ground state before formation of cyclopropane. On the other hand, the cycloelimination reaction to yield ketene and ethene arises from the singlet excited state.Irradiation of cyclopentanone in aqueous and frozen aqueous solutions has been examined and the influence of applied magnetic fields assessed. Photodecarbonylation in the crystalline phase of the ketone (3) at 310 nm takes place stereospecifically with the formation of the cyclopentane derivative (4). The latter can be readily transformed into racemic herbertenolide (5). ... 

The process for the industrial production of permethric acid by Roussel-Uclaf starts from racemic trons-chrysanthemic acid, which is in turn accessed by the Martel synthesis (cf. 1,3-cycloelimination). After separation of the enantiomers with an amino-alcohol, the (IR)-enantiomer is subjected to ozonolysis. Basic epimerisation gives (IR)-cis-caronaldehyde hemiacetal. Water is added to the (IS)-enantiomer in the presence of a catalytic amount of sulfuric acid then the carboxylic add function is epimerised with the formation of a lactone. Magnesium bromide-catalysed ring-opening leads to (lR)-c s-chrysanthemic acid, which is converted into (IR)-ds-caronaldehyde hemiacetal by an analogous route. [105]... 

The acetal, on treatment with hydrochloric acid in methanol, undergoes ring opening by the formation of the ester and chlorination to 161, which is the right substrate for 1,3-cycloelimination to cis/trans caronaldehyde dimethylacetal 162 [258]. 

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