Elimination giving three-membered heterocyclic rings

5 ELIMINATION GIVING THREE-MEMBERED HETEROCYCLIC RINGS 

A wide variety of three-membered heterocyclic rings are formed by a 1,3-elimination of HX . Such reactions occur with particular facility when the heteroatom carries the hydrogen atom and X is a halogen or an ester function. Typical examples are the conversions of 2-halogenothiols, 2-halogenoalcohols and 2-halogenoamines into cyclic sulphides, epoxides and aziridines, respect- 

Either initial proton abstraction by the base or a concerted 1,3-elimination now competes with the internal displacement . Cyclisation is accompanied by inversion of configuration at the carbon bearing the leaving group, as exemplified by a-lactam formation (264) . 

The greater separation of the leaving groups in 1,3-elimination than in 1,2-elimination increases the probability of carbanion relative to concerted reactions. It would seem a sound approach to utilise the response of some of the carbanion 1.3-eliminations to changes in the reaction conditions as models for estimating the carbanion character in the 1,2-processes. 

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When lithiated, the ring strain of the three-membered heterocycle remains important, and this strain, combined with a weakening of the a-C-O bond, due to its greater polarization, make metalated epoxides highly electrophilic species [2], They react with strong nucleophiles (often the base that was used to perform the a-deprotonation) to give olefins following the elimination of M2O (Scheme 5.2, Path B), a process often referred to as reductive alkylation . 

Numerous carbo- and heterocyclic compounds of various sizes have been prepared by HR-type monocyclization. Cyclopropanes are formed only when neopentylpalladium intermediates are formed in the absence of anions. Intramolecular carbopalladation of the alkenyl triflate 201 generates the neopentylpalladium 202, and its 3-exo cyclization and jd-H elimination afford the cyclopropane 203 [95]. Intermolecular carbopalladation of the alkyne 204 generates 205. Its 5-exo cyclization provides the neopentylpalladium 206, which undergoes 3-exo cyclization to construct the three-membered ring 207. Finally y3-H elimination gives rise to the cyclopropane 208 [96]. 

Additions to Electron-Deficient Species. Diazomethane will also add to highly electrophilic species such as sulfenes or im-minium salts to give the corresponding three-membered ring heterocycles. When the reaction is performed on sulfenes, the products are episulfones which are intermediates in the Ramberg-Backlund rearrangement, and are therefore precursors for the synthesis of alkenes via chelotropic extrusion of S02- The sulfenes are typically prepared in situ by treatment of a sulfonyl chloride with a mild base, such as Triethylamine (eq 47). Similarly, the addition of diazomethane to imminium salts has been used to methyle-nate carbonyls. In this case, the intermediate aziridinium salt is treated with a strong base, such as n-Butyllithium, in order to induce elimination (eq 48). 

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