Stereoelectronic control fragmentation reactions

Because of the absence of a suitably positioned C—H bond, the alkoxy radical derived from 546 cannot undergo heterocyclization. -Fragmentation therefore ensues to give 547 along with a small amount of parent ketone. An empirical predictive rule has been developed to account for the stereoelectronic control observed in such reactions... 

The stereoelectronically controlled reaction of hydroxide ion with an 0-labeled tertiary amide(J8 ) (Fig. 3) should give the intermediate 19 which can fragment in only two ways, yielding the starting labeled amide J8 or the hydrolysis products direct cleavage of W to give unlabeled amide 18 cannot take place with the help of the primary electronic effect. In order to form the unlabeled amide J8 with stereoelectronic control, intermediate Jj) must first be converted into another conformer such as 20. Oxygen... 

In another study, Buchanan and McLay (104) found that the equatorial oicy- ri clic ketotosylate 36S gave the seven-membered olefinic ester 367 while the,. axial isomeric tosylate 368 gave the seven-membered olefinic ester 371. Again, the reaction of the equatorial tosylate occurs through the fragmen- tation of intermediate 366. The transformation 368- 371 can be explained by the stereoelectronically controlled retro-Dieckmann fragmentation of 369 followed by the elimination of the tosylate group from 370. 

Radical fragmentation reactions are also stereoelectronically controlled. For the (3-elimination or fragmentation of carbon-carbon centred radicals, the... 

As a result of an analysis of numerous data on the direction of hydrolysis reactions of ethers and amides which proceed via formation of the type XXVII intermediates, a simple phenomenological theory was evolved of stereoelectronic control of fragmentation of tetrahedral intermediates [120, 121]. This theory rests on the classical notion of the hybrid orbitals of electron lone pairs (ELP) of heteroatoms. 

An equilibrium between the isomeric hydroxysulfones, VIII/44 and VIII/45, was observed in the presence of butyllithium. Treatment of VIII/44 with potassium ferf-butoxide gave only the ring enlarged (E)-isomer VIII/46. The ( -configuration of the double bond is the result of the stereoelectronic course of the fragmentation. Under the same reaction conditions the formation of the isomeric VIII/47 from VIII/45 was not observed only compound VIII/46 was isolated. This must happen via thermodynamically controlled epimerization of VIII/45 —> VIII/44. For similar reactions, see ref. [28]. 

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