Anchi meric assistance

Methanolysis of the syn isomer of 117 (see Section III,B) is anchi-merically assisted and gives the rearranged ether (118). Silver ion-assisted methanolysis of anti- l is also attended by rearrangement the product is formulated as the 1-benzazepine (119). Mechanisms are suggested for both of these rearrangements. ... 

Since evo-2-norbornyl arenesulfonates apparently do solvolyze with anchi-meric assistance, the less strained 2-bicyclo[2.2.2]octyl derivatives (92) are of interest. In this system no comparison of the. rates of exo and endo derivatives can be made, since a substituent exo to one bridge is endo to t fief other. Stereochemical studies are, however, possible. If the carbonium ion (93) were formed from chiral 92 by participation of the —C6 bond, it would be chiral. Scheme 5 shows that attack at C2 of 93 should give retained bicyclo[2.2.2.]octyl solvolysis products (92a), and attack at Cx should give chiral exo-2-bicyclo[3.2.1]octyl... 

Intramolecular nucleophilic displacements, such as those in lactone formation, have faster reaction rates than intermolecular SN2 reactions because the latter require two species to collide. The neighboring participant is said to furnish anchi meric assistance. 

One of the most important lessons taken from this body of work has been the excellence of carbohydrates as a teaching ground for many of the fundamental concepts of modem organic chemistry, including the concepts of ion pairs, of transient intermediates, of neighboring group participation and of its sister anchi-meric assistance, of torsional strain and interactions, of so-called stereoelectronic effects, and of double diastereoselection. 

Another method " for predicting the rates of cyclic secondary systems (17) has recently become available. The method is a refinement of the a-Me/H ratio method of estimating the magnitude of solvent and anchi-meric assistance. Since a linear or plot (Taft plot) is observed for the solvolysis of acyclic tertiary derivatives (18) in various solvents, by placing... 

Like the methoxy group, the hydroxy group shows weak HO-3 anchi-meric assistance when the neighboring group is on a primary carbon [cf. Table 12 compare (183) to (184) and (186) to (188)]. The enhanced assistance by the hydroxyl group in (188) relative to (184), k(i88)A(i84) = 698 is said to be a result of steric interactions of the adjacent methyl groups. ... 

Finally, Paquette and Dunkin found no evidence indicating oxygen participation (laticyclic participation) in the solvolysis of (220). Thus anchi-meric assistance by the n bond occurs, leading to the intermediate (221), and an intermediate such as (222), if it exists, is not indicated by kinetic or product studies. 

In related studies of these types of metal-mediated epoxide-opening chemistry, Carreira s group has reported that, when chloro epoxide 110 was treated with ZrCl4, anchi-meric assistance of chlorine atoms occurred to give configurationally scrambling chloride 114, whereas no scrambling took place when it was treated with TiCl(Oi-Pr)3 (Scheme 43.16)." The authors have postulated that a... 

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