Antiperiplanar migration

This is attributed to the unfavourable steric interactions which arise in the transition state that is required for antiperiplanar migration of the exocyclic substituent.143 Some examples of synthesis of alcohols by hydroboration-oxidation are included in Scheme 4.8. More vigorous oxidizing agents such as Cr(VT) reagents effect replacement of boron... 

The reaction of trienyl azide 76 was noteworthy as the first reported example of a combined Diels-Alder/Schmidt reaction of any type (see Section 7.7 above). In addition, the isolation of compound 77b represented the first known observation of a bridged lactam from an intramolecular Schmidt reaction (in contrast to the formation of bridged ortho-amide 62 shown in Scheme 7.40). Mechanistically, an endo Diels-Alder leads to 77a and 77b (box at bottom of Scheme 7.43), while an exo transition state leads to 77c (transition state not shown). The formation of fused or bridged lactam depends on antiperiplanar migration of carbon from the equatorially or axially disposed N2 groups in the intermediates shown. 

This is attributed to the unfavorable steric interactions that arise in the transition state for antiperiplanar migration of the exocyclic substituent. ... 

Steric and conformational factors are also important, especially in cyclic systems.233 There is a preference for the migration of the group that is antiperiplanar with respect to the peroxide bond. In relatively rigid systems, this effect can outweigh the normal preference for the migration of the more branched group.234... 

This is clearly demonstrated in the pinacolinic deamination (cf. p. 114) of an optically active form of the amino-alcohol (50). Such reactions proceed from a conformation (antiperiplanar 50a or 50b) in which the migrating (Ph) and leaving (NH2 as N2 cf. p. 114) groups are TRANS to each other. Rearrangement via a bridged carbocation would necessarily lead to 100% inversion at the migration terminus in the product ketone (5lab), whichever initial conformation, (50a) or (50b), was involved ... 

Artemisyl, Santolinyl, Lavandulyl, and Chrysanthemyl Derivatives.— The presence of (41) in lavender oil has been reported earlier. Poulter has published the full details of his work (Vol. 5, p. 14) on synthetic and stereochemical aspects of chrysanthemyl ester and alkoxypyridinium salt solvolyses (Vol. 3, pp. 20—22) and discussed its biosynthetic implications. Over 98% of the solvolysis products are now reported to be artemisyl derivatives which are formed from the primary cyclopropylcarbinyl ion (93) which results from predominant (86%) ionization of the antiperiplanar conformation of (21)-)V-methyl-4-pyridinium iodide the tail-to-tail product (96 0.01%) may then result from the suprafacial migration of the cyclopropane ring bond as shown stereochemically in Scheme 3. This is consistent with earlier work (Vol. 7, p. 20, ref, 214) reporting the efficient rearrangement of the cyclobutyl cation (94) to (96) and its allylic isomer, via the tertiary cyclopropylcarbinyl cation (95). ... 

The preferred formation of lactone 73 was explained by the orientation of the non-bonded electron pairs of the hydroxyl group in the corresponding tetrahedral intermediate and by assuming that the migrating C —C bond must be antiperiplanar to the RC00 - 0 bond. They have also assumed that the per-acid reacts on the least hindered face of the carbonyl group in 72. 

The reaction is thought to proceed as shown in Figure B4.2. After hydroboration and addition of hydroxide anion to the boron atom, an iodonium species is formed which creates an electrophilic terminus for alkyl migration. (The migration occurs with retention of configuration of the alkyl group.) Finally an antiperiplanar elimination reaction gives the (Z)-alkene. 

This is an example of a reductive hydroxy-tosylate 1,2-rearrangement. For such a rearrangement to proceed, the migrating bond must be antiperiplanar to the C—O bond of the leaving group. Initially, treatment of 2 with L-selectride6 leads to aldehyde 18, which is then reduced in situ to 1 (Scheme 11.6). 

The stereoelectronic requirements proposed for the migration step are an antiperiplanar arrangement of the C— Rm bond and the breaking O—O bond. It has been further suggested that one of the hydroxy nonbonding electron pairs must be antiperiplanar to the migrating carbon atom, as in (3). ... 

Cyclopentene oxide is very unreactive (k < l( itcyclohexene oxide), an observation clearly not in accord with expected Sn2 reactivity it gives cyclopentanone as the only observed product. This ketone cannot be formed by antiperiplanar hydride displacement of bromide from the tra/is-bromohydrin, whereas the c/5-bromohydrin could easily adopt the necessary conformation for hydride migration. Almost certainly the latter is formed by slow, perhaps rate-determining 5n2 attack by bromide on the initially formed trans-bromohydrin salt, as outlined in equation (121). This secondary reaction may be more important with Lil than with LiBr, although this has not been proven (see also equation 83 and discussion of MgX2-catalyzed reactions). 

The inverse relationship between the micellar effects on direct displacement and hydrogen shift has been confirmed with esters of isoleucine (200)194 (Fig. 5). Apparently the two competing reactions are controlled by the preferential mode of solvent approach on conformation (203) of the diazonium ion, with migrating hydrogen and leaving group antiperiplanar. In dilute solution the solvent approach is preferentially... 

The first cinchona rearrangement is also feasible without Ag +, provided stringent stereochemical and experimental conditions are fulfilled. In the preferred conformation of epi-la-OMs, the C9-OMs leaving group and migrating C7-C8 0-bond are antiperiplanar (Scheme 12.41). Under optimized conditions with NaOBz as a buffer (for the liberated methanesulfonic acid), a-amino ether 107-OMe was formed in 81%... 

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