Norbomyl acetate

The acetolyses of both ero-2-norbomyl brosylate and e do-2-norbomyl brosylate produce exclusively exo-2-norbomyl acetate. The exo-brosylate is more reactive than the endo isomer by a factor of 350. Furthermore, enantiomerically enriched exo-brosylate gave completely racemic ero-acetate, and the endo-brosylate gave acetate that was at least 93% racemic. 

With Robert s data one should consider the criticism of his results by Deno According to the latter the equalization of carbon and hydrogen atoms that precedes the reaction of the carbonium ion with the solvent results from intramolecular rearrangements so it should be regarded neither as a proof of the equivalence of carbon atoms in carbonium ion structures nor as a confirmation of resonance structures. Hence, Robert s evidence that on conversion of 2-norbornyl tosylate into norbomyl acetate all the atoms except C become nearly equivalent does not favour the structure of the nonclassical ion 5 because the experimental data on isotopic equalization indicate only relative rates of carbonium ion rearrangements. 

The hydrogenolysis of 3-acetoxynortricyclene (481) using Hj-Pt in the presence of traces of perchloric add afforded 7-acetoxynorbornane (68 %) and 2-exo-norbomyl acetate (32%) the formation of (482) using Dj-Pt indicates >95% stereospecific deuterium incorporation into the 2,6-endo-positions. Similar deuterolysis of 7-acetoxyquadricyclane gave the 5yn- ndo,anti-exo-tetradeuterio-derivative (483), which is also formed from 7-acetoxynorbornadiene in fact valence isomerization of the quadricyclane to the norbornadiene is a prelude to the deuterium incorporation. 

The deamination of oido-norbomylamine yields 87% norbomyl acetates ( 99% exo), 10% norbomanols (98% exo) and 2% of the norbomyl nitratesThe exo acetate was 89% racemic this feet... 

The firmest conclusion that can be drawn with regard to the structural problem is that both the exo- and endo-amines yield a substantial amount of classical norbomyl ions during deamination. If stabilization by non-classical bridging is in competition with classical ion formation, one might deduce from the optical purity of the exo-acetate produced from the exo-amine that it amounts to less than T4 kcal mole . On the other hand all the data could be reconciled without invoking the non-classical species provided that a small barrier is proposed for the Wagner-Meerwein shift. 

As an example (Scheme 12), exposure of racemic 72 to sodium methoxide resulted in a smooth fragmentation of the norbomyl backbone to achieve olefinic methyl ester 73, whose elaboration then afforded densely functionalized cyclopentane framework 74. Sodium acetate-mediated displacement of the iodo functionality within 74, followed by elimination of the mesylate group and base-promoted deacetylation finally provided the unsaturated carbasugar 75, which nicely served as a building block for the preparation of a set of neplanocin analogues. 

Let us now consider Scheppele s discussion of the solvolysis of 2-d-exo- and -endo-bicyclo[2.2.1] hept-2-yl p-nitrobenzoates and p-toluenesulfonates. A maximum a isotope effect of 1 22 is suggested for kH/kD for the limiting solvolysis of simple secondary sulphonates (solvolyses where the rate is not affected by the nucleophilicity of the medium). The endo-esters yield a similar value which might be taken to imply the absence of steric hindrance in their reaction. However, whereas the maximum isotope effect for the simple sulphonates is believed to arise partly from an activated complex in the conversion of the internal ion pair into a solvent-separated ion pair, B - C, the effect in the endo-norbomyl compounds is suggested to arise during the transition state involved in the formation of the intimate ion pair, A - B. The latter is deduced from the fact that polarimetric and titrimetric rate constants are the same in acetic acid, aqueous acetone and ethanol (Winstein and Trifan, 1949 and 1952). If this is the case, the endo-effect of 1-20 should be... 

As depicted in the following scheme, a homoallyl alcohol derived from a norbomyl a-diketone underwent a lead(IV) acetate reaction in MeOH, resulting in the formation of a novel methoxy substituted spirocyclic tetrahydrofuran <07CC4239>. It is believed that the addition of the methoxylead(IV) acetate species across the alkene from the less sterically hindered side to form a plumbonium cation leads to the major product after subsequent cyclization and reductive elimination. Moreover, construction of tetrahydrofurans by a Pd(II)/Pd(IV)-catalyzed aminooxygenation of homoallyl alcohols was also reported <07AGE5737>. 

The asymmetric transesterification of cyclic me o-diols, usually with vinyl acetate as an irreversible acyl transfer agent, includes monocyclic cycloalkene diol derivatives, bicyclic diols, such as the ej o-acetonide in eq 12, bicyclic diols of the norbomyl type, andorganometallic l,2-bis(hydroxymethyl)ferrocenepossessing planar chirality. 

Howe and Winstein have shown if the skeleton of the endo-norbomyl brosylate molecule is so much deformed that theo-bonds C —and C —OBs become nearly transcollinear, then the a-participation of the C —C bond in the solvolysis of endo-ester becomes possible. Thus, the acetolysis of endo-brosylate 31 yielded 22.5 % of endo-acetate 33 this is explained by the formation of an intermediate nondassical ion 32). Solvolysis without anchimeric assistance would have yielded exo-acetate as the main product since the exo side in this ion is far less sterically hindered than the endo side it is, however, only a side product of 6.5%. 

Table 1. Calculated and Experimental Isotopic Distribution for the 2-Norbornyl Acetate Formed on Acetolysis of 2-exo-Norbomyl [2,3- C]brosylate ...

These results agree with the formation of a classical cation in either case considering the shielding group effect and the solvent backside attack which was supposed to result in an inverted configuration the predominance of exo acetate 117, however, indicates the 2-norbomyl cation to be less accessible from the endo side than from the exo side. The formation of 34% of endo isomer 116 from exo tosylate 113 testifies... 

In this case, the participation leads to the formation of the norbomyl cation, which is captured as the acetate. More will be said later about this important cation in Section 5.12. 

This intermediate was postulated as a result of several facts regarding the acetolysis of 2-norbomyl arenesulfonates. First, the exo-isomer reacts more rapidly than the corresponding e t/o-isomer, presumably from heightened levels of anchimeric assistance. Regardless of which isomer endolexo) is utilized as the reactant, the exo-acetate is produced exclusively. Finally, optically active exo-starting materials react to give complete... 

FIGURE 21.50 An Energy versus Reaction progress diagram for the SnI solvolysis of fx )-2-norbomyl tosylate. A rapid ionization precedes a slower formation of acetate. 

The maneonenes 50 are tricyclic diethers found in the marine alga Laurencia nidifica [97-99] (Fig. 6). Synthetic analogues like 54 have been prepared by Renaud and Vionnet through radical addition to ketene acetal 51 (Scheme 7) [100]. This generates radical 52 which isomerizes into 7-oxa-2-norbomyl radical 53. Allylic quenching of radical 53 with allyltributyltin produces tricyclic compound 54. 

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