Bridgehead positions rearrangements

Rearrangement of the 3,5-dimethylated carbene Id would yield the destabilized cyclopropene 3d with a methyl group in the bridgehead position 1, and consequently no detectable amount of cyclopropene 3d is formed during irradiation of Id. Indeed, whereas the 3,5-dimethyl substituted carbene Id is 3.4 kcal mol-1 more stable than the 2,6-dimethyl isomer lb, the stability is reversed for the cyclopropenes, as 3d is found to be 6.5 kcal mol-1 higher in energy than 3b at the B3LYP/6-31G(d) level of theory (Table 3). 

S.2.8. The 1,3,5,7-Tetramethyl- and 1,2,3,5,7-Pentamethyl-2-adamantyl Cations. The nature of the 2-adamantyl cation 571 has been difficult to study under stable ion conditions since it undergoes facile rearrangements to the more stable 1-adamantyl cation 49.182 This difficulty was circumvented by Lenoir, Schleyer, Saunders, and co-workers999 by blocking all four bridgehead positions by methyl groups in a study of 1,3,5,7-tetramethyl- and l,2,3,5,7-pentamethyl-2-adamantyl cations 572 and 573. 

First, as discussed earlier in connection with the aluminum halide catalyzed rearrangements of hydrocarbons (Section II. A. 2), intermolecular hydride transfer reactions appear to be fairly unselective processes. Apparently, charge development in the transition states of these reactions is minimized a penta-coordinate carbon intermediate may be involved. As a result, the strong preference for the bridgehead positions exhibited by most ionic substitution reactions is partially overcome. 

The conversions summarized in Scheme 21 and equations (5) and (6) ow clearly the syndetic va-satility of using bicyclo[3.1.0]hexan-2-ones as precursors of divinylcyclopropanes. Indeed, bicy-clo[3.2.1]octa-2,6-dienes possessing alkyl substituents at either bridgehead position (148a,148c) or at C-4 with endo (141,148b) or exo orientation (146) are readily prepared. Furthermore, the rearrangement... 

The hydrolytic stability of the [Co(diCLsar)]3+ cation toward conversion to the [Co(diHOsar)]3+ dihydroxysarcophaginate is due to the difficulty of forming a carbocation at a planar bridgehead position. It was noted that the nitrosation of [Co(diAMsar)]3+ sarcophaginate occurring without rearrangement involves compe-... 

Quasi-Favorskii rearrangement Skeletal rearrangement of bicyclic a-halo ketones in which the halogen is located at the bridgehead position to afford carboxylic acids or carboxylic acid derivatives. 370... 

The cyclobutyl-cyclopropylmethyl rearrangement of 2-oxabicyclo[4.2.0]octan-3-ones 7 is strongly dependent on their substitution pattern at the bridgehead positions.When unsubstituted or monomethyl-substituted -lactones are treated with acetic acid or 4-toluenesulfonic acid under reflux, no rearrangement occurs. However, under similar conditions dialkyl-sub-stituted lactones lead to an almost quantitative yield (at 20-40% conversion) of y-lactones 8. 

When the isoxazole (105.1) is treated with an excess of diazomethane at laboratory temperature, a [1, S]sigmatropic shift occurs and the benzoyl group migrates from the bridgehead position to an adjacent carbon (that derived from diazomethane)—a move which resembles the Van Alphen-Hauttel rearrangement [B-4]. 

Rearrangement of polycyclic systems. (4, 432-435 5,585-587). Paquette has reviewed shver ion-catalyzed rearrangements of cyclic systems, particularly those that are useful synthetically. A few of these are formulated. One is the preparation of homotrophidene (2) from 3-norcarene (1) shown in equation I. Note that rearrangement of (3), with a methyl group at a bridgehead position. 

Valence tautomerism of the bullvalene skeleton appears to be implicated in the rearrangement processes that occur during the formation of potential bullvalenyl-carbinyl carbocations. " Thus, for example, solvolysis of the esters [(154) and (155)] in aqueous acetone affords the alcohol (156) as the exclusive product. This result is consistent with the mode of attack pictured in (154), or the equivalent alternative formulation which places the carbinyl ester function at the cyclopropane bridgehead position [i.e. the valence tautomer of (154)]. In the case of the positionally isomeric... 

When quinamine was refluxed in dilute acetic acid for a prolonged period it afforded the ketoamine, quinamicine, which may have been the result of a direct loss of water or more likely an allylic rearrangement of the tertiary hydroxyl group in the hydroxy-methyleneindoline form then elimination (the hydroxyl cannot be lost to form the iminium ion because of the bridgehead position of the quinuclidine nitrogen). The formulation of quinamicine is the only reaction of these compounds which has any parallel in... 

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