Transannular hydride transfer

Intrigued by facile transannular hydride transfers in medium-sized rings,959-963 Saunders et al56 examined the 2,6-dimethylheptyl cation 544. Even at the lowest... 

Several instructive iUnstrations taken from the field of natural products will serve to conclude the Bsent discussion of acid-catalyzed epoxide isomerization. Mwdi has been said about the ooeurrenoe of simple i,2-hydride transfers in boron trifluoiide-CBta yzed epoxy steroid rearrangements. Although initiated by a mineral acid rather than a Lewis acid, an instance of transannular hydride transfer has been reported for a 9/3,11 -epoxy steroid, as shown in Eq. <484. Such migrations can in principle occur elsewhere. 

The latter results from an unusual intramolecular Cannizzaro-tjrpe reaction involving a transannular hydride transfer from C-20 to C-7. Oxidation of LXXVI followed by reduction gave the epimeric hydroxyl lactam LXXVII with the hydroxyl (p ax 1682 cm i) in the original configuration of ajaconine. The hydroxyl in LXXVI is thus trans to the i r-bridge and in LXXVII is cis thereto. Further evidence on this point is provided by the behavior of the methiodide (LXXVIII) of the azo-methine alcohol (LXVII). The product liberated from this salt by hot, methanolic alkali is a hydroxyl-free base (LXXIX, 10) and hence... 

Medium ring compounds commonly show enhanced solvolysis rates and solvolysis products derived from transannular hydride transfer. Ionisation in these systems may take place with some degree of transannular C—H bond participation, the extent of bridging in the transition state and competitive nucleophilic solvent assistance (A s-process) being broadly and independently variable. 

The rearrangement of [13] bears a close resemblance to the transannular reactions observed in medium sized rings that have been reviewed by Prelog and Traynham (1963) and Cope et al. (1966). Recently Sorensen and coworkers have studied medium sized cycloalkyl cations under stable ion conditions in non-nucleophilic media and demonstrated that their structures are ji.-hydrido-bridged. The bonding situation in these ions contrasts sharply with that in the ions described above and rather corresponds to that of transition states (or intermediates) for intramolecular hydride transfer in these ions. 

Highly unsaturated cyclic compounds likewise do not react to give di-or tricarboxylic acids, but tertiary monocarboxylic acids are obtained with CO/HgO by a transannular reaction with hydride transfer and formation of bi- or tricyclic systems at the bridgehead. Thus, cyclodecadiene-1,5 reacts to give a mixture of cis- and trans decalin carboxylic acid [624] and cyclododecatriene-1,5,9 reacts to give a mixture of isomeric tertiary per-hydroacenaphthene carboxylic acids. 

Potassium tert-butoxide Isomerization of cyclic hydroxyketones Transannular 1,4-hydride transfer... 

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