Cationic rings

The rather high basic strength of 3,5-diazaindole [91] (p/sTa = 6T) is not due to the protonation of the imidazole ring (for benzimidazole p g = 5-53), but rather of the pyridine ring. Cation [92] is formed and this is stabilized by the same kind of resonance as... 

A rationalization of the generally selective formation of the undesired C-16 -C-14 PREF relative stereochemistry on reaction of the chloro-imine-chloroindoline alkene 17a,b with vindoline (3) may be found in a preferred conformation, 25a or 25b, of the cationic intermediate 25, where the C-3 methylene group hinders approach to one face of the nine-mem-bered ring (Fig. 2). A conformationally more flexible nine-membered ring cationic intermediate 29a,b would be expected to be formed from the chloro derivative of a D-secocleavamine 30. From reaction of such a compound with vindoline (3), under the usual protic acid conditions for cou-... 

The fundamental types of thermally and photochemically induced intramolecular transformations are summarized in Scheme 1. All reactions of this class involve intermediates in which aromaticity is lost hence they are most common in the classes of less aromaticity, i.e. polyhetero rings, cationic rings, rings containing carbonyl groups. However, polysubstitution, especially by bulky groups, can also induce reactions by strain relief in transition states. Most of the reactions known are photochemical. 

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