Bisiminium salts

Treatment of a tetrahydrofuran solution of 46 with water led to formation of a new product, noranhydrovinblastine (49, navelbine, NVB), in 27% yield. The formation of this new product can be explained as follows Addition of 2 mol of water to the bisiminium salt (46) gives an unstable diol (50) which then undergoes deformylation at N-6, rendering N-6 nucleophilic. Bond formation between N-6 and C-8 occurs after elimination of water and addition to the exocyclic carbon of 51. 

The intramolecular acylation of 2-aryloxybenzonitriles allowed the synthesis of molecules with dixanthone skeleton.363 The procedure, in fact, is an intramolecular Houben-Hoesch reaction364 to afford the intermediate bisiminium salts [Eq. (5.144)] which, after hydrolysis, gave the final diketo products. 

With a few exceptions the bases are colorless or pale yellow, as are the dications, which must exist as bisiminium salts. In contrast the monocations are intensely colored, commonly purple or blue. Formation of the monocation involves setting up a stable 6-7r-electron vinamidinium system such systems have stabilization energies of the order of 20 kcal- mol-1 [72CI(L)335]. It has been noted that when a solution of the purple 2,4-dimethylbenzodiazepinium hydrochloride was basified, a yellow solution resulted that turned colorless after a few seconds. This was attributed to initial formation of the conjugated base (2), which rapidly tautomerized to the usual diimine form (40HCA1147). 

Similar stable intermediate dication structures cannot be drawn for electrophilic attack at the 5- and 7-positions. This is reflected in the enormous difference in reactivity toward electrophiles between the 6-position and the 5- and 7-positions.25 27 Kinetic studies of the bromination27 and deuteriation25 of dihydrodiazepines and their salts indicate a ratio of reactivities of at least 1 109, and probably greater. Indeed 6-methyl-dihydrodiazepinium salts undergo attack by bromine at the 6-position, despite the presence of the substituent group at that position, rather than at the 5(7)-position.27 The resultant products are hydrolysis products since the dication intermediate (14) has no mesomeric stabilization and cannot gain such stabilization by loss of a proton, and, as a bisiminium salt, is readily hydrolyzed. Similarly, when 6-methyldihydrodiazepinium... 

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