Betaines, heterocyclic salts

Quaternization of harman (235) with ethyl bromoacetate, followed by cyclization of the pyridinium salt 236 with 1,2-cyclohexane-dione in refluxing ethanol yielded an ester which on hydrolysis gave the pseudo-cross-conjugated mesomeric betaine 237. Decarboxylation resulted in the formation of the alkaloid Sempervirine (238). The PCCMB 237 is isoconjugate with the 11/7-benzo[u]fluorene anion—an odd nonalternant hydrocarbon anion—and belongs to class 14 of heterocyclic mesomeric betaines (Scheme 78). 

Salt elimination, heterocycles from, 259, 260 Salts, betaine, 49-51, 61-64 Selectivity in oligomerization, 168, 169, 211-215... 

Other polyaza derivatives of aromatic systems, such as the blue tria-zaphentalene derivative (478), are also adequately described as meso-meric betaines or inner salts. Ohta and Kato, in a section entitled Bridged Heterocyclic Compounds (ref. 9b, page 241), have referred to seven types of compounds and the stereochemically unacceptable structures (479-485) which had been allocated to them. The possibility was considered by Ohta and Kato that some of these structures might have to be replaced by their meso-ionic equivalents. However, we have now established that this is not the case, and the relation between the original structures (479-485) and the correct structures (486-492) is given in Fig. 10. ... 

The preceding sections demonstrate two important general routes to six-membered heterocyclic mesomeric betaines. These are (i) deprotonation of appropriate quaternary salts and (ii) valence tautomerism of bicyclic isomers. Both approaches have been used to prepare isoquinolinium-4-olates (170) (Scheme 7). 

Examples of these betaines (293) are the bridged heterocycles 295, which have been made by condensation of l,2,4-triazin-5-one derivatives 294 with aldehydes or ketones.2-2is yj g molecular structure of compound 295 (R = H, R2 = R = Me, R" = SMe) has been determined by X-ray crystallography. Acetylation gives derivatives 295 (R = Ac) which with methyl iodide give the deep red salts 296 in high yield. 

Heterocyclic iminium salts, oxidative transformation, 41, 275 Heterocyclic mesomeric betaines and analogs in natural product chemistry, 85, 67 Heterocyclic oligomers, 15, 1 Heterocyclic products, natural, synthesis of by hetero Diels-Alder cycloaddition reactions, 42, 245... 

In another example of heterocyclic ring synthesis, pyridinium salts 816 require only Et3N for deprotonation, the resulting ylides 817 add to esters of 2-bromo-2-alkenecarboxylic acids or analogous benzonitriles to give intermediate betaines 818 which, via intramolecular nucleophilic attack, give indolizines 819 (Scheme 174) <1999JOC7618>. 

Heterocyclic Betaines Pyridinium (Imidazolium) Azolate Inner Salts with Several Interannular Linkages ... 

Pyridinium(imidazolium) inner azolate salts and molecules with a betaine character of general type 1 are attractive substrates from the viewpoint of structural chemistry, as mentioned in the Introduction. This ensemble of compounds offers the possibility of two terminal heterocyclic rings, joined through several spacers, with extreme characteristics within heteroaromatic systems a 7r-deficient nucleus (cation) and a 7r-excessive nucleus (anion). The high dipolar character is the distinctive feature offered by these compounds and has a powerful influence on their physical and chemical properties. 

Selective synthesis and cycloaddition reactions of new azomethine imines 109 containing a 1,2,4-triazine ring have been reported. 4,5-Dihydro[l,2,4]triazolo[3,4-c]benzo[l,2,4]triazines 108 with aromatic aldehydes gave stable iminium salts which were deprotonated to give new mesomeric betaines 109. These underwent 1,3-dipolar cyclization reactions affording tetra- and pentacyclic heterocycles 110 <05EJO3553 05H1889>. 

---