Cross-conjugated betaines

Cross-conjugated mesomeric betaine - Cross-conjugated heterocyclic mesomeric betaines are cyclic mesomeric betaines in which the positive and negative charges are exclusively restricted to separate parts of the Tt-electron system of the molecule <1985T2239>. For examples see Sections 2.2.1.2.2 and 2.3.1.2.1. 

Clearly, in the case of (66) two amide tautomers (72) and (73) are possible, but if both hydroxyl protons tautomerize to the nitrogen atoms one amide bond then becomes formally cross-conjugated and its normal resonance stabilization is not developed (c/. 74). Indeed, part of the driving force for the reactions may come from this feature, since once the cycloaddition (of 72 or 73) has occurred the double bond shift results in an intermediate imidic acid which should rapidly tautomerize. In addition, literature precedent suggests that betaines such as (74) may also be present and clearly this opens avenues for alternative mechanistic pathways. 

Tliis chapter covers nitrogen-containing fulvalenes that can be obtained by replacement of CH=CH and/or CH, for example, types 1-3 starting from compounds 1-6. Compounds in which nitrogen atoms are arranged on the periphery of the cross-conjugated system as in 15 or 16, as well as derivatives in which the central double bond contains heteroatoms as in 17, are not included. For azoniafulvalenes of type 17 and related heterocyclic betaines see (94AFIC197). 

D. Pseudo-cross-conjugated Heterocyclic Mesomeric Betaines. 131... 

Trigollenine (18) and the pseudo-cross-conjugated Homarine (19) are isoconjugate with the isopropenylbenzene anion, which is an odd alternant hydrocarbon anion. Therefore, these betaines belong to class 9 and 13, respectively. 

Oxidation of berberine (49) with hot dilute nitric acid yields berberidic acid (220) (58MI1) which can form a cross-conjugated and a pseudo-cross-conjugated mesomeric betaine on deprotonation as shown in Scheme 72. 

Methylisoquinolinium 2-carboxylate (230), originally prepared by Quast (70LA64), was recently identified as a defensive betaine from Photuris versicolor fireflies (99JNP378). It is a pseudo-cross-conjugated mesomeric betaine isoconjugate to the odd alternant hydrocarbon 2-isopropenyl-naphthalene anion which is an odd alternant hydrocarbon anion. This compound therefore is a member of class 13, which is very rare. The UV absorption maxima Imax (methanol) were found at 235 (4.35), 320 (shoulder, 3.97), and 326 (3.99) nm. This compound undergoes similar reactions as Homarine 19 (Scheme 75). The NMR data are presented in Table VIII. 

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). 

Vincarpin and Dihydrovincarpin are two additional examples of pseudo-cross-conjugated mesomeric betaines. They were isolated from Vinca major elegantissima (76TL4887) (Scheme 82). 

Structurally related to Nigellicine (234) (cf. Scheme 77) which is a pseudo-cross-conjugated mesomeric betaine. 

Oxidation yields the cross-conjugated mesomeric betaine 217, but no zwitterionic species, which would result by deprotonation of the indole NH group (88LA1111). The corresponding 5,6-unsaturated indole, however, gave the zwitterion. The same publication describes a total synthesis of Flavoserpentine. 

The Diels-Alder cycloaddition potential of fused 4-aryldihydropyrimidine mesomeric betaines has been studied. The cross-conjugated thiazinium betaine 317 underwent 1,4-dipolar cycloaddition with electron-rich dipolaro-philes, and thus 1-diethylaminoprop-l-ine gave the pyrido[l,2-tf]pyrimidine 318 by loss of carbonyl sulfide (Equation 34). Reaction of 317b with 1,1-diethoxyethene resulted in the 8-ethoxy analogue of 318 (R = H) <1997JOC3109>. 

In general, conjugated heterocyclic mesomeric betaines are associated with 1,3-dipoles and cross-conjugated heterocyclic mesomeric betaines are associated with 1,4-dipoles. The dipolar cycloaddition reactions of both types of heterocyclic mesomeric betaines have been widely investigated and its use for the preparation of a diverse variety of heterocyclic compounds was duly covered in CHEC-II(1996) <1996CHEC-II(8)747>. 

Synthesis of Cross-Conjugated and Pseudo Cross-Conjugated Mesomeric Betaines... 

Synthesis of cross-conjugated mesomeric betaines containing l-oxo-l//-pyrazolo[l,2-tf]pyrazol-4-ium-3-olate, 1-oxo-1 //-pyrazolof 11.2,4]triazol-4-ium-3-olate, 7-oxo-7//-pyrazolo[l,2-tf][l,2,4]triazol-4-ium-5-olate, 7-oxo-7.//-pyrazolo[ 1.2-//][ 1.2.3 ]mazol-4-ium-5-olate, 3-oxo-3//-pyrazolo[l,2- ][l,2,3,5]thiatriazol-8-ium-l-olate, and 2-hydroxy-pyrazolof 1.2-//][ 1.2.5]m.izin-5-iumH-olate structures was duly covered in CHEC-II(1996) <1996CHEC-II(8)747> and... 

C. Heterocyclic Betaines Isoelectronic with Reducible Anions (Cross-Conjugated Betaines)... 

Further examples of cross-conjugated betaines (535-537) have been reported. 88-394 Particularly notable is the thermal ring opening of compounds of the general type 538 to give the ketene intermediates 539. ... 

---