Mesomeric betaines structure

Mesomeric betaine structure 7, which is a rough simplification of the real situation indescribable by a single structure, represents another degree of saturation. The real structure of mesomeric betaines is a resonance hybrid of several dipolar structures <1977T3203>. Additional unsymmetrical substitution or aza substitution increases the number of possible distinct dipolar structures. 

The bright red betaines 269 form salts with dilute hydrochloric acid and are dealkylated by hot ethanolic potassium hydroxide. The mesomeric betaine structure (269) is fully supported by spectroscopic properties. ... 

This mesomeric betaine structure is known only in the form of its 5,5-dioxides (305). They are prepared in good yields from corresponding trimethylsilyl pyrazoles and chlorosulfonyl isocyanate and subsequent heating to 60°C (Equation (45)) <83H(20)845>. 

Sydnones contain the 1,2,3-oxadiazole ring they can be represented by a mesomeric betaine structure and 3-substituted sydnones may be sulfonated in the 4-position by treatment with sulfur trioxide-dioxan complex in dichloromethane at 20-40... 

The structure of malonyl-a-aminopyridine (cf. 121) has been discussed by Snyder and Robinson/ who interpreted the infrared and ultraviolet spectra and the fact that it could be converted into a monochloro derivative (122, R = Cl) to indicate that the intra-molecularly hydrogen-bonded hydroxy form 122 (R = OH) was predominant. However, comparison of the basicities of the methoxy compound 122 (R = OMe), the mesomeric betaine 123 (R = Me), and the parent compound indicates that in aqueous solution the last exists mainly in the zwitterion form 123 (R = H), ... 

In contrast to the alkaloids mentioned so far, heterocyclic mesomeric betaines are defined as neutral conjugated molecules which can be represented only by dipolar structures in which both the negative and the positive charges are delocalized within the rr-electron system (38JCS824, 85T2239). The first heterocyclic mesomeric betaine was prepared unknowingly by Emil Fischer (1882LA316). The real structure remained unknown... 

This review demonstrates that representatives of all four major classes of heterocyclic mesomeric betaines were isolated from natural sources. The profound differences in the electronic structures of these distinct classes can be realized by a closer look at the canonical formulae, the frontier orbital profile, the isoconjugate relationships, physico-organic properties, and the... 

Leptopinine 82 possesses the 3,4-dihydro-isoquinolin-7-ol moiety without further conjugation and was isolated as yellow powder from Hypecoum leptocarpum after several extractions with hydrochloric acid so that this alkaloid was finally isolated as a chloride (Scheme 29). The formation of a mesomeric betaine from this dihydroisoquinoline derivative is unlikely because on addition of base no significant changes of the UV spectra were observed (99P339). A similar structure is Pycnarrhine Pycnarrhena longifolia), which was isolated as a hydroxide (81P323). 

Demethylvasconine (85) (9-methoxy-5-methyl-phenanthridin-8-olate) presented in Scheme 31 was found in Crinum kirkii (95P1291) (Amaryllidaceae). Although published as cation, no information about the anion of this alkaloid is given. Its relationship to other alkaloids of this class, however, makes a betainic structure more than likely and this is confirmed by a comparison of the NMR data of 85 with the cationic and betainic alkaloids presented in Table III. This betaine is isoconjugate with the 2-methylphenanthrene anion and thus defined the alkaloid as a member of class 1 (odd alternant hydrocarbon anions). Whereas substitution of the isoconjugate phenanthridinium moiety at the 1-position with an anionic fragment results in zwitterions (cf. Section III.D), the phenanthridinium-2-olate is a mesomeric betaine. 

Benzo[c]phenanthridine alkaloids are widespread in Papaveraceae, Fumariaceae, and Rutaceae. Fagaridine (118), the structure of which had to be revised, is a derivative of the unknown 5-methyl-benzo[c]phenan-thridine-8-olate (119) which is isoconjugate with the 2-methyl-chrysene anion (Scheme 43). Thus, Fagaridine is a member of class 1 of conjugated heterocyclic mesomeric betaines, which are isoconjugate with odd alternant hydrocarbon anions. 

As shown in Scheme 26, Papaverine hydrochloride yields a separable mixture of Protopapaverine (67) and the salt norpapaverinium chloride (68) when heated slightly beyond its melting point for several minutes. Molecule 67 can exist as conjugated mesomeric betaine 67A (7-hydroxy form) or as 2-substituted-2//-isoquinolin-6-one 67B (6-hydroxy form) (66TL1177). Similar structures were described as zwitterionic pentacyclic... 

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

No single structure can uniquely describe the bonding of mesomeric betaines and their true structure is a resonance hybrid of several dipolar structures. The possible dipolar structures for basic compounds of this class were discussed in CHEC-II(1996) <1996CHEC-II(8)747>. 

Reaction of dimethyl acetylenedicarboxylate (DMAD) with extremely unstable mesomeric betaine 96, generated in situ from 95, gives in 30-36% yield of a 1 2 adduct, the structure of which was originally determined as 97 <1978CL1093>. However, a more recent reinvestigation based on the H and 13C NMR spectroscopy shows that the actual product is pyrazolo[l,5- ]azepine 98, formed probably by mechanism shown in Scheme 6 <1995JCM338>. 

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... 

Baker,67 Ollis,68 Ramsden,69 and other authors70 defined mesoionic systems as five-membered rings that cannot be represented by normal covalent structures. Following Katritzky,71 they are now universally named systematically as mesomeric betaines. 

Pyrylium 3-oxides 54 (Scheme 25) should be considered as heteroaromatic six-membered mesomeric betaines. A diketo-oxepine 44 (Scheme 26) has two equivalent identically polarized limiting structures... 

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