Vinylogous carboxylic acids

The Claisen-type condensation reaction of cyclic vinylogous carboxylic acid triflates with lithium enolates and their analogues has provided acyclic alkynes bearing a 1,3-diketone-type moiety.19 The reaction mechanism has been proposed to proceed via a 1,2-addition of the enolate to the vinylogous acyl triflate, followed by fragmentation of the aldolate intermediate (Scheme 2). 

Vogel and coworkers have described the synthesis of anti-1,6 7,12-/ wmethano[ 14]annulene 44 for studies of its -electron structure.32 Incorporation of AIBN into decarboxylative bromination of vinylogous carboxylic acid 45 via Barton esters increased the efficiency of this reaction, Scheme 16b. Harvey... 

Figure 1.19 The insertion of a carbon-carbon double bond between the OH and the C=0 of the carboxylic acid creates the vinylogous carboxylic acid on the right. ...

Similar resonance forms in a vinylogous carboxylic acid also account for an acidic hydrogen. ... 

Vitamin C is another enol-based redox system like the hydroquinones and tyrosine, but it has no aromatic character. The enediol component is stabilized by a conjugated lactone group. The oxidation (Ej = +58 mV) occurs in two one-electron steps, but the ascorbate radical is not as stable as the semiquinone radical (Bielski, and Richter, 1977). The anion radical disproportionates via an initial dimerization in a similar way as semiquinone radicals (Bielski et al., 1981 Sawyer, et al., 1982). The lifetime of the radical is in the order of microseconds. The acidity of ascorbic acid (pk = 4.2) stems from the OH group in the P position to the lactone carbonyl group. It corresponds to the OH group of a vinylogous carboxylic acid (Scheme 7.2.11). Its UV maximum occurs at 260 nm (e = 1 x 1(T). 

Other acidic functional groups will also undergo reaction with diazomethane. Thus phosphonic acids (eq 5) and phenols (eq 6) are methylated in high yields, as are hydroxytropolones (eq 7) and vinylogous carboxylic acids (eq 8). The origin of the selectivity in eq 6 is due to the greater acidity of the A-ring phenol. 

Vinylogs (Vinylogous) s. azolides, vinylogous carboxylic acid amides, -Dieckmann cyclization, -l,2-dithiole-3-thiones, -urethans, -... 

The first discussions of tropolones made no mention of possible contributions of dipolar forms such as (XVI) involving tropylium ion character in the seven-membered ring. It was however pointed out that tropolone can be regarded as an extended or vinylogous carboxylic acid (XVII). ... 

The acidity of tropolones derives from their being extended or vinylogous carboxylic acids, as mentioned earlier in this chapter, and other facets of the chemistry of the hydroxyl group resemble carboxylic acids. Thus conversion of a tropolone into its alkyl ether corresponds to an esterification reaction and it may be brought about by means of the usual esterification reagents, e.g. with alcohols and mineral acid [278], alkyl sulphates [279], or diazomethane [245,274, etc.] or by treating a metal salt of tropolone with an alkyl halide [245,280]. (9-Alkylation of unsymmetric-ally substitut ed tropolones leads to two isomeric alkoxytropolones ... 

The oxicams comprise a class of vinylogous carboxylic acids, which were first launched in the 1990s. [183] The basic structure is a 4-hydroxy-2-methyl-l,l-dioxo-5,6-dihydrothiazine-3-carboxamide. Most ofthese substances show along duration of action and are used in rheumatism therapy. 

Reactions with vinylogous carboxylic acid amides Chloral condensation... 

Fluoroborates Vinylogous amidinium salts from vinylogous carboxylic acid amides and alkoxyaminomethinium salts... 

Enamines from thioimino-esters. - Vinylogous carboxylic acid amides. Startg. thioimino-ester dissolved in triethyl phosphite, and warmed 20 hrs. at 60° under Ng -> product. 

Syntheses with vinylogous carboxylic acid chlorides... 

Synthesis of a,/ -ethyleneketones from vinylogous carboxylic acid amides... 

Vinylogous carboxylic acid halides s. 17, 806 Condensations with l,3-dioxan-4,6-diones Isopropylidene malonate s. 17, 807 Modified azlactone synthesis s. 18, 804... 

Vinylogous a-diazoketones from vinylogous carboxylic acid chlorides— 4-Acyl-5-diazomelhylisoxazoles, condensed isoxazoles s. 17, 854... 

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