Carboxylic acid esters Decarboxylation

Chromone-5-carboxylic acid, 7-hydroxy-2-methyl-decarboxylation, 3, 711 Chromonecarboxylic acids decarboxylation, 3, 710 Chromone-2-carboxylic acids esters... 

Pyrrole-2-carboxylic acid esters have been prepared from ethyl chloroformate and pyrrolylmagnesium bromide1 2 or pyrrolyllithium,3 by hydrolysis and decarboxylation of dimethyl pyrrole-1,2-dicarboxylate followed by re-esterification of the 2-acid4 and by oxidation of pyrrole-2-carboxaldehyde followed by esterification with diazomethane.4... 

Almotriptan has also been synthesized via decarboxylation of the carboxylic acid intermediate 65, but a detailed preparation of 65 was not provided in the patent literature (Scheme 22)." The patent indicates that the carboxy indole 65 was prepared according to the method of Gonzalez.°° Thus, (2-oxo-tetrahydro-3-furanyl)-glyoxylic acid ethyl ester (62) was heated in aqueous H2SO4 to give 2-oxo-5-hydroxypentanoic acid in situ, which was treated with hydrazine 59 to produce hydrazone 63. Fischer cyclization of 63 using HCl gas in DMF gave the lactone 64, which was converted to carboxylic acid 65. Decarboxylation of 65 was catalyzed by cuprous oxide in quinoline at 190 °C to afford almotriptan (5)." ... 

Indole-2-carboxylic acid, 3-methyl-ethyl ester chlorination, 4, 215 Indolecarboxylic acids decarboxylation, 4, 286 esterification, 4, 287 Indole-2-carboxylic acids esters... 

Thermal cyclization of the isopropylidene ester of the malonates (70) was accompanied by decarboxylation at position 3, whereby 4-oxo-4H-pyrido-[l,2-fl]pyrimidines (74 R1 = H) were obtained.79,142,143 When the cyclization was performed in phosphoryl chloride-polyphosphoric acid and the reaction mixture was treated with alcohol, 4-oxo-4H-pyrido[l,2-a]pyri-midine-3-carboxylic acid esters were isolated.151 Treatment of the reaction mixture with water gave carboxylic acids. 

It is rather difficult to convert carboxylic acids to decarboxylative alcohols. However, treatment of O-acyl esters (2) in the presence of Sb(SPh)3 and molecular oxygen, followed by hydrolysis, generates the corresponding decarboxylative alcohols. Eq. 8.11 shows the preparation of a sex pheromone of the citrus mealybug from (+ )-ds-pinonic acid [30-32]. When 1802 instead of 1602, is used in this reaction, 180-alcohols can be obtained. 

Fig. 6.26. Acylation of alcohols with an acyl Meldrum s acid (A). The reaction product obtained after in situ decarboxylation is a /l-keto carboxylic acid ester D.

Saponification of the isoxazoline ring and subsequent decarboxylation of compound (243) afforded the 5-cyano-l,4-dihydropyridine-3-carboxylic acid ester (244) <89JOC5585>. If the acid (243) was first reesterified with ( — )-menthol, the resulting diastereomeric esters were separable. Saponification of the individual enantiomers followed by decarboxylation afforded a method to prepare 3-cyano-l,4-dihydropyridines (244) stereospecifically. 

Hydrolysis (especially with bromoacetic acid)28 of the dicarboxylic esters can be followed by decarboxylation and it is possible to remove one carboxyl group at a time to prepare the imidazole-4-carboxylic acid. The decarboxylation of imidazolecarboxylic acids has been discussed by Schipper and Day.2... 

A cyclopropyl to allyl radical rearrangement could also be induced in 20 % yield by the thermal decomposition (in carbon tetrachloride at 32 °C with sonication) of the A-hy-droxypyridine-2-thione ester of l-fluoro-2,2-diphenylcyclopropane-l-carboxylic acid (Barton decarboxylation reaction). ... 

Therefore, diethyl malonate is deprotonated but not ethyl acetate. Moreover, the ethoxide ion is strong enough to deprotonate the diethyl malonate quantitatively such that all the diethyl malonate is converted to the enolate ion. This avoids the possibility of any competing Claisen reaction since that reaction needs the presence of unaltered ester. Diethyl malonate can be converted quantitatively to its enolate with ethoxide ion, alkylated with an alkyl halide, treated with another equivalent of base, then alkylated with a second different alkyl halide (Fig.R). Subsequent hydrolysis and decarboxylation of the diethyl ester yields the carboxylic acid. The decarboxylation mechanism (Fig.S) is dependent on the presence of the other carbonyl group at the P-position. 

Since electrophilic substitution on pyrroles occurs so easily, it can be useful to block substitution with a removable substituent. This is usually done with an ester group. Hydrolysis of the ester (this is particularly easy with t-butyl esters—see Chapter 23) releases the carboxylic acid, which decarboxylates on heating. There is no doubt that the final electrophilic substitution must occur at C2. 

An alternative procedure for reductive decarboxylation without the use of trialkyltin hydrides as hydrogen atom donors has been developed Alkane carboxylic acid esters derived from AT-hydroxypyridine-2-thione decomposed to alkyl radical, which can readily accept a hydrogen atom from t-BuSH (equation 74) to give alkanes. This reaction can be conveniently performed as a one-pot experiment wherein the acid chloride of an alkane carboxylic acid, the sodium salt of thiohydroxamic acid, t-BuSH and 4-dimethyl-aminopyridine (DMAP) in benzene solution are heated to reflux. This procedure works well for COOH groups attached to primary and secondary carbon atoms. Instead of AT-hydroxypyridine-2-thione, one can use other thiohydroxamic acids, viz. iV-hydroxy-AT-methylthiobenzamide, 3-hydroxy-4-methylthiazole-2(3if)-thione (equation 75) and l-iV-hydroxy-3-AT-methylbenzoylenethiourea for decarboxylation reactions. 

Application of this technique to the identification of methyl esters of the organic acids obtained by the controlled oxidation of bituminous coal allowed the more volatile benzene carboxylic acid esters to be identified (Studier et al., 1978). These were esters of benzene tetracarboxylic acid, tere-phthalic acid, toluic acid, and benzoic acid. Decarboxylation of the total acid mixture was shown to afford benzene, toluene, Cj-benzenes (i.e., ethylbenzene or xylenes), Cj-benzenes, butylbenzenes, Cj-benzenes, Cybenzenes, naphthalene, methylnaphthalene, C2-naphthalene, biphenyl, methylbi-phenyl, C3-biphenyl, indane, methylindane, Cj-indane, phenanthrene, and fluorene. 

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