Thienyl carbonate

Esterification of carboxylic acids with alcohols, including bulky secondary ones, by equimolar di-2-thienyl carbonate (2-DTC) in the presence of a catalytic amount of 4-(dimethylamino)pyridine in toluene solvent at room temperature followed by addition of a catalytic amount of hafnium(IV) trifluoromethanesulfonate, Hf(OTf)4, afforded the corresponding esters in good to high yields. In step 1 (Scheme 1), interaction of the acid and 2-DTC (1) produces the thienyl ester (2) with evolution of CO2 and formation of 2(5H)-thiophenone (3). In step 2, the added Hf(OTf)4 forms with (2) an activated complex (4), alcoholysis of which yields the ester (5) and a further molecule of 2(5H)-thiophenone.1 The procedure was also effective for converting [Pg.48]

RCO2H, R OH, Di-2-thienyl carbonate, F, DMAP, 57-91% yield. This reagent is also suitable for macrolactonization. ... 

Di-2-thienyl carbonate 102, a new reagent for the esterification of carboxylic acids, has been developed and was prepared by treatment of 5//-thiophen-2-one 103 with triphosgene in the presence of a suitable base <04CL552>. 

Ester condensation under mild conditions was realized using 2-thienyl carbonate and then transesterification was carried out using hafnium catalyst as shown in Equation 70 [76]. Carboxylate (160) was converted into the corresponding thienyl ester (162) by the reaction with thienyl carbonate (161) in the presence of DMAP as a catalyst, followed by condensation with allyl alcohol with a catalytic amount of Hf(OTf)4 to produce allyl ester in high yield. Various carboxylates and alcohols are applicable for the reactions, including sterically hindered alcohols, such as i-Pr2CHOH, t-BuMeCHOH, and (-)-menthol. 

Esterification of carboxylic acids with alcohols, including bulky secondary substrates, was achieved by using an equimolar amount of di-2-thienyl carbonate (2-DTC) in the presence of a catalytic amount of 4-(dimethylamino)pyridine (DMAP). This was followed by the addition of a catalytic amount of Hf(OTf)4 to give the products in good to excellent yields (eq 9). 4... 

The 4-carboxylic acid in all instances is the more stable. Heated to 220°C 2-(a-thienyl)-4.5-thiazoledicarboxylic acid lost 1 mole of carbon dioxide to give the 4-monoacid,... 

The acid cleavage of the aryl— silicon bond (desilylation), which provides a measure of the reactivity of the aromatic carbon of the bond, has been applied to 2- and 3-thienyl trimethylsilane, It was found that the 2-isomer reacted only 43.5 times faster than the 3-isomer and 5000 times faster than the phenyl compound at 50,2°C in acetic acid containing aqueous sulfuric acid. The results so far are consistent with the relative reactivities of thiophene upon detritia-tion if a linear free-energy relationship between the substituent effect in detritiation and desilylation is assumed, as the p-methyl group activates about 240 (200-300) times in detritiation with aqueous sulfuric acid and about 18 times in desilylation. A direct experimental comparison of the difference between benzene and thiophene in detritiation has not been carried out, but it may be mentioned that even in 80.7% sulfuric acid, benzene is detritiated about 600 times slower than 2-tritiothiophene. The aforementioned consideration makes it probable that under similar conditions the ratio of the rates of detritiation of thiophene and benzene is larger than in the desilylation. A still larger difference in reactivity between the 2-position of thiophene and benzene has been found for acetoxymercuration which... 

On the other hand, the electron-attracting properties (—I and —M) of the 2-thienyl groups should also facilitate prototropic reactions, where the rate-determining step is the removal of a proton from the a-carbon and which thus is facilitated by electron-attracting sub-... 

It is thus obvious that the 2-thienyl group is both a better electron donator and electron acceptor than the phenyl group, facilitating both nucleophilic and prototropic reactions at the a-methylene carbon. 

The thenyl cyanides are of great importance for the preparation of thiophene derivatives. Because of the acidifying effects of both the thienyl and of the cyano groups, carbanions are easily obtained through the reaction with sodamide or sodium ethoxide, which can be alkylated with halides, carbethoxylated with ethyl carbonate, or acylated by Claisen condensation with ethyl... 

A mixture of 1.0 g of 6,6,9-trimethyl-9-azabicyclo[3.3.1 ] nonan-3/3-ol, methyl 0i,0i-di-(2-thienyD-glycollate and 30 mg of metallic sodium is heated at 80°C to 90°C for about 2 hours under reduced pressure. After cooling, ether is added to the reaction mixture. The mixture is extracted with 10% hydrochloric acid. The aqueous layer is alkalified with sodium carbonate and reextracted with ethyl acetate. The extract is washed with water, dried and concentrated to dryness. The residue thus obtained is treated with hydrogen chloride by conventional manner. 2.0 g of the 0i,0i-di-(2-thienyl)glycollate of 6,6,9-trimethyl-9-azabicyclo-(3.3.1 ] nonan-3/3-ol hydrochloride are obtained. Yield 83%. 

The 2-(2-pheriylvinyl) derivative 18 and the thienyl compound 20 cyclize exclusively at the alkene carbon, and at the thiophene ring, to give 3,4-diphenyl-l//-2-benzazepine (19) and 4-phenyl-6-//-thieno[3,2-e]-2-benzazepine (21), respectively.48 A mechanistic rationale for these results has been offered. This method has been extended to the synthesis of 7Z7-pyrido[3,4-t/]-, 7//-pyrido[2,3-t/]- and 77/-pyrido[4,3-r/]benzazepincs and to other thieno- and furo-fused 2-benzazepines.244... 

Similar coupling and iodination reactions are observed with thienyl iodide, as shown in Eq. 2.38 [35]. Thus, carbon—carbon bond formation occurs with the first molecule of thienyl iodide, and subsequent Cu/I exchange occurs with the second molecule. 

The amphetamine and mescaline analogs which can be obtained by replacing carbon atoms with O, N, or S atoms may also be active, but little work has been done on this. For the synthesis of thienyl analogs see JACS 64,477(1942). 

A richer structural variety is realized when the propargyl carbonates 90 are treated with cyclopropyl(cyano)thienyl cuprates [56] such as 141, allowing the preparation of allenes with up to four different substituents (Scheme 5.19) [57]. 

Reduction of 1-adamantanecarbonyl chloride at carbon or mercury leads to the quantitative formation of 1-adamantanecarboxaldehyde [80]. Mubarak [81] investigated the reduction of 2-thiophenecarbonyl chloride at both carbon and mercury the starting material appears to undergo a two-electron reduction to form an acyl anion, which leads to l,2-di(2-thienyl)ethene-l,2-diol di(2-thiophenecarboxylate) as the major product and to 2-thiophenecarboxaldehyde as the minor product. 

This approach has recently been extended to keto tautomers (Bassetti et al., 1988). Fourier-transform nmr spectroscopy is needed to record the signals of the small amounts of the keto isomers that are present. The carbonyl carbons of the keto form are to lower field of the corresponding atoms in the enol by ca. 10 ppm. The data were analysed into substituent factors relative to pentane-2,4-dione. Replacing the methyl groups of this compound with 2-thienyl, phenyl and t-butyl groups caused upfield shifts to the a-carbon of —3.14, —4.4 and —6.5 ppm, respectively, when the substituents were introduced at this site. 

Let us now direct our attention to the P=C bond in phosphaalkene ion-radicals. The literature contains data on two such anion-radicals in which a furan and a thiophene ring are bound to the carbon atom, and the 2,4,6-tri(tert-butyl)phenyl group is bound to the phosphorus atom. According to the ESR spectra of anion-radicals, an unpaired electron is delocalized on a n orbital built from the five-membered ring (furanyl or thienyl) and the P=C bond. The participation of the phosphaalkene moiety in this MO was estimated at about 60% and some moderate (but sufficient) transmission of the spin density occurs through the P=C bridge (Jouaiti et al. 1997). Scheme 1.6 depicts the structures under discussion. 

Oxidation of silyl enol ethers leading to carbon-carbon bond formation [85JCS(CC)420 87JCS(P1)559] finds an interesting application in the synthesis of furans. For example, l,4-di(3-thienyl)-l,4-butanedione (65), which... 

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