The reaction of an aromatic heterocycle with a proton is called a protonation. One of articles about this theory is 《Derivatives of pyrrylazobenzenearsonic acids》. Authors are Muic, N.; Fles, D..The article about the compound:Ethyl 3,5-Dimethyl-2-pyrrolecarboxylatecas:2199-44-2,SMILESS:O=C(C1=C(C)C=C(C)N1)OCC).Recommanded Product: Ethyl 3,5-Dimethyl-2-pyrrolecarboxylate. Through the article, more information about this compound (cas:2199-44-2) is conveyed.
cf. C.A. 45, 9526a. The synthesis is essentially the same as previously described. 2,4-Dimethyl-3,5-carbethoxypyrrole (I) was prepared by the method of Knorr. I was saponified in 10% KOEt, and converted to 2,4-dimethyl-3-carbethoxy-5-pyrrolecarboxylic acid (II) by the method of Küster, et al. (C.A. 16, 3895). Decarboxylation of II by dry distillation gave 2,4-dimethyl-3-carbethoxypyrrole (III). I was also treated with concentrated H2SO4 by the method of Fischer and Walach (C.A. 20, 1620) to give 2,4-dimethyl-5-carbethoxy-3-pyrrolecarboxylic acid, which was decarboxylated by heating at atm. pressure to 2,4-dimethyl-5-carbethoxypyrrole (IV). Attempts to couple diazotized 4, 3-H2N(O2N)C6H3AsO3H2 and III were not successful; a resinous product, which could not be purified, was obtained, and III was isolated from the reaction mixture Attempts to couple a salt of diazotized 3,4-H2N(HO)C6H3AsO3H2 (V) with IV were also unsuccessful. p-H2NC6H4AsO3H2 (4.34 g.) in 50 cc. H2O containing 1.63 cc. concentrated H2SO4 was diazotized with 20 cc. N NaNO2 at 0-5° and the product filtered into 3.34 g. III in 200 cc. absolute EtOH. 4-(3-Carbethoxy-2,4-dimethyl-5-pyrrylazo)benzenearsonic acid (VI) precipitated as an orange-yellow powder. VI was filtered, rinsed with water, dissolved in aqueous NaOH, and the solution clarified with active C; acidification with dilute HCl gave 4.3 g. VI, orange-yellow microcrystals, decompose 210°, slightly soluble in water, somewhat more soluble in EtOH, nearly insoluble in C6H6 and ether, and soluble in dioxane; crystallization from dioxane gave well-formed needles. VI was precipitated from alk. solution with dilute acids. VI was stable in air under light. V (4.66 g.) in 70 cc. H2O containing 5.8 cc. concentrated H2SO4 was diazotized as above and the product filtered into 3.34 g. III in 200 cc. absolute EtOH. 2-(3-Carbethoxy-2,4-dimethyl-5-pyrrylazo)-1-phenol-4-arsonic acid (VII) precipitated, and addnl. amounts of VII were obtained on diluting with H2O. VII was then dissolved in N NaOH, the solution clarified with active C, added to 0.1 N HCl with constant stirring, and the precipitate was filtered, washed with H2O, dried, and recrystallized twice from dioxane to yield 7.2 g. VII, yellow needles, decompose 160°. VII was stable in air under light. (p-H2NC6H4)2As(:O)OH (1.46 g.) in 30 cc. H2O containing 3.7 cc. concentrated HCl was diazotized as above with 10 cc. N NaNO2 and the solution added dropwise at 5° or lower to 1.67 g. III in 70 cc. EtOH containing 5 g. NaOAc, previously dissolved in a small volume of H2O, to yield di-Et 5, 5′-[arsinobis(p-phenyleneazo)]bis[2,4-dimethyl-3-pyrrolecarboxylate] (VIII). VIII was filtered, washed with cold H2O, dried in vacuo, and recrystallized from dioxane and then from ether to yield 1 g. VIII, dark orange microcrystals, m. 151° (decomposition). VIII was soluble in EtOH, dioxane, and CHCl3. p-H2NC6H4AsO3H2 (2.17 g.) in 25 cc. H2O containing 0.81 cc. concentrated H2SO4 was diazotized with 10 cc. N NaNO2 and the product filtered into 1.67 g. IV in 200 cc. absolute EtOH; when the solution was clear 15 g. NaOAc in a small amount of H2O was added with cooling, and, after 1 hr., 4 l. H2O was added to precipitate 4-(5-carbethoxy-2,4-dimethyl-3-pyrrylazo)benzenearsonic acid (IX), yellow-orange powder. IX was twice dissolved in alkali and reprecipitated by dilute HCl, washed with water, and dried in vacuo to yield 1.2 g. IX, darkens 100°, m. 185° (decomposition), IX was soluble in EtOH and dioxane, and stable in air under light.
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Reference:
Thiazolidine – Wikipedia,
Thiazolidine – ScienceDirect.com