A. A. Shatrova

Stereochemical behavior of 77Se‐1H spin‐spin coupling constants in pyrazolyl‐1,3‐diselenanes and 1,2‐diselenolane

Conformational study of five derivatives of 2‐(pyrazol‐4‐yl)‐1,3‐diselenane together with related 1,2‐diselenolane in respect to the stereochemical trends of geminal and vicinal 77Se‐1H spin‐spin coupling constants has been carried out by means of high‐level theoretical calculations in combination with experiment. The marked dihedral angle dependences for both types of couplings accounted for the lone pair effect in the case of geminal coupling constants and the Karplus‐type relationship for vicinal couplings have been established, which is of major importance for the stereochemical analysis of saturated selenium containing heterocycles. Copyright

Microwave-assisted synthesis of 2,5-diarylthiazolo[5,4-d]thiazoles from benzaldehydes and dithiooxamide

One-pot condensation of dithiooxamide with aromatic aldehydes and subsequent oxidation of intermediate 2,5-dihydro[1,3]thiazolo[5,4-d][1,3]thiazoles with selenium dioxide afforded 2,6-diaryl[1,3]thiazolo-[5,4-d][1,3]thiazoles which were characterized by 1H and 13C NMR, IR, and X-ray diffraction data.

Synthesis and properties of pyrazole carbaldehyde bis(2-hydroxyethyl)-dithioacetal hydrochlorides*

A simple method has been developed for the synthesis of water-soluble pyrazole derivatives, namely 4-[bis(2-hydroxyethylsulfanyl)methyl]pyrazoles hydrochlorides, by the reaction of a series of pyrazole carbaldehydes with 2-mercaptoethanol in the presence of trimethylchlorosilane. When treated with aqueous ammonia solution the pyrazole-4-carbaldehydes bis(2-hydroxyethyl)dithioacetal hydro-chlorides are converted to the 4-[bis(2-hydroxyethylsulfanyl)methyl]pyrazole free bases.

Synthesis of the first representative of 2-pyrazolyl-1,3-diselenanes, 3,5-dimethyl-1-(prop-2-en-1-yl)-4-(1,3-diselenan-2-yl)-1H-pyrazole

Selenoacetals are used as reagents in organic synthesis, and they play a significant role in the development of organoselenium chemistry [1–4]. Up to now, a few selenoacetals derived from cyclic carbaldehydes have been reported [1–10]. The known methods for the preparation of selenoacetals are multistep and laborious, and their scope is restricted by accessibility and stability of initial selenols. For example, the synthesis of 1,3-diselenanes has not received wide application due to difficult preparation and extreme instability of propane-1,3-diselenol and its substituted derivatives. Propane-1,3-diselenol reacts with aliphatic aldehydes in strongly acidic medium [8, 9], and only reactions of propane-1,3-diselenol and its substituted derivatives with benzaldehyde and some aliphatic aldehydes occur under milder conditions, in the presence of ZnCl2 [1–3, 10]. Prior to our studies hetaryl-1,3-diselenanes were unknown. We previously synthesized 2-(1,3-diselenan2-yl)thiophenes in 12–57% yield from propane-1,3-diselenol and thiophenecarbaldehydes [11]. Even under mild conditions (–3 to –5°C, Me3SiCl as catalyst) these reactions were accompanied by oligomerization. In the present communication we report for the first time on the reaction of 1-allyl-3,5-dimethyl-1H-pyrazole-4-carbaldehyde (I) with propane-1,3-diselenol (II) in the presence of Me3SiCl. The reaction direction was not obvious. For example, pyrazolecarbaldehydes reacted with 2-sulfanylethanol in the presence of Me3SiCl to give mainly open-chain bis(2-hydroxyethyl) dithioacetals even at an equimolar reactant ratio [12], in contrast to analogous reactions of aromatic aldehydes [13]. We have found that the reaction of aldehyde I with propane-1,3-diselenol (II) in Me3SiCl at room temperature yields only 1-allyl-4-(1,3-diselenan-2-yl)-3,5dimethyl-1H-pyrazole which was isolated as hydrochloride III. Neither open-chain bis(3-selanylpropyl) diselenoacetals nor addition products of diselenol II at the allyl group of pyrazolecarbaldehyde I were detected. Owing to protonation of the N atom, signals from methyl protons in the H NMR spectrum of hydrochloride III are broadened and displaced downfield relative to the corresponding signals of free base IV. Furthermore, the 3-Me signal appears in a weaker field relative to the 5-Me signal. Protonation of the N atom makes its resonance more upfield by 80–90 ppm. Hydrochloride III was converted into free base IV during chromatography on silica gel. According to the H NMR data, the six-membered 1,3-diselenane ring adopts a chair conformation with equatorial orientation of the pyrazolyl substituent. Propane-1,3-diselenol (II) was prepared by reductive cleavage of the Se–Se bond in poly(trimethylene diselenide) in the system N2H4 · H2O–KOH [14]. 4-(1,3-Diselenan-2-yl)-3,5-dimethyl-1-(prop-2en-1-yl)-1H-pyrazole hydrochloride (III). Propane1,3-diselenol (II), 0.404 g (2 mmol), was added dropwise under stirring at room temperature to a solution of 0.328 g (2 mmol) of pyrazolecarbaldehyde I in 1.304 g (12 mmol) of Me3SiCl. The reaction was complete in 5 min and was accompanied by heat evolution. The reaction mixture divided into two layers. The viscous material was separated from the liquid part by decanting, washed with hexane, and dried under reduced ISSN 1070-4280, Russian Journal of Organic Chemistry, 2012, Vol. 48, No. 9, pp. 1259–1260.

Synthesis of functionalized pyrazoles from pyrazolecarbaldehydes and amino alcohols

Abstract1,3,5-Trialkyl-1H-pyrazole-4-carbaldehydes reacted with amino alcohols at ambient temperature in the absence of a catalyst to give the corresponding Schiff bases which failed to undergo cyclization to 2-(pyrazol-4-yl)-1,3-oxazolidines on heating in the presence of dehydrating agents.

Quantum-chemical study of mechanisms of reaction between 1,3,5-trimethyl-1H-pyrazole-4-carbaldehyde with 2-mercaptoethanol

By quantum-chemical method MP2/6-311+G**//B3LYP/6-31+G** competing reaction channels were studied of the formation of pyrazolyl-1,3-oxathiolane and/or bis(2-hydroxyethyl)dithioacetal from 1,3,5-trimethyl-1H-pyrazole-4-carbaldehyde and 2-mercaptoethanol. The kinetically controlled formation of bis(2-hydroxyethyl) dithioacetal is governed by the concentration in the reaction mixture of 2-mercaptoethanol dimers. The reaction by stages of 1,3,5-trimethyl-1H-pyrazole-4-carbaldehyde with the molecules of mercaptoethanol results in the prevailing formation of 2-(pyrazol-4-yl)-1,3-oxathiolane structures.

Fragmentation of Pyrazolecarbaldehyde Thio- and Dithioacetals under Electron Impact and Chemical Ionization

The mass spectra of 1-substituted 3,5-dimethyl-1H-pyrazole-4-carbaldehyde bis(2-hydroxyethyl) dithioacetals and thioacetals were studied for the first time. The main fragmentation pathways of their molecular ions generated under electron impact and chemical ionization were similar. Primary decomposition of the molecular ions of bis(2-hydroxyethyl) dithioacetals involves elimination of 2-sulfanylethanol molecule with formation of the corresponding 1,3-oxathiolane radical cation. Fragmentation of the molecular ions [M]+ · and [M + H]+ derived from 2-(3,5-dimethyl-1H-pyrazol-4-yl)-1,4,6-oxadithiocanes includes cleavage of the eight-membered heteroring and elimination of C4H9OS ·. Substituents in the heteroring of pyrazolecarbaldehydes inhibit decomposition processes related to the aldehyde group.

First example of the synthesis of bis(2-hydroxyethyl)dithioacetals from nitrobenzaldehydes and mercaptoethanol in the presence of methylchlorosilanes

Aldehydes are known to readily convert into the corresponding 1,3-oxathiolanes and 1,3-oxathianes reacting with 1,2and 1,3-mercaptoalkanols in the presence of acid catalysts: scandium trifl ate [1], boron trifl uoride [2], p-toluenesulfonic acid [3], vanadium hydrogen sulfate [4], polyphosphoric acid on silica gel [5], etc. [6–8]. We formerly obtained open-chain dithioand diselenoacetals of thiophenecarbaldehydes with the use of trimethylchlorosilane [9–11]. In this study we discovered an uncommon formation of bis(2-hydroxyethyl)dithioacetals in the reaction of nitrobenzaldehydes with 2 equiv of 2-mercaptoethanol under the action of trimethylchlorosilane Me3SiCl. The process was carried out at room temperature by stirring the reagents in the 5–6-fold molar excess of Me3SiCl.

Microwave Activation of Reaction between 1,3,5-Trisubstituted Pyrazole-4-carbaldehydes and Sterically Hindered Aminoalcohols

Structure of products of reaction between 1,3,5-trisubstituted pyrazole-4-carbaldehydes and 2-aminoalkan-1-oles is determined by the structure of the alkyl chain of aminoalcohol. The reaction with 2-aminobutan-1-ol proceeds for 2 h in conditions of microwave activation at 150°C with the formation of 2-hydroxyalkylimines of pyrazole-4-carbaldehyde. At reaction of 1,3,5-trisubstituted pyrazole-4-carbaldehydes with 2-amino-2-methylpropan-1-ol mixtures form of the corresponding 2-hydroxyalkylimines and pyrazol-4-yl-1,3-oxazolidines in ratios 66 : 33–80 : 20. Heating of 2-hydroxyalkylimines of pyrazole-4-carbaldehyde in the presence of dehydrating agents, in particular, trimethylchlorosilane, does not result in 1,3-oxazolidines.

Microwave synthesis of new azolyl-substituted thiazolo[5,4- d ]thiazoles

One-pot condensation of dithiooxamide with 4,5-dichloro-1,2-thiazole-3-carbaldehyde and 5-phenyl-1,2-oxazole-3-carbaldehyde, followed by oxidation of intermediate 2,5-dihydro[1,3]thiazolo[5,4-d]-[1,3]thiazoles with selenium dioxide, afforded previously unknown 2,5-bis(4,5-dichloro-1,2-thiazol-3-yl)- and 2,5-bis(5-phenyl-1,2-oxazol-3-yl)[1,3]thiazolo[5,4-d][1,3]thiazoles as first representative of bis-isothiazolyl- and bis-isoxazolyl thiazolothiazoles.