Microwave-assisted cyclizations promoted by polyphosphoric acid esters: a general method for 1-aryl-2-iminoazacycloalkanes

Abstract

SummaryThe first general procedure for the synthesis of 5 to 7-membered 1-aryl-2-iminoazacycloalkanes is presented, by microwave-assisted ring closure of ω-arylaminonitriles promoted by polyphosphoric acid (PPA) esters. 1-Aryl-2-iminopyrrolidines were easily prepared from the acyclic precursors employing a chloroformic solution of ethyl polyphosphate (PPE). The use of trimethylsilyl polyphosphate (PPSE) in solvent-free conditions allowed for the synthesis of 1-aryl-2-iminopiperidines and hitherto unreported 1-aryl-2-iminoazepanes. The cyclization reaction involves good to high yields and short reaction times, and represents a novel application of PPA esters in heterocyclic synthesis.

Microwave-assisted synthesis of 2-substituted 4,5,6,7-tetrahydro-1,3-thiazepines from 4-aminobutanol

“…Instead of the target compounds, the reactions led to the corresponding N-acylpyrrolidines, as disclosed by Hoogenboom et al [55]. These results were intriguing, since the synthesis of some seven-and even eightmembered 1,3-diheterocycles had already been achieved by our group using the PPA ester/MW system [48,50,52,53].…”

“…As part of an ongoing program on the applicability of PPE and PPSE to the synthesis of nitrogen-containing heterocycles [48][49][50][51][52][53][54], we developed a general method for the preparation of fiveand six-membered cyclic iminoethers (2-oxazolines and 5,6dihydro-4H-1,3-oxazines, respectively), by MW-assisted ring closure of amido alcohols promoted by PPA esters [54]. PPE and PPSE were then tested for the synthesis of the more challenging seven-membered heterocycles, namely 4,5,6,7-tetrahydro-1,3-oxazepines.…”

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“…Moreover, the microwave assisted synthesis of 2-amino-4-arylthiazoles in aqueous medium has been recently developed 11 . Therefore, it is appealing to develop the corresponding strategy for the green synthesis of the 4-Aryl-2-hydra-zinothiazole nucleus of the proposed research [12][13][14][15] . Many attempts were carried out to prepare 1-(4-mtolylthiazol-2-yl)hydrazine (B) where R = CH 3 for derivatives prepared, the first attempt was carried out by the conventional method under reflux for 2hrs., by refluxing thiocarbohydrazide with, 2bromo-1-p-tolylethanone in glacial acetic acid 16,17,18,19 .…”

“…Abdellattif MH, Hussien MA and Alzahrani E: New approaches of 4-aryl-2-hydrazinothiazole derivatives synthesis, molecular docking, and biological evaluations. Int J Pharm Sci & Res 2018; 9(12): 5060-78. doi: 10.13040/IJPSR.0975-8232.9 (12).5060-78.…”

Syntheses of 3,4- and 1,4-dihydroquinazolines from 2-aminobenzylamine

“…We had previously reported the MW-assisted synthesis of 1,4,5,6-tetrahydropyrimidines and their homologues by cyclodehydration reactions promoted by polyphosphoric acid (PPA) esters PPE and PPSE [ 58 – 60 ]. This strategy was successfully applied to the synthesis of 1-aryl-2-imino-1-azacycloalkanes [ 61 ], 2-oxazolines, 5,6-dihydro-4 H -1,3-oxazines and 4,5,6,7-tetrahydro-1,3-oxazepines [ 62 ].…”