Iodine-Catalysed Bohlmann-Rahtz Cyclodehydration Reactions

Abstract

The cyclodehydration of Bohlmann-Rahtz aminodienones is catalysed by iodine in ethanol at room temperature to give 2,3,6-trisubstituted pyridines in excellent yield, with total regiocontrol and without the need for chromatographic purification.

The Bohlmann-Rahtz Pyridine Synthesis: From Discovery to Applications

“…These reactions were then repeated in the presence of sodium thiosulfate (Scheme 15, Table 11) and the yields were compared. 15 Clearly from these experiments, the NIS-mediated heteroannelation was largely unaffected by the presence of thiosulfate, whereas the iodine-mediated cyclization failed, giving a 91% return of starting material. Thus it was concluded that the NIS itself is capable of catalyzing the Bohlmann-Rahtz cyclodehydration reaction as a Lewis acid.…”

Regioselective Microwave-Assisted Synthesis of Substituted Pyrazoles from Ethynyl Ketones

“…Pyridines, 1 pyrazoles, 2-5 isoxazoles, 2,5 triazoles 5 and pyrimidines 2,4,6 have all been prepared from these intermediates, with applications in the synthesis of non-proteinogenic amino acids, 6 complex thiopeptide natural products, 7 terpyridine a-helix mimetics, 8 non-steroidal anti-inflammatory agents, 9 and heterocyclic combinatorial libraries. [10][11][12] Following our own studies on the development of more efficient and expedient procedures for the synthesis of pyridines, [13][14][15][16][17][18][19][20][21] pyrimidines, 12,22 and pyrido [2,3-d]pyrimidines 14,23,24 from ethynyl ketones, we investigated methods for the preparation of pyrazoles from these versatile precursors. Although the cyclocondensation of hydrazine and ethynyl ketones proceeds efficiently under well-established conditions, 2 the regioselectivity of reactions involving substituted hydrazines can be poor [2][3][4][5] and is often contradictory in the literature.…”

Synthetic Use of Molecular Iodine for Organic Synthesis

“…74 Cyclodehydration of aminodienones 198 is catalyzed by iodine in ethanol to give 2,3,6-trisubstituted pyridines 199 in high yields (Scheme 94). 75 Treatment of 3-(amino)methyleneindoline-2-thione (200) with iodine in pyridine generates brassilexin 201 (Scheme 95). 76 Pyridine reacts with 2-alkyl-1,4-naphthoquinones 202 in the presence of iodine to afford naphtho [2,3-b]indolizine-6,11-diones derivatives 203 (Scheme 96).…”