Issue in Honor of Prof. M. Makosza ARKIVOC 2004 (iii) 74-84
Vicarious nucleophilic substitution of hydrogen vs. ANRORC-type ring transformation in reactions of 1,2,4-triazines with a-halocarbanions. Novel route to functionalized pyrazoles1
Andrzej Rykowski,a* Ewa Wolinska,a Danuta Branowska,a and Henk C. Van der Plasb
a Department of Chemistry, University of Podlasie, 08-110 Siedlce, Poland E-mail:
[email protected] b Laboratory of Organic Chemistry, Wageningen University, Dreijenplein 8, 6703 HB Wageningen, The Netherlands E-mail:
[email protected]
Dedicated to Professor Mieczyslaw Makosza on the occasion of his 70th birthday (received 06 Oct 03; accepted 11 Dec 03; published on the web 15 Dec 03)
Abstract
A novel route to pyrazole derivatives bearing sulfonyl, aminosulfonyl, alkoxy- and aryloxysulfonyl groups at C-3 by ring transformation of 3-chloro-6-aryl-1,2,4-triazines 1a-c with a-halocarbanions 2a-h is described.
Keywords: Ring transformation, 1,2,4-triazines, a-halocarbanions, pyrazoles
Introduction
Ring transformations of functionalized 1,2,4-triazines are valuable entries in the synthesis of several other heterocyclic systems. The triazines are frequently used as azadiene equivalents in the Diels-Alder cycloaddition/retro-cycloaddition reactions to form pyridine or pyrimidine derivatives.2 They also may undergo ring interconversions into five and six membered azaheterocycles when reacted with nucleophilic reagents.3 These transformations usually take place via several steps: (1) Addition of Nucleophile; (2) Ring Opening; and (3) Ring Closure of the resulting open-chain intermediate, [ANRORC mechanism]. An ANRORC – type mechanism has recently been observed when 3-X-1,2,4-triazines (X=nucleofugal group) react with activated methylene compounds, featuring the presence of cyano functionality at the carbanionic center. Functionalized 3-aminopyridazine derivatives are obtained.4 Such ANRORC – type ring transformations, initiated by the addition of the carbanionic center at C-5 may provide a new entry to the preparation of functionalized heterocycles, which could otherwise hardly be prepared by other methods.
ISSN 1424-6376 Page 74 ©ARKAT USA, Inc
