Pyranopyrazoles. II. Synthesis and reactions of 1H,6H‐pyrano[2,3‐c]pyrazol‐6‐ones

Abstract

Various 1H,6H‐pyrano[2,3‐c]pyrazol‐6‐ones (III‐XXIII) were obtained from β‐keto esters and 1H‐pyrazol‐5‐ones or hydrazines. Nitrations, chlorinations and brominations of these pyranopyrazoles were also carried out giving the corresponding derivatives (XXIV‐LXIV). The pyrone ring is the more reactive one in these reactions and the preferred position of attack is the 5‐position. The substitution products are formed by the addition‐elimination route.

Condensation of Pyrazol-5-ones Unsubstituted in the Position 1 with β-Keto Esters. Synthesis of Pyrano[2,3-c]pyrazol-6-ones

“…It has different characteristics and its structure has been rigorously proved [5]. The problem of the choice between structures 3 and 4 was resolved by 1 H NMR spectroscopic analysis, the condensation product actually corresponding to structure 3, which is in agreement with data in [3,4]. For a final proof of this question we attempted to use 4-ethyl-3-methyland 4-isopropyl-3-methylpyrazol-5-ones (6 and 7) in this condensation.…”

“…Later, the preparation and some reactions (nitration, chlorination, and bromination) of pyrano [2,3-c]pyrazol-6-ones were reported in [4] in which the synthesis of the latter was also carried out using the Wolf method. The authors based this on the use of various pyrazol-5-ones with different substituents in position 1 and only aceto-and benzoylacetic esters as the β-keto esters.…”

Synthesis and chemistry of N‐arylated pyrano[2,3‐c]pyrazoles

“…Pyrano[2,3-c]-pyrazol-6-one 5 was prepared from 4 [5] by the method of Khan and coworkers [2]. Due to the greater steric demand of the phenyl group at C3 and its electronwithdrawing nature, N-arylation proceeded in very low yield (10%) with 80% of the starting material recovered.…”

“…We became interested in this fused ring system for potential agrichemical use as part of our discovery program. Although there are numerous examples of substitution at N1 of 1 [2], which are prepared using substituted hydrazines, there are few reports which discuss alkyl substitution at N2 [1a,1c,3] and the literature is silent on the synthesis and characterization of N2-arylated pyrano-[2,3-c]pyrazol-6-ones 2 which were of particular interest to us. Two studies featuring the N-alkylation of 1 indicate a preference for incorporation of the alkyl group at N2, but reaction is by no means selective with ratios of N2:N1 alkylation approximately 2:1 [1a,1c].…”

Ring-closing metathesis as a key step to construct 2,6-dihydropyrano[2,3-c]pyrazole ring system

“…1) is very limited, and their chemistry and biological properties remain largely unexplored. Derivatives of pyrano [2,3-c]pyrazol-6(1H)-one 9 ( Fig. 1, F) are structurally similar and are known as analgesic, antiinflammatory 10 and antiviral agents.…”