Zur Kenntnis der Umlagerung 3.3‐disubstituierter Pyrazolenine

Abstract

Bei der durekatalysierten, alkalischen und thermischen Umlagerung 3.3-disubstituierter Pyrazolenine in Pyrazole handelt es sichsoweit die Wanderung desSubstituenten an das C-Atom 4 erfolgt -um cine Sextettumlagerung. Dies folgt aus dem Vergleich der Wanderungsgeschwindigkeit verschiedener Substituenten.Die i m Jahre 1943 von JAN VAN ALP HEN^) beschriebene Pyrazolenin-Pyrazol-Umlagerung

Reactions of methoxyvinyllithium. Synthesis and rearrangement of 4-hydroxyisopyrazoles

“…These results prove that the stereochemistry of the thermal reaction [1,5] is identical with t h a t of the acid-catalyzed process [1,2]. If the reasonable assumption is made that this latter process is of t h e general class of [1,2] rearrangements known…”

“…A similar [1,5]-sigmatropic shift is reasonable for the purely thermal rearrangement of the 4-hydroxyisopyrazoles 3, which Scheme I followed by a tautomerization provides the 2-pyrazolin-4-ones, Scheme 11. In order to examine t h e stereochemistry involved Scheme I1 in this [1,5] shift we required a simple degradation of the rearrangement products and t o this end examined some possible routes. An attempted ring cleavage reaction on t h e N-chloro derivative 8, generated by reaction with tert-butyl hypochlorite (-78 "C), gave, by action of ethanolic sodium ethoxide, the ether 9, presumably by way of t h e dehydropyrazole 10.…”

“…This same reductive dealkylation was observed in the sec-butyl substituted analogue of pyrazolone 11. Eventually we were successful in establishing a stereochemical cycle which related the stereochemistry of the purely thermal [1,5]-sigmatropic shift t o a [1,2] shift of t h e type characteristic of alkyl migration t o an electron-deficient terminus. This is described in Scheme 111.…”

“…17, 19762875 to proceed with retention of the migrating groups then i t follows that the thermal path [1,5] 14 to 15 also proceeds with retention. Furthermore, we demonstrated the essential thermal nature of this [1,5] rearrangement, 14 to 15, by observing n o change in t h e rate of this conversion (NMR) in pyridine upon addition of trifluoroacetic acid. An acid-catalyzed process was thereby eliminated.…”

“…Recently we described the convenient acyl anion equivalent methoxyvinyllithium (MVL) and some of its reacti0ns.l-3 I n the course of a study of the reactions of MVL with esters we hydrolyzed the initial adducts 1 t o t h e hydroxy diones, as 2,l and then treated these substances with hydrazine in alcohol R, P H R, P H Me0 * OMe which could also be obtained by simply heating 5.6 These later authors6 described this reaction as a thermal [1,5] sigmatropic change, which in the suprafacial mode should proceed with retention of configuration of t h e migrating group,7 Scheme I. A similar [1,5]-sigmatropic shift is reasonable for the purely thermal rearrangement of the 4-hydroxyisopyrazoles 3, which Scheme I followed by a tautomerization provides the 2-pyrazolin-4-ones, Scheme 11.…”