Homologation d'ester de Kowalski

L'homologation d'ester de Kowalski est une réaction chimique de type homologation pour les esters[1],[2], c'est-à-dire une réaction ayant pour but d'allonger la chaîne carbonée des esters d'un atome de carbone.

The Kowalski ester homologation
The Kowalski ester homologation

Elle a été proposée par Conrad J. Kowalski en 1985[1] comme alternative plus sûre à la réaction d'Arndt-Eistert.

Mise en œuvre

L'ester est mis à réagir avec le dibromométhane en milieu basique (tétraméthylpipéridure de lithium — LiTMP), puis avec l'acétate de lithium en présence de bis(triméthylsilyl)amidure de lithium (liHMDS). On fait ensuite réagir avec sec-butyllithium, puis le n-butyllithium et enfin le chlorure d'acyle dans l'éthanol.

Mécanisme réactionnel

Il a été proposé pour cette réaction le mécanisme suivant, mécanisme toutefois sujet à discussion :

Mécanisme proposé de l'homologation d'ester de Kowalski
Mécanisme proposé de l'homologation d'ester de Kowalski

Variantes

Une variante de ce procédé a été proposée en 2000 par Smith III, Kozmin, et Paonequi permet d'obtenir de meilleurs rendements[3].

En changeant le réactif dans la seconde étape, cette réaction permet de préparer également des éthers de silyle-ynol[4].

Notes et références

  1. a et b Kowalski, C. J.; Haque, M. S.; Fields, K. W. J. Am. Chem. Soc. 1985, 107, 1429. (DOI 10.1021/ja00291a063)
  2. Organic Syntheses, Coll. Vol. 9, p.426 (1998); Vol. 71, p.146 (1993). (Article)
  3. Smith III, A. B.; Kozmin, S. A.; Paone, D. V. J. Am. Chem. Soc. 2000, 122, 4984. (DOI 10.1021/ja000430p)
  4. (en) Smith, III, A. B., Kozmin, S. A. et Paone, D. B., « Total Synthesis of (-)-Cylindrocyclophane F », J. Am. Chem. Soc., American chemical society, vol. 121,‎ , p. 7423 (DOI 10.1021/ja991538b, lire en ligne)

Voir aussi

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