Alkoxycarbonyl‐ and Carboxylate‐Group Migrations in the Benzilic Acid Rearrangement of Ethyl Cyclopropane‐2, 3‐dioxopropionate

Abstract

SummaryIn alkaline medium C3H5-CO-C (OH),-COOEt (lb) is transformed into C3H5-C (OH)(COOH), (2a). Labelling experiments show that the cyclopropyl group is not shifted, but only ROOC and/or -0,C groups. GC./MS. and NMR. analysis after incomplete reaction show that both ROOC-and -02C-groups migrate; at higher pH (ca. 14) the ester group rearrangement seems to be more important than at pH ca. 9-10. a, 8-Dioxocarboxylic esters form rather stable hydrates in the presence of water; these can be attacked by sufficiently basic nucleophiles, e.g. OH-, at three different positions (Scheme): A: at C(1), leading to hydrolysis of the ester group; B: at C(2), leading to hydrate-anions which undergo a benzilic acid type rearrangement via migration of a ROOC or -0,C group; C: at C(3), forming a bishydrate-anion which can be cleaved in a retro-aldol-type reaction. Path C prevails if the substituent R at C(3) is electron-attracting, e.g. aryl of CF3, favouring the hydration of the P-carbonyl group, whereas paths A and B are used when the hydration of the B-carbonyl group is diminished by electron donation (R= alkyl) or by steric effects (R= mesityl) [2]. The balance between paths A and B depends upon pH: at high pH (ca. 14) the hydrate at C (2) is transformed into its anion triggering the migration of COOR' (B), whereas at pH ca. 10 the ester group is hydrolysed first, followed by migration of the -0OC group [3].The cyclopropyl group is considerably electron.-releasing without important steric constraint, but is at the same time a good migrating group in the benzilic acid rearrangement [4]; so we were interested to see how it influences the balance between reaction paths A, B and C.