[2,3]-Wittig Sigmatropic Rearrangement of γ-Allyloxy-β-Enaminoesters

Abstract

The dianions derived from γ-allyloxy-β-enaminoesters undergo a [2,3]-Wittig sigmatropic rearrangement leading to γ-hydroxy-β-enaminoester derivatives, which can be subsequently lactonized to the corresponding 4-aminofuran-2(5H)-ones.

β-ketoesters - β-enaminoesters - lithiation - Wittig rearrangement - lactones

References

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Although the deprotonation of tertiary β-enaminoesters at the γ-position (including N,N-disubstituted 4-aminofuran-2-(5H)-ones)⁴ is well-documented, the double metalation of primary or secondary ones has not been reported to our knowledge. See:

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9

Due to their moderate stability on silica gel, the β-enamino-esters 5a-c and 8 were conveniently engaged in the rearrangement step without further purification and therefore an excess of n-BuLi (4 equiv) was routinely used in order to ensure an efficient metalation process of these crude compounds at -78 °C. The nucleophilic addition of
n-BuLi to the ester carbonyl group of lithiated β-enamino-esters, and/or N,O-dilithiated β-enaminoesters arising from the [2,3]-sigmatropic rearrangement, has never been observed as a side reaction. The fact that these species may be considered as vinylogous lithiated carbamates probably accounts for their low reactivity towards n-BuLi.

10

Due to the complexity of the NMR spectra of the mixture of 6c and 6′c (presence of Z/E isomers in equilibrium for each diastereomer), the diastereomeric ratio (6c/6′c) was evaluated by GC-MS after calibration with authentic samples prepared by condensation of (R)-α-methyl-benzylamine with the γ-trimethylsilyloxy-β-ketoester 10 resulting from the [2,3]-Wittig rearrangement of 1 and subsequent silylation with TMSCl (Scheme [8] ). [6]

11

The diastereomeric ratio (7c/7′c)was determined by ¹H NMR. These two diastereomers were separated and hydrolyzed to the corresponding enantiomeric tetronic acids (+)-11 and (-)-11 respectively. The (5R) absolute configuration of tetronic acid (+)-11 was assigned after hydrogenation, by correlation with the known optically pure (5R)-5-propyltetronic acid (+)-12 (Scheme [9] ). [1a]

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