Abstract
The optically pure tricarbonyl(η6-2-substituted benzaldehydes)chromium are used as chiral auxiliaries in the condensation with a series of doubly lithiated carboxylic acids. The intramolecular ring closure of the obtained Cr(CO)3 complexed β-hydroxyacids affords the optically pure β-lactones in satisfactory yield.
Highlights
As a part of our studies on the application of chiral tricarbonyl(arene)chromium derivatives in asymmetric synthesis of products with potential biological activity [4]-[7], we were interested in the application of these organometallic substrates for the synthesis of some optically pure 2-oxetanones
In this paper we report the results on the stereoselective synthesis of some Cr(CO)3 complexed β-hydroxyacids 3 and their cyclization to new Cr(CO)3 complexed βlactones 4 using both the racemic and the optically pure tricarbonyl(2-methoxybenzaldeyde) [13] or (2chlorobenzaldehyde) chromium [14] 1 and the dianion of the appropriate acid 2 (Scheme)
We have repeated the reaction in the usual experimental conditions starting first from the uncomplexed 2-methoxybenzaldehyde and t-butyl propionic acid, obtaining a 60:40 threo/ erythro ratio, exactly the same for the unsubstituted benzaldehyde
Summary
The synthesis and reactivity of β-lactones in racemic and enantiopure form have received in the last years a growing interest due to different reasons: i) the 2-oxetanone ring is a basic component in some biologically active natural products [1]; ii) their high reactivity toward a large series of electrophilic and nucleophilic species makes these compounds useful building blocks in the organic synthesis [2]; iii) their use as monomers in the preparation of biodegradable polymers [3].As a part of our studies on the application of chiral tricarbonyl(arene)chromium derivatives in asymmetric synthesis of products with potential biological activity [4]-[7], we were interested in the application of these organometallic substrates for the synthesis of some optically pure 2-oxetanones. The addition of optically pure benzaldehyde 1 (R=OMe or R=Cl) to a dianion of isobutyric acid (R1=R2=Me), generated with LDA at –50 °C in THF solution, affords in a few minutes and after usual work-up products 3a,b respectively in good yields and each as a single diastereoisomer.
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