Synthesis 2006(14): 2418-2439  
DOI: 10.1055/s-2006-942447
FEATUREARTICLE
© Georg Thieme Verlag Stuttgart · New York

N-Heterocyclic Carbene-Catalyzed Conjugate Umpolung for the Synthesis of γ-Butyrolactones

Christian Burstein, Serena Tschan, Xiulan Xie, Frank Glorius*
Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Straße, 35032 Marburg, Germany
Fax: +49(6421)2825629; e-Mail: glorius@chemie.uni-marburg.de;
Further Information

Publication History

Received 30 March 2006
Publication Date:
07 July 2006 (online)

Abstract

The N-heterocyclic carbene-catalyzed conjugate Umpolung of differently substituted α,β-unsaturated aldehydes, e. g. cinnamaldehydes, α-methylcinnamaldehydes, and crotonaldehydes, is described. Coupling of these compounds with a variety of electrophilic aldehydes and ketones results in the selective formation of highly substituted β- and γ-butyrolactones.

16

Under a different set of reaction conditions (t-BuOH, DBU, THF) and using two equivalents of electrophilic aldehyde, Bode et al. reported improved yields: ref. 7d.

17

The stereochemistry of the lactones 1, 7, 12, and 20 was assigned by X-ray and NMR analysis. The 1H NMR data for the lactone ring hydrogens of 1a-h is very similar within the cis and trans series. The stereochemistry of the products 1 was therefore assigned by comparison of the NMR data for these compounds with that of cis-1d, whose structure was unequivocally established by X-ray structural analysis. The stereochemistry of lactones 7, 12, and 20 was assigned in an analogous manner. CCDC-246600 (cis-1d), CCDC-250238 (u-7b), CCDC-603356 (12c-I) and CCDC-603357 (20c-II) contain the crystallographic data (excluding structure factors) for this paper. [25] This data can be obtained free of charge from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 (1223)336033; e-mail: deposit@ccdc.cam.ac.uk.

18

Bode et al. found that when using cis-cinnamaldehyde the same stereochemical outcome as with trans-cinnamaldehyde was observed. Moreover, stopping the reaction of cis-cinnamaldehyde prior to completion a significant amount of trans-cinnamaldehyde was observed, showcasing the importance of homoenolate resonance structure IIb: ref. 7d.

20

Crotonaldehyde and α-methylcinnamaldehyde were reported to be unreactive under the conditions employed by Bode et al.: ref. 7 of ref. 7d.