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Abstract
The trans-selective catalytic asymmetric formation of β-lactones constitutes an attractive surrogate for anti-aldol additions. Recently, we have reported the first catalyst which is capable of forming trans-β-lactones with high enantioselectivity from aliphatic (and aromatic) aldehyde substrates by cyclocondensation with acyl bromides. In that previous study the concepts of Lewis acid and organic aprotic ion pair catalysis were combined in a salen-type catalyst molecule. Since a pyridinium residue on the salen periphery is essential for high trans- and enantioselectivity, we were interested in the question of whether substituents on the pyridinium rings could be used to further improve the catalyst efficiency, as they might have a significant impact on the effective charges within the heterocycles. In the present study we have thus compared a small library of aluminum salen/bispyridinium catalysts mainly differing in the substituents on the pyridinium residues. As one result of these studies a new catalyst was identified which offers slightly superior stereoselectivity as compared to the previously reported best catalyst. NBO calculations have revealed that the higher stereoselectivity can arguably not be explained by the variation of the effective charge.
Highlights
Introduction βLactones, are attracting the interest of scientists for mainly two reasons: (1) a number of natural and synthetic β-lactones are known to act as specific enzyme inhibitors [1,2,3,4]
Phenols 9 were formylated with paraformaldehyde in the presence of
Catalysts for the asymmetric formation of trans-β-lactones are of major interest, since trans-β-lactones offer a divergent and atom-economic access to the important class of anti-aldol products
Summary
Introduction βLactones (systematic name 2-oxetanones), are attracting the interest of scientists for mainly two reasons: (1) a number of natural and synthetic β-lactones are known to act as specific enzyme inhibitors [1,2,3,4]. Ring opening with hard nucleophiles offers the possibility to a divergent access to aldol products [9,10,11,12,13], whereas treatment with soft nucleophiles can be utilized to synthesize β-functionalized carboxylic acid derivatives [9,10,12]. In both cases the stereoinformation of the β-lactone ring can be completely transferred into the ring opening product and the acyl-oxygen bond cleavage with hard nucleophiles typically proceeds with retention of configuration. The catalytic asymmetric formation of ester or amide aldol derivatives via β-lactones is attractive for the high divergency of accessible aldol structures, and for the high atom-economy of the overall process, since no preformation of silylketene acetals or related nucleophiles is necessary [14]
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