Two onium ion-tagged prolines, imidazolium bis(trifluoromethylsulfonyl)imide-substituted proline 6 and butyldimethylammonium bis(trifluoromethylsulfonyl)imide-substituted proline 7, were synthesised and their catalytic activity in the direct asymmetric aldol condensation was studied in ionic liquids. For the reaction of acetone with various aldehydes, using 5 % of the catalyst, the yields of the aldols varied between 50-85 % while the ee values were in the 80-85 % range. Other ketones were studied too, the yields obtained in those cases being in the 35-78 % range while the enantioselectivities varied between 75-94 %.

Abstract:The installation of an imidazolium tag via acetate connection to the C-4 of cis-4-hydroxy-lproline provides a highly efficient catalyst for the direct asymmetric aldol reaction, that works in a remarkably low catalyst loading (0.1 mol%) affording TONs up to 930 in the case of electron-poor aromatic aldehydes with ee up to > 99%.

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Catalyst 5, an ion pair consisting of a hydrophilic cation and a lipophilic anion, fulfils the solubility requirements needed to couple efficiency (enantioselectivities and anti-diastereoselectivities up to > or = 99%) and catalyst recyclability in asymmetric aldol reactions under aqueous biphasic conditions.

[Structure: see text] [4R-[4alpha(S),5alpha]]-2,2-Dimethyl-4-(2-oxo-5-vinyl[1,3]dioxolan-4-yl)oxazolidine-3-carboxylic acid tert-butyl ester 5a, obtained in excellent yield and diastereoselectivity by the alpha-hydroxyallylation of the Garner aldehyde (4), is exploited in a novel high-yield synthesis of D-ribo-phytosphingosine (8), using microwave-enhanced cross metathesis as the key step in the chain elongation.