Prolinal dithioacetal

Aminothiourea-prolinal dithioacetal (234), in the presence of PhC02H, can catalyse Michael addition of ketones R CH2COR and aldehydes to nitroalkenes at 3 mol% loading to afford the 3yn-configured products with <99 1 dr and <99% ee under solvent-free conditions at room temperature. The related carbohydrate-derived thiourea is believed to activate both 8-diketones and nitroalkenes via coordination (235) the Michael adducts were obtained in <89% ee " Another variant of the thiourea motif with a cinchona alkaloid scaffold exhibited higher stereocontrol in the same reaction (<98% ee), carried out in MeCN at —40°C ... 

A highly enantioselective proline-catalysed intramolecular Morita-Baylis-Hillman reaction of hept-2-enedial (111) has been reported. Addition of imidazole to the mixture resulted in an unusual inversion of enantioselectivity.149 The first example of a TiCU-mediated Morita-Baylis-Hillman-type reaction of cy-acetyl cyclic ketene dithioacetals with arylaldehydes has been described.150... 

Numerous 2-substituted pyrrolidine organocatalysts have been prepared from L-proline and its derivatives, and have been proven to be highly efficient for many asymmetric reactions. Representative organocatalysts have been selected and categorised on the basis of the 2-substituted group that includes di- and tri-amine (la-m), dithioacetal (2a-f), guanidine (2g-i), sulfonamide (3a-j), amide and thioamide (3k-n), urea (4a and 4e), thiourea (4b-d, f-j) and heterocycles such as tetrazole (5a,b), triazole (5c-g), imidazole (5h-j) and benzoimidazole (5k) (Figure 9.1). 

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