Hydroxyacetone bond formation

It is believed that the hydroxyl group present in hydroxyacetone (39) stabilizes the (Z)-enamine via hydrogen bond formation, and thus the use of O-protected derivatives as donors in the direct aldol reaction required, at best, tuning the reaction conditions. Only a few catalysts were applied to the reaction of O-Bn-hydroxya-cetone (51) with various aldehydes [103, 58]. Luo and Cheng s catalyst 38 also... 

At even higher temperature, the polysaccharides decompose further by extensive C-C bond breaking. This leads to the formation of C2 4 oxygenates such as glycol aldehyde, acetic acid and hydroxyacetone (CH3-CO-CH2OH). The formation of these products can be rationalized by a series of reactions that include,... 

The excellent regioselectivity was explained by highly regioselective enamine formation by the hydroxyacetone, because of the hydroxy group. This n-donating hydroxyl group stabilizes the hydroxyl enamine by interacting with the n -orbital of the C=C double bond [93]. 

Following immediately the initial efforts on primary amino acids catalyzed aldol reactions, the application of primary amine acid in Mannich reaction has also been attempted. Cordova reported that simple primary amino acids and their derivatives could catalyze the asymmetric Mannich reactions of ketones with comparable results to those obtained in the catalysis of proline[28]. Later, Barbas [29] and Lu [30] independently reported that L-Trp or 0-protected L-Thr could catalyze anti-selective asymmetric Mannich reactions of a-hydroxyacetones with eiflier preformed or in-situ generated imines. The preference of anii-diastereoselectivity was ascribed to the formation of a Z-enamine, with the assistance of an intramolecular H-bond (Scheme 5.15). 

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