Hydroxyproline, enantioselective addition

Yamaguchi et al. also showed that Rb-L-prolinate catalyzes enantioselective addition of nitroalkanes to several acyclic and cyclic enones [25, 26]. For acyclic enone acceptors the best result, i.e. 74% yield and 68% ee of the S product, was achieved in the addition of 2-nitropropane to -3-penten-2-one (40a, Scheme 4.13) [25]. Screening of several proline derivatives and cyclic amino acids of other ring size resulted in the identification of the O-TBDMS-derivative of 4-hydroxyproline as the best catalyst for addition of nitrocyclohexane to cycloheptenone. In this particular reaction 74% yield and 86% ee were achieved [26]. 

Chiral aminoalcohols (45), derived from (2S, 4S)-4-hydroxyproline and (S)-proline, respectively, were found to be superior catalysts for the enantioselective 1,4-addition of arylthiols to 2-cyclohexen-l-one to yield 3-arylthiocyclohexanones (46) 82). 

Surprisingly few studies have been directed towards the development of noncinchona alkaloid-based catalysts for the alcoholative ASD of meso-anhydrides, or indeed any of the enantioselective alcoholysis processes. Uozumi has reported a series of (2S, 4R)-4-hydroxyproline-derived 2-aryl-6-hydroxyhexahydro-lfi-pyr-rolo[l,2-c] imidazolones which mediate the methanolytic ASD of ds-hexa-hydrophthalic anhydride in up to 89% ee when employed at the 10 mol% level for 20 h at —25 °C in toluene [186]. Additionally, Nagao has described the use of a bifunctional chiral sulfonamide for the thiolytic ASD of meso-cyclic anhydrides in up to 98% ee when employed at the 5 mol% level for 20 h at rt in ether [187]. 

A chiral aminoalcohol 7, derived from i-4-hydroxyproline, is found to be a superior catalyst for the enantioselective 1,4-addition of arylthiols to 2-cyclohexenrl-one to yield 3-arylthio-cyclohexanones in high optical purities. ... 

Recently, and co-workers [41] Zhang reported the first highly efficient protocol for the synthesis of a-amino esters with prolinol-derivated catalyst. The (9-pivaloyl 7ra 5-4-hydroxyproline derivative (Cat. 10) achieved best results. Crucial for efficiency was the addition of small quantities of pentanoic acid. Through this approach, a broad range of chiral a-amino esters were synthesized in good yields (up to 97%) and with high levels of enantioselectivity (up to 93%) (Scheme 15.14). 

The enantioselective Michael addition of aldehydes R CH2CHO to nitroalkenes R CH=CHN02 in aqueous solution was compared with the reaction performed under solvent-free ball-milling conditions. From the range of pyrrolidine-derived organocatalysts (325), O-lauroyl-trans-4-hydroxyproline worked best in the aqueous media, whereas (325) proved to be most efficient in the ball-milling technique. ... 

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