Anti-aldols

Several approaches based on nitro-aldol for the synthesis of amino sugars have been reported. Alumina-catalyzed reaction of methyl 3-nitropropanoate with O-benzyl-D-lactaldehyde gives the D-ribo-nitro-aldol (anti, anti isomer) in 63% yield, which is converted into L-daunosamine17b 105 (see Section 3.3). Jager and coworkers have reported a short synthesis of L-acosamine based on the stereoselective nitro-aldol reaction of 2-O-benzyl-L-lactaldehyde with 3-nitropropanal dimethyl acetal as shown in Scheme 3.10.106 The stereoselective nitro-aldol reaction is carried out by the silyl nitronate approach as discussed in Section 3.3. 

Our final highlight in the discodermolide synthesis is the use of reagent control to get what we want and not what the molecules want. The combination of enol borinate 276 and an aldehyde 277 featuring a cis double bond, led to the formation of aldols anti- and syn-278. A model study had shown that the inherent selectivity with these enals could be improved by the use of ( )-Ipc groups on boron instead of cyclohexyl but it is not the selectivity that was wanted. (+)-Ipc groups were able to turn around the selectivity and improve the yield of the reaction while they were at it.50... 

The proline-catalyzed aldol reaction of cyclohexanone (57) with 4-nitrobenzal-dehyde (2a) in DMSO affords the desired aldol anti-SSa with good yield but rather low stereoselectivity (Scheme 3.15) [14]. However, the addition of water led to an increase both in the yield and enantioselectivity, though the reaction rate remained [17]. 

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