Galacto stereoisomer

Following the general route shown, N-acetylated methyl 7-deoxy-a-DL-lin-cosaminide (370), an eight-carbon atom sugar related to the antibiotic linco-mycin, was synthesized starting from 5,6-dihydro-2-methoxy-6-propionyl-2H-pyran (333,R = Et) the last step involved c -hydroxylation and also gave— besides the desired "galacto" product—some of the alio" stereoisomer (371). 

By means of 250 MHz H n.m.r. spectra, the solution conformations of stereoisomers of peracetylated aldohexose dimethyl acetals and diethyl acetals have been determined. With the exceptions of the T>-galacto- and D-man/io-isomers, which were extended planar, all isomers were in the sickle conformation. The results were interpreted on the basis of avoidance of parallel 1,3-substituent interactions. 

The 3-(D-galacto-pentitol-l-yl)pyrrole (90) was obtained by addition of the zwitterionic reagent (91) to the nitroalkene (92) analogous results were obtained for the D-manno-stereoisomers. ... 

Armstrong and Sutherland have reported a combinatorial approach to C-disaccharides in which one sugar unit was kept constant while the other was varied to provide all possible stereoisomers [77]. To illustrate this concept, the C-allylated glucose derivative 158, was chain-extended to give the diene ester 192 in four conventional steps (Scheme 46). Dihydroxylation and DIBALH reduction afforded four stereoisomeric lactols with the o/L-galacto and the Dfh-ido configurations... 

A unique example of a planar chiral catalyst displaying an imine as reactive function was reported by Kunz et al. [29] in 2007 (Scheme 8.10). Readily accessible pseudo-gem aldehyde-methyl ester Pc [30] in racemic form was condensed with fully protected galacto-pyranosylamine to give diastereoisomeric imines with respect to the Pc chirality. HPLC separation delivered the (Rp) stereoisomer in pure form, which showed a remarkable ability to perform highly enantioselective Strecker reactions (up to 99% ee). It is assumed that the imine catalyst acts as a Br0nsted base toward hydrocyanuric acid produced in situ and generates an iminium-cyanide pair that can interact with the imine substrate and direct cyanide addition on a unique face of the latter. 

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