A-Selective galactosylation

A. Imamura, H. Ando, S. Korogi, G. Tanabe, O. Muraoka, H. Ishida, and M. Kiso, Di-ferf-butylsily-lene (DTBS) group-directed a-selective galactosylation unaffected by C-2 participating functionalities, Tet. Lett., 44 (2003) 6725-6728. 

FIGURE 5 Typical sugar donors for a-selective galactosylation. 

High a-selectivities were observed when the D-galactosyl donor 10 was used for the coupling with secondary hydroxy groups in carbohydrate acceptors including 9 (see Scheme 5). With the trichloroacetimidate method [5], the yields of disaccharides derived from 6 and 7 were 85% (10 1, a/p) and 87% (only a), respectively. The condensation of 10... 

The coupling reaction with diacetone galactosyl acceptor 33 and the donor 29 was examined in various solvents (O Scheme 9) [32]. In all solvents the reaction constantly provided perfect a-selectivity, whereas the coupling yield differed greatly. The best result was obtained when the reaction was carried out in CCI4 to produce 34 in 46% yield, which was accompanied with the competitive formation of 2,3-en derivative (33%). 

In most cases, the a-anomer of mono-galactosylated ceramides can be easily separated from the p-one by colunm chromatography. In addition, galactosylation reaction of a ceramide proceeds kinetically with high a-selectivity, and... 

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