- Discriminative Glycosylation of 3-(Aryloxy)propane-1,2-diols by Choice of a Glycosyl Donor
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Regioselective glycosylation of rac-guaifenesin (1A) with various glycosyl donors viz., β-D-glucosepentaacetate (2), pyridyl 2,3,4,6-O-tetra-O-benzyl- 1-thio-β-D-galactopyranoside (9), 1-O-acetyl-2,3,5-tri-O-benzoyl-α/ β-D-ribo- (13) and xylofuranoside (17) is reported. Glycosyl donors 2, 13 and 17 bearing ester protecting groups is reported to exhibit high regioselectivity to form the corresponding diastereomeric mixture of 1-O-glycosylated guaifenesin derivatives 3A, 14A and 18A, respectively; formation of diglycosylated derivatives 5A, 15A and 19A is not observed. While no such selectivity is observed when the donor 9 bearing ether protecting groups is used in the coupling reaction with 1A, resulting in the formation of digalactosylated derivatives 10A. That the regioselectivity is not dependent upon substituents present on the aromatic ring is shown by coupling IB with 2 to isolate 1-O-glycosylated derivative 3B; formation of diglycosylated derivative 5B was not observed. Applicability of this finding is shown by preparation of enantiopure guaifenesin [(R)-1 (98% ee) and (S)-1 (98% ee)] by separation of their corresponding diastereomers (R)-3 and (S)-3, respectively.
- Mereyala, Hari Babu,Mamidyala, Sreeman Kumar,Chigurapati, Krishna Prasad,Srinivasa, Sastri R.
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