- Regioselective acetylation of carbohydrates and diols catalyzed by tetramethyl-ammonium hydroxide in water
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A novel method for an efficient regioselective acetylation of carbohydrates and diols in aqueous solution is described. Treatment of substrates with 1-acetylimidazole, and tetramethyl-ammonium hydroxide (TMAH) in water under mild conditions gave highly regioselective acetylation for primary hydroxyl groups. This discovery provides an eco-friendly way for selective acetylation of non-protected glycosides and diols in water, avoiding the use of toxic organic solvents and the necessity of pre-protection of secondary hydroxyl groups. This journal is
- Lu, Yuchao,Wei, Peng,Pei, Yuxin,Xu, Hengfu,Xin, Xiaoting,Pei, Zhichao
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p. 4510 - 4514
(2014/12/10)
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- 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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p. 2378 - 2384
(2007/10/03)
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- Kinetic resolution of acyclic 1,2-diols using a sequential lipase- catalyzed transesterification in organic solvents
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A method for the kinetic resolution of 3-(aryloxy)-1,2-propanediols rac- 1a-n without additional protection-deprotection steps using a lipase- catalyzed sequential transesterification with lipase amano PS has been developed. In the first step of this one-pot procedure the racemic 1,2-diols are acylated regioselectively at the primary hydroxy group without enantioselection. The subsequent acylation at the secondary hydroxy group of the formed primary monoacetate is responsible for high enantioselection. The enantioselectivity of this transformation depends significantly on the substitution pattern of the aryl ring and the organic solvent used. 3- (Aryloxy)-1,2-propanediols with substituents in the para-position show a much higher enantioselectivity than the corresponding derivatives with ortho- substituents. Among other substrates, the pharmaceuticals Mephenesin, Guaifenesin, and Chlorphenesin have been resolved. The replacement of the aryloxy by an alkyl substituent causes a dramatic decrease of enantioselectivity.
- Theil,Weidner,Ballschuh,Kunath,Schick
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p. 388 - 393
(2007/10/02)
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