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 1B 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.
本文报道了消旋的
愈创甘油醚(1A)与各种糖供体,即
β-D-葡萄糖五乙酸酯(2)、
吡啶基2,3,4,6-O-四苄基-1-
硫代-β-
D-半乳糖吡喃糖苷(9)、1-O-乙酰基-2,3,5-三-O-苯甲酰基-α/β-
D-核糖-(13)和
木糖呋喃糖苷(17)的区域选择性糖基化反应。具有酯保护基的糖供体2、13和17被报道表现出高度的区域选择性,分别形成相应的1-O-糖基化
愈创甘油醚衍
生物3A、14A和18A的非对映异构体混合物;未观察到形成二糖基化衍
生物5A、15A和19A。然而,当使用具有醚保护基的供体9进行与1A的偶联反应时,未观察到这种选择性,导致形成了二半
乳糖基化衍
生物10A。通过将1B与2偶联并分离出1-O-糖基化衍
生物3B,表明区域选择性不依赖于芳环上的取代基;未观察到形成二糖基化衍
生物5B。这一发现的适用性通过分离其相应的非对映异构体(R)-3和(S)-3,分别制备了具有高对映体纯度的
愈创甘油醚[(R)-1(98% ee)和(S)-1(98% ee)]。