Synthesis of α-<scp>d</scp>-Glcp-(1→3)-α-<scp>d</scp>-Galf-(1→2)-α-<scp>l</scp>-Rhap constituent of the CPS of Streptococcus pneumoniae 22F. Effect of 3-O-substitution in 1,2-cis α-<scp>d</scp>-galactofuranosylation
作者:Gabriel Gola、Carola Gallo-Rodriguez
DOI:10.1039/c3ra45658g
日期:——
The synthesis of the trisaccharide α-D-Glcp-(1â3)-α-D-Galf-(1â2)-L-Rhap (3) constituent of Streptococcus pneumonia 22F was achieved with complete diastereoselectivity. This is the first example of a synthesis of an internal α-D-Galf containing oligosaccharide of a pathogen microorganism. Allyl α-D-galactofuranoside, used as novel precursor of the internal Galf, allowed the introduction of an orthogonal group at O-3. The trichloroacetimidate method was used for the construction of 1,2-cis-α-D-galactofuranosyl linkage. The influence of the 3-O-substituent (PMB, Bz, PFBz, PMBz, TIPS) was evaluated in benzylated galactofuranosyl trichloroacetimidate donors in terms of yield and selectivity of α-D-Galf-(1â2)-α-L-Rhap product as well as donor rearrangement by-product. Complete stereoselectivity was observed with all protecting groups used in the Galf donor, but the 3-O-benzoyl substitution gave the best yield. Protective groups were also evaluated in the rhamnoside acceptor, benzyl substitution was a requirement for complete stereoselectivity.
以完全非对映选择性合成了肺炎链球菌 22F 的三糖δ±-D-Glcp-(1â3)-δ±-D-Galf-(1â2)-L-Rhap (3)。这是首次合成病原微生物内部含有δ-D-Galf 的寡糖。使用烯丙基δ±-D-半乳糖呋喃糖苷作为内部 Galf 的新型前体,可以在 O-3 处引入一个正交基团。三氯乙酰亚氨酸法用于构建 1,2-顺式-δ±-D-半乳糖呋喃糖基连接。根据δ-D-Galf-(1â2)-δ-L-Rhap 产物以及供体重排副产物的产量和选择性,评估了苄基半乳糖呋喃糖基三氯乙酰亚胺供体中 3-O 取代基(PMB、Bz、PFBz、PMBz、TIPS)的影响。在 Galf 供体中使用的所有保护基团都具有完全的立体选择性,但 3-O 苯甲酰基取代的产率最高。还对鼠李糖苷受体中的保护基团进行了评估,发现苄基取代是完全立体选择性的必要条件。