Synthesis of Carbohydrate-based Chiral Crown Ethers as Ligands in Asymmetric Hydrogenation
作者:Franziska Faltin、Volker Fehring、Renat Kadyrov、Antonio Arrieta、Thomas Schareina、Rüdiger Selke、Ralf Miethchen
DOI:10.1055/s-2001-12357
日期:——
Starting from phenyl 2,3-di-O-allyl-4,6-O-benzylidene-β-d-glucopyranoside (1) the chiral crown ethers 6 and 7, containing a 1,4-bridged α-d-glucopyranoside moiety, were synthesized in four steps via phenyl 2,3-O-allyl-6-O-benzyl-β-d-glucopyranoside (2). To build up the corresponding polyethylene glycol side chain at 4-position, compound 2 was subsequently alkoxylated with bis(2-chloroethyl)ether and diethylene glycol or triethylene glycol yielding via 3 the polyethylene glycol derivatives 4 and 5, respectively. On a similar way phenyl 2,3-di-O-allyl-6-O-benzyl-4-O-2-[Ï-hydroxypenta(oxyethylene)ethyl]}-β-d-galactopyranoside (15) was prepared from phenyl 4,6-O-benzylidene-β-d-galactopyranoside (10) via the intermediates 11, 12 and 13. The chiral crowns 6, 7, and 16 were obtained in yields of 26-38% by intramolecular transglycosylation of 4, 5, and 15, respectively. Whereas a high α-stereoselectivity was found for the cyclization of the 1,4-bridged d-glucose crowns 6 and 7, galactose derivative 15 gave the β-glycosidic linked crown 16. In order to obtain the rhodium chelates 18 and 20 as precatalysts for asymmetric hydrogenations, the gluco-crown ethers 6 and 7 were deallylated to 8 and 9 and phosphorylated under anaerobic conditions giving the bis(phosphinic esters) 17 and 19. The latter were used as ligands for 18 and 20. Finally, asymmetric hydrogenations of amino acid precursors 21 a - d were investigated in the presence of the rhodium chelates 18 and 20. Under hydrogen, they show as catalysts in different solvents a diminished range of enantioselectivity in comparison with an analogous complex without such a crown ether ring. This can be explained by a stiffening effect of the anellated ring on the chelate ring conformation which is confirmed by the unusually uniform CD-spectra of 20 in solvents of different polarity.
从苯基2,3-二氧基-阿利尔-4,6-氧-苄基-β-D-葡萄糖吡喃苷(1)开始,通过苯基2,3-氧-阿利尔-6-氧-苄基-β-D-葡萄糖吡喃苷(2),合成了含有1,4桥接α-D-葡萄糖吡喃苷部分的手性冠醚6和7,整个过程共进行了四步。为了在4位构建相应的聚乙烯醇侧链,化合物2随后与双(氯乙基)醚和二乙ylene醇或三乙ylene醇进行醚化反应,得到聚乙烯醇衍生物4和5。以类似的方式,从苯基4,6-氧-苄基-β-D-半乳糖吡喃苷(10)经中间体11、12和13制备了苯基2,3-二氧基-阿利尔-6-氧-苄基-4-氧-2-[ω-羟基戊氧基(聚乙烯醇)乙基]}-β-D-半乳糖吡喃苷(15)。通过4、5和15的分子内转糖基化反应,分别获得了手性冠醚6、7和16,产率为26-38%。尽管在1,4桥接D-葡萄糖冠醚6和7的环化过程中发现了较高的α立体选择性,但半乳糖衍生物15则生成了β-糖苷键合的冠醚16。为了获得铑螯合物18和20作为不对称氢化的前催化剂,手性冠醚6和7被去阿利尔化为8和9,并在厌氧条件下进行磷酸化,得到双(磷酸酯)17和19。后者被用作18和20的配体。最后,在铑螯合物18和20存在下研究了氨基酸前体21 a-d的非对称氢化。在氢气条件下,它们作为催化剂在不同溶剂中显示出较低的对映选择性范围,相较于没有这种冠醚环的类似配合物。这可以通过环状结构对螯合环构象的强固效应来解释,这一结论通过20在不同极性溶剂中的异常均匀的CD光谱得到了证实。