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Title: Highly stereoselective synthesis of α-glycosylated carboxylic acids by phenanthroline catalysis
Carbohydrate molecules with an alpha-glycosylated carboxylic acid motif provide access to biologically relevant chemical space but are difficult to synthesize with high selectivity. To address this challenge, we report a mild and operationally simple protocol to synthesize a wide range of functionally and structurally diverse alpha-glycosylated carboxylic acids in good yields with high diastereoselectivity. While there is no apparent correlation between reaction conversion and the pKa of carboxylic acids, there is a notable trend in selectivity. Carboxylic acids with a pKa ranging from 4 to 5 exhibit high selectivity, whereas those with a pKa of 2.5 or lower do not display the same level of selectivity. Our strategy utilizes readily available 2,9-dibutyl-1,10-phenanthroline as an effective nucleophilic catalyst to displace a bromide leaving group from an activated sugar electrophile in a nucleophilic substitution reaction, forming phenanthrolinium intermediates. The attack of the carboxylic acid takes place from the alpha-face of the more reactive intermediate, resulting in the formation of alpha-glycosylated carboxylic acid. Previous calculations suggested that the hydroxyl group participates in the hydrogen bond interaction with the basic C2-oxygen of a sugar moiety and serves as a nucleophile to attack the C1-anomeric center. In contrast, our computational studies reveal that the carbonyl oxygen of the carboxylic acid serves as a nucleophile, with the carboxylic acid-OH forming a hydrogen bond with the basic C2-oxygen of the sugar moiety. This strong hydrogen bond (1.65 Å) interaction increases the nucleophilicity of the carbonyl oxygen of carboxylic acid and plays a critical role in the selectivity-determining step. In contrast, when alcohol acts as a nucleophile, this scenario is not possible since the -OH group of the alcohol interacts with the C2-oxygen and attacks the C1-anomeric carbon of the sugar moiety. This is also reflected in alcohol-OH's weak hydrogen bond (1.95 Å) interaction with the C2-oxygen. The O(C2)-HO (carboxylic acid) angle was measured to be 171° while the O(C2)-HO (alcohol) angle at 122° deviates from linearity, resulting in weak hydrogen bonding.  more » « less
Award ID(s):
1856437
PAR ID:
10612970
Author(s) / Creator(s):
; ; ;
Publisher / Repository:
Royal Society of Chemistry
Date Published:
Journal Name:
Organic Chemistry Frontiers
Volume:
11
Issue:
20
ISSN:
2052-4129
Page Range / eLocation ID:
5769 to 5783
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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