✍ Scribed by Ziegler, Thomas ;Pantkowski, Guido
No coin nor oath required. For personal study only.
The application of the 2‐(chloroacetoxymethyl)benzoyl (CAMB) group to the O‐protection of position 2 in glycosyl donors is described. Saponification of the phthalide and subsequent chloroacetylation of 2‐(hydroxymethyl)benzoic acid (1) gave 2‐(chloroacetoxymethyl)benzoic acid (2). Treatment of 2 with thionyl chloride afforded the acyl chloride 3. Acylation of O‐2 of 1,3,4,6‐tetra‐O‐acetyl‐α‐D‐glucopyranose (4a), galactopyranose (4b) and of the amino group of 1,3,4,6‐tetra‐O‐acetyl‐2‐amino‐2‐deoxy‐β‐D‐glucopyranose (4c) afforded the fully protected pyranose derivatives 5a–c in high yields. Compounds 5a and 5b were converted with HBr in acetic acid into the corresponding 2‐O‐CAMB‐protected bromides 6a, b in excellent yields. Silver trifluoromethanesulfonate promoted glycosylation of the latter with 2‐hydroxyethyl benzoate yielded the β‐glycosides 8a (74%) and 8b (69%). Direct activation of the glucosamine derivative 5c with ferric chloride gave the glucoside 8c (54%). Furthermore, the galactosyl bromide 6b was coupled with methyl 2,3,4‐tri‐O‐benzoyl‐β‐D‐glucopyranoside (11) affording the 2′‐O‐CAMB‐protected methyl glycoside disaccharide 12 (63%). Similarly, condensation of the glucosyl bromide 6a with 4a gave the β‐(1→2)‐linked disaccharide 13 (45%). Treatment of compounds 8a and 13 with thiourea resulted in the deblocked glycosides 9a and 14. Similar deprotection of 8b gave 2‐benzoyloxyethyl 3,4,6‐tri‐O‐acetyl‐β‐D‐galactopyranoside (9b) and the 2‐O‐[2‐(hydroxymethyl)benzoyl] derivative 10. Reaction of 8c with thiourea afforded solely the dechloroacetylated glucoside 9c. magnified image
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