Regioselective opening of 3,5-O-di-tert-butylsilylene-D-galactofuranosides: Synthesis of (1→5)-b-D-galactofuran derivatives

CAROLA GALLO -RODRIGUEZ; MARIANO J. TILVE

New Orleans

Simposio; XXVIII International Carbohydrate Symposium; 2016

Intenational Carbohydrate Organization

The use of thiogalactofuranosides as donors for the construction of oligosaccharides containing internal Galf is limited. This fact could be attributed to the difficulty to functionalize thiogalactofuranoside derivatives showing O-2, O-3 and O-5 with similar reactivities. In view of our experience with 3,5-O-di-tert-butylsilylene-D-galactofuranosyl derivatives,1,2 the differentiation between the secondary hydroxyl groups 3-OH and 5-OH was envisioned. An efficient method for complete regioselective 5-O-opening of 3,5-O-di-tert-butylsilylene-D-galactofuranoside derivatives was developed. The use of a solution nBu4NF (1.1 equiv) in CH2Cl2 on 3,5-O-DTBS derivative 1 gave the 5-OH derivative 2 which was confirmed by acetylation. 3-O-Di-tert-butylhydroxysilyl derivative 2 was a stable compound after purification and under glycosylation reactions. Preactivation of conformationally restricted thioglycoside 1 employing p-NO2-benzensulfenyl chloride/AgOTf promoting system3 followed by condensation over the 5-OH thioglycoside derivative product 2 gave the disaccharide 4 without autocondensation by-product. The disaccharide 4 was also used as a donor to afford the corresponding trisaccharide 5. Moreover, regioselective 5-O-deprotection was also successfully performed on disaccharide 4 and trisaccharide 5.