ABSTRACT
Experimental Methods ........................................................................................... 189 General Methods ............................................................................................... 189
Phenyl 2,3,4,6-Tetra-O-Acetyl-1-Thio-β-d-Glucopyranoside (2) ................ 189 4,5-Dihydro-1,3-Thiazol-2-yl 2,3,4,6-Tetra-O-Acetyl-1-Thio-β-dGlucopyranoside (3) ..................................................................................... 190 Ethyl 2,3,4,6-Tetra-O-Acetyl-1-Thio-β-d-Glucopyranoside (4) .................. 190 p-Tolyl 2,3,4,6-Tetra-O-Acetyl-1-Thio-β-d-Glucopyranoside (5) ............... 191
Acknowledgment ................................................................................................... 191 References .............................................................................................................. 195
Alkyl/aryl thioglycosides. Thioglycosides are the most versatile glycosyl donors for oligosaccharide synthesis.1-4 They survive many conventional reaction conditions that are often employed in carbohydrate chemistry and can be readily activated with mild electrophilic promoters. Moreover, thioglycosides can be transformed into other common glycosyl donors such as glycosyl halides, sulfoxides, and imidates (via hemiacetals). First attempts to activate 1-thioderivatives employed rather harsh reaction conditions and resulted in only modest yields and stereoselectivities.5-7 Subsequently introduced milder promoters have been in common use for the past 2 decades.3 Many successful strategies for expeditious oligosaccharide synthesis have been developed based on thioglycosides.4,8
Thioimidates. 2-Thiopyridyl derivatives were among the ¥rst thioimidates to be investigated as glycosyl donors.9 Hanessian based his concept of “remote activation”10 on 2-pyridyl glycosides and the scope of this technique was expanded by Mereyala who investigated MeI-promoted glycosylation conditions.11 Demchenko and coworkers have recently introduced S-benzoxazolyl (SBox) and S-thiazolinyl (IUPAC preferred name: 4,5-dihydro-1,3-thiazolyl) (STaz) glycosides as novel ef¥- cient glycosyl donors for stereoselective glycosylation and expeditious oligosaccharide synthesis.12,13 High anomeric selectivity and yields were achieved for a variety of systems in the presence of stoichiometric amounts of promoters, such as MeOTf or AgOTf. Both armed and disarmed glycosyl thioimidates can be selectively activated
in the presence of other common glycosyl donors such as S-alkyl/aryl and O-pentenyl glycosides, as well as ¥t into a variety of expeditious strategies for oligosaccharide synthesis based on selective or chemoselective activations.8,9
This contribution describes transformation of 1,2,3,4,6-tetra-O-acetyl-β-dglucopyranose (1) into phenyl 2,3,4,6-tetra-O-acetyl-1-thio-β-d-glucopyranoside (2)14 and 4,5-dihydro-1,3-thiazol-2-yl 2,3,4,6-tetra-O-acetyl-1-thio-β-d-glucopyranoside (3)15 in the presence of BF3-etherate, as well as into ethyl 2,3,4,6-tetra-O-acetyl1-thio-β-d-glucopyranoside (4)16 and tolyl 2,3,4,6-tetra-O-acetyl-1-thio-β-dglucopyranoside (5)17 in the presence of zirconium(IV) chloride.
