Preparation of acylated pyranoid glycals in neutral aqueous medium by using chromium(II) complexes as reagents

Reactions of 2,3,4,6-tetra-O-acetyl-alpha-D-glucopyranosyl bromide (3) with chromium(II)L complexes (L = EDTA, NTA, IDA, GLY, MAL) in neutral (5 < pH < 7) aqueous DMF gave 3,4,6-tri-O-acetyl- 1,5-anhydro-2-deoxy-D-arabino-hex -1-enitol (13) in 70-90% yields and >95% purity. Complexes of Cr(II) with L = EDTA, NTA were similarly efficient with 2,3,4,6-tetra-O-acetyl-alpha-D-g lucopyranosyl chloride (2) in furnishing glucal 13, while with L = IDA, GLY , MAL hydrolysis of 2 could not be suppressed. Under the same conditions [C r-II(EDTA)(2-) also efficiently gave the corresponding glycals 14-19 from 2 ,3,4,6-tetra-O-benzoyl- (4) and 2,3,4,6-tetra-O-pivaloyl-alpha-D-glucopyran osyl bromide (5), per-O-acetylated alpha-D-galactopyranosyl chloride (6) an d bromide (7), alpha-D-mannopyranosyl chloride (8), alpha-D-xylopyranosyl c hloride (9), and bromide (10), beta-D-arabinopyranosyl bromide (11), and al pha-L-rhamnopyranosyl chloride (12). (C) 1999 Elsevier Science Ltd. All rig hts reserved.