An improved preparation of 2,3,5-tri-O-acyl-β-d-ribofuranosyl azides by the Lewis acid-catalysed reaction of β-d-ribofuranosyl acetates and trimethylsilyl azide: an example of concomitant formation of the α anomer by trimethylsilyl triflate catalysis
✍ Scribed by Anton Štimac; Jože Kobe
- Publisher
- Elsevier Science
- Year
- 1992
- Tongue
- English
- Weight
- 514 KB
- Volume
- 232
- Category
- Article
- ISSN
- 0008-6215
No coin nor oath required. For personal study only.
✦ Synopsis
An improved synthesis ' of 2,3,5-tri-O-benzoyl-/3-o-ribofuranosyl azide (3) involves the reaction of trimethylsilyl azide in the presence of a Lewis acid with l-0-acetyl-2,3J-tri-O-benzoyl-@-o-ribofuranose 2-4 (1) rather than the action of sodium azide upon the corresponding halide 5. Titanium tetrachloride, aluminum chloride, and boron trifluoride etherate were suggested * to be catalysts that are as equally effective as trimethylsilyl trifluoromethanesulfonate 3 (triflate) and stannic chloride 4Z6. 2,3,5-Tri-O-acyl-P-Dribofuranosyl azides were formed exclusively from the corresponding glycosyl acetates 2-6 However, excessive proportions of either . trimethylsilyl azide and/or the catalyst were employed in most of these reactions, which is not only wasteful but makes the isolation step more difficult.
We now report that the preparation of 3 can be simplified by using a 10% excess of trimethylsilyl azide and 0.05 mol equiv of catalyst. Dichloromethane was selected as a reaction medium owing to its favourable solvent effect *. With stannic chloride as the catalyst, the rate of reaction was almost independent of the concentration above 0.03 mol equiv, and the use of 0.05 mol equiv was more effective than 0.1 mol equiv of titanium tetrachloride, trimethylsilyl triflate, aluminum chloride, boron trifluoride etherate, and magnesium bromide etherate in promoting the reaction of 1 with trimethylsilyl azide to give 3 (Figs. 1 and2). With stannic chloride, there was a 99.4% conversion within 4 h (cf. 96% with titanium tetrachloride in 30 h and 99.6% with trimethylsilyl triflate in 9 days). Boron trifluoride etherate, magnesium bromide etherate, and aluminum chloride were