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- Title
Glycosylidene Carbenes. Part 2. Synthesis of O-Aryl Glycosides.
- Authors
Briner, Karin; Vasella, Andrea
- Abstract
Phenol, 4-methoxyphenol, 4-nitrophenol, methyl orsellinate ( 1), and 2,6-di( tert-butyl)-4-methylphenol (BHT; 2) have been glycosylated by thermal reaction (20-60°) with various glycosylidene-derived diazirines. 4-Methoxyphenol reacted with the D-glucosylidene-derived diazirine 3 to give O-glucosides ( 4 and 5, 69%, 3:1) and C-glucosides ( 6 and 7, 16%, 1:1). Similarly, phenol yielded O-glucosides ( 10 and 11, 70%, 4:1) and C-glucosides ( 12 and 13, 13%, 1:1). 4-Nitrophenol gave only O-glycosides, 3 leading to 14 and 15 (75%, 3:2; Scheme 1), and the D-galactosylidene-derived diazirine 17 to 22 and 23 (52% (from 16), 65:35; Scheme 2). The reaction of phenol with 17 yielded 58% (from 16) of the O-galactosides 18 and 19 (4:1) and 14% of the C-galactosides 20 and 21 (1:1). From the D-mannosylidene-derived diazirine 25, we predominantly obtained the α-D-configurated 26 (38 % from 24). These results are interpreted by assuming that an intermediate (presumably a glycosylidene carbene) first deprotonates the phenol to generate an ion pair which combines to give O- and - with electron-rich phenolates - also C-glycosides. A competition experiment of 3 with 4-nitro- and 4-methoxyphenol gave the products from the former ( 14 and 15) and the latter phenol ( 4-7) in almost equal amounts. Differences in the kinetic acidity of OH groups, however, may form the basis of a regioselective glycosidation, as evidenced by the reaction of 3 with methyl orsellinate ( 1) yielding exclusively the 4- O-monoglycosylated products 27 and 28 (78%, 85:15), although diglycosidation is possible ( 27→ 31 and 32; 67%, 4:3; Scheme 3). Steric hindrance does not affect this type of glycosidation; 3 reacted with the hindered BHT ( 2) to afford 33 and 34 (81 %, 4:1). The predominant formation of 1,2- trans -configurated O-aryl glycosides is rationalized by a neighbouring-group participation of the 2-benzyloxy group.
- Publication
Helvetica Chimica Acta, 1990, Vol 73, Issue 6, p1764
- ISSN
0018-019X
- Publication type
Article
- DOI
10.1002/hlca.19900730621