“…Compounds 21 − 23 were then subjected to the common operations, i.e., selective removal of the anomeric acetate or benzoate (NH 3 , THF/MeOH, 0 °C) and addition with N -phenyl trifluoroacetimidoyl chloride (K 2 CO 3 , acetone, rt), to provide glycosyl trifluoroacetimidates 6a − c in satisfactory yields. 6- O -Acetyl-2,3,4-tri- O -benzoyl-α- d -glucopyranosyl bromide ( 6d ), prepared from 12 , was converted into 1-OH sugar 24 under the action of Ag 2 CO 3 in aqueous acetone, which was then transformed readily into fluoride 6e (DAST, THF), trifluoroacetimidate 6f , and trichloroacetimidate 6g under usual conditions. 4 Preparation of Donors 6 a − g a a Reagents and conditions: (a) 90% CF 3 COOH, 0 °C to rt, 77%; (b) TBSCl, imidazole, DMF, rt, 86% for 22 , 88% for 23 ; (c) NH 3 , THF/MeOH, rt; (d) PhNCClCF 3 , K 2 CO 3 , acetone, rt, 74% for 6a (two steps), 62% for 6b (two steps), 86% for 6c (two steps), 97% for 6f ; (e) Ac 2 O, pyridine, rt; (f) 33% HBr/HOAc, CH 2 Cl 2 , rt, 80% (two steps); (g) Ag 2 CO 3 (1.5 equiv), acetone/H 2 O, rt, 98%; (h) DAST, THF, −30 °C to rt, 86%; (i) CCl 3 CN, K 2 CO 3 , acetone, rt, 90%. …”
Section: Resultsmentioning
confidence: 99%
“…Our attention was then focused on the 6- O -acetyl-2,3,4-tri- O -benzoyl- d -glucopyranosyl donors 6d − g . The most frequently employed protocol for glycosylation of phenols relies on the S N 2 displacement of the anomeric bromide with phenolates generated in basic conditions. , However, attempted coupling of toralactone 5 with glycosyl bromide 6d under literature conditions, including the use of tetrabutylammonium bromide (TBAB) as a phase-transfer reagent 29a-c and the use of CdCO 3 as a base, ,29d,e all failed to give considerable yields of the desired product 28 (Table , entries 6−8). Peracetylglucosyl fluoride under the action of BF 3 ·OEt 2 and DTBMP has been reported as a successful protocol for glycosylation of phenols .…”
Toralactone 9-O-beta-d-glucopyranosyl-(1-->6)-beta-d-glucopyranosyl-(1-->3)-beta-d-glucopyranosyl-(1-->6)-beta-d-glucopyranoside (1, cassiaside C(2)), isolated from Cassia obtusifolia L. and showing strong antiallergic activity, was concisely synthesized employing glycosyl trifluoroacetimidates as glycosylation agents. The unique naphtho-alpha-pyrone structure of toralactone (5) was constructed by condensation of orsellinate 8 with pyrone 9 in the presence of LDA as developed by Staunton and co-workers. The naphthol of toralactone showed minimal reactivity as an acceptor and was screened with various glycosyl donors. It is finally concluded that sacrifice of an excess amount of the trifluoroacetimidate or trichloroacetimidate donors (6f/6g, 6.0 equiv) in the presence of a catalytic amount of TMSOTf (0.05 and 0.3 equiv, respectively) afforded excellent yields of the coupling product, which was otherwise only a minor product under a variety of conditions examined.
“…Compounds 21 − 23 were then subjected to the common operations, i.e., selective removal of the anomeric acetate or benzoate (NH 3 , THF/MeOH, 0 °C) and addition with N -phenyl trifluoroacetimidoyl chloride (K 2 CO 3 , acetone, rt), to provide glycosyl trifluoroacetimidates 6a − c in satisfactory yields. 6- O -Acetyl-2,3,4-tri- O -benzoyl-α- d -glucopyranosyl bromide ( 6d ), prepared from 12 , was converted into 1-OH sugar 24 under the action of Ag 2 CO 3 in aqueous acetone, which was then transformed readily into fluoride 6e (DAST, THF), trifluoroacetimidate 6f , and trichloroacetimidate 6g under usual conditions. 4 Preparation of Donors 6 a − g a a Reagents and conditions: (a) 90% CF 3 COOH, 0 °C to rt, 77%; (b) TBSCl, imidazole, DMF, rt, 86% for 22 , 88% for 23 ; (c) NH 3 , THF/MeOH, rt; (d) PhNCClCF 3 , K 2 CO 3 , acetone, rt, 74% for 6a (two steps), 62% for 6b (two steps), 86% for 6c (two steps), 97% for 6f ; (e) Ac 2 O, pyridine, rt; (f) 33% HBr/HOAc, CH 2 Cl 2 , rt, 80% (two steps); (g) Ag 2 CO 3 (1.5 equiv), acetone/H 2 O, rt, 98%; (h) DAST, THF, −30 °C to rt, 86%; (i) CCl 3 CN, K 2 CO 3 , acetone, rt, 90%. …”
Section: Resultsmentioning
confidence: 99%
“…Our attention was then focused on the 6- O -acetyl-2,3,4-tri- O -benzoyl- d -glucopyranosyl donors 6d − g . The most frequently employed protocol for glycosylation of phenols relies on the S N 2 displacement of the anomeric bromide with phenolates generated in basic conditions. , However, attempted coupling of toralactone 5 with glycosyl bromide 6d under literature conditions, including the use of tetrabutylammonium bromide (TBAB) as a phase-transfer reagent 29a-c and the use of CdCO 3 as a base, ,29d,e all failed to give considerable yields of the desired product 28 (Table , entries 6−8). Peracetylglucosyl fluoride under the action of BF 3 ·OEt 2 and DTBMP has been reported as a successful protocol for glycosylation of phenols .…”
Toralactone 9-O-beta-d-glucopyranosyl-(1-->6)-beta-d-glucopyranosyl-(1-->3)-beta-d-glucopyranosyl-(1-->6)-beta-d-glucopyranoside (1, cassiaside C(2)), isolated from Cassia obtusifolia L. and showing strong antiallergic activity, was concisely synthesized employing glycosyl trifluoroacetimidates as glycosylation agents. The unique naphtho-alpha-pyrone structure of toralactone (5) was constructed by condensation of orsellinate 8 with pyrone 9 in the presence of LDA as developed by Staunton and co-workers. The naphthol of toralactone showed minimal reactivity as an acceptor and was screened with various glycosyl donors. It is finally concluded that sacrifice of an excess amount of the trifluoroacetimidate or trichloroacetimidate donors (6f/6g, 6.0 equiv) in the presence of a catalytic amount of TMSOTf (0.05 and 0.3 equiv, respectively) afforded excellent yields of the coupling product, which was otherwise only a minor product under a variety of conditions examined.
Synthesis of a New Phenol Glycoside, Neohancoside C from Cynanchum hancockianum.-The synthesis of a new phenol glycoside (V) is presented, key step being the phase-transfer catalyzed glycosylation of acetophenone (III) with glucose derivative (II). The best protecting group for the regioselective introduction of the xylose unit proved to be the acetyl group. -(KONDA, Y.; IWASAKI, Y.; TAKAHATA, S.; ARIMA, S.; TOIDA, T.; KAJI, E.; TAKEDA, K.; HARIGAYA, Y.; Chem. Pharm. Bull. 45 (1997) 4, 626-630; Sch. Pharm. Sci., Kitasato Univ., Minato, Tokyo 108, Japan; EN)
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