O-Glycosyl Donors
3.4
595
2.8.5 Intramolecular Aglycon Delivery from n-Pentenyl Glycofuranosides
An approach to α-D-fucofuranosyl glycosides that makes use of the intramolecular aglycon
delivery (IMAD) [93,94,95,96] starting from an n-pentenyl fucofuranoside has been described
( > Scheme 25) [97]. n-Pentenyl fucofuranoside 178 bearing a free 2-OH group was attached to
a 4-O-PMB-protected galactopyranoside 179, upon treatment of the mixture with DDQ. The
unstable tethered compound 180, could be activated with NIS, in the absence of even catalytic
amounts of acid, to undergo an efficient p-methoxybenzyl-assisted aglycon delivery [95] leading to the desired glycoside 181. The unusual structure 181, resulting from quenching of the
benzylic cation with N-succinimide, was then processed to α-D-fucofuranoside 182.
2.8.6 Intramolecular C-Glycosylation of NPGs
The intramolecular C-glycosylation of NPGs has been studied by Martin’s group in the course
of their approaches to bergenin [98] and related natural products [99,100]. The treatment of
pentenyl β-D-glucopyranose 183 with IDCP promoted an internal, Friedel–Crafts type, C-arylation reaction in excellent yield ( > Scheme 26a). The resulting product was exclusively the
kinetically favored, cis-fused tricyclic system 184. Treatment of the latter with an oxophilic
Lewis acid (BF 3 ·OEt 2 ) led to the trans-fused (β-linked) 185. On the contrary, analogous reac⊡ Scheme 26
Intramolecular C-glycosylation of NPGs
3.4
595
2.8.5 Intramolecular Aglycon Delivery from n-Pentenyl Glycofuranosides
An approach to α-D-fucofuranosyl glycosides that makes use of the intramolecular aglycon
delivery (IMAD) [93,94,95,96] starting from an n-pentenyl fucofuranoside has been described
( > Scheme 25) [97]. n-Pentenyl fucofuranoside 178 bearing a free 2-OH group was attached to
a 4-O-PMB-protected galactopyranoside 179, upon treatment of the mixture with DDQ. The
unstable tethered compound 180, could be activated with NIS, in the absence of even catalytic
amounts of acid, to undergo an efficient p-methoxybenzyl-assisted aglycon delivery [95] leading to the desired glycoside 181. The unusual structure 181, resulting from quenching of the
benzylic cation with N-succinimide, was then processed to α-D-fucofuranoside 182.
2.8.6 Intramolecular C-Glycosylation of NPGs
The intramolecular C-glycosylation of NPGs has been studied by Martin’s group in the course
of their approaches to bergenin [98] and related natural products [99,100]. The treatment of
pentenyl β-D-glucopyranose 183 with IDCP promoted an internal, Friedel–Crafts type, C-arylation reaction in excellent yield ( > Scheme 26a). The resulting product was exclusively the
kinetically favored, cis-fused tricyclic system 184. Treatment of the latter with an oxophilic
Lewis acid (BF 3 ·OEt 2 ) led to the trans-fused (β-linked) 185. On the contrary, analogous reac⊡ Scheme 26
Intramolecular C-glycosylation of NPGs