514
3
Chemical Glycosylation Reactions
prepared with the help of closely related trichloroacetimidate building blocks and a similar
strategy, such as for instance for woodrosin I [295] and tricolorin F [296]. Structurally related,
highly bioactive antiviral compounds, such as for instance cycloviracin B 1 or glucolipsin A
were isolated and successfully synthesized [297,298,299]. The approach to the synthesis of
glucolipsin A [299] is exhibited in > Scheme 44. Glucosylation of fatty acid derivative 44a
with O-glycosyl trichloroacetimidate 44b gave the desired glycoside 44c in good yield. Ester
hydrolysis (→ 44d) and dimerization to 44e and then hydrogenolysis of the O-benzyl groups
led to target molecule 44f which had physical data in accordance with the natural product
(see [299]).
Saponins are steroid- or triterpenoid-based glycolipids which are found in terrestrial and
marine plants. They possess various biological activities [300]. Several papers reported on
successful glycosylations at the 3-hydroxy group [301,302,303,304,305,306]. When this
hydroxy group is sterically hindered due to disubstitution at C-4, this glycosylation is a difficult task. However, with O-glycosyl trichloroacetimidate donors good yields and anomeric
selectivities have also been obtained for this glycosylation step [307,308,309,310,311,312].
Particularly worth mentioning is the total synthesis of QS 21 A published by Gin et al. [25]
where the decisive step is carried out with a trisaccharide donor.
Most carbohydrates contain ethane-1,2-diol fragments, therefore they were also successfully included into crown ether macrocycles [313]. Polyvalency of carbohydrates was studied
based on dendrimeric structures which were generated on successful glycosylations of highly
branched polyethylene glycol units [314].
5.8 Cyclooligosaccharides
Cyclodextrins (CDs) have gained a lot of interest because they provide useful cavities for the
generation of inclusion complexes. Therefore, branched CDs and CDs with various carbohydrate residues have been synthetic targets. The trichloroacetimidate method was successfully
employed to reach such goals. Mannosyl, galactosyl, lactosyl, and the Galβ(1–4 )-lactosyl
residues were selectively introduced at O-6 of β- and γ -CD, respectively [315,316]. Also
building blocks for oligomannoside attachment to CDs were prepared [317]. Most remarkable is the synthesis of a O-6 branched cyclo(1–3)-glucohexaose and octaose [318,319]. The
ring closure of the prepared nona- and dodecasaccharide worked extremely well based on
O-glycosyl trichloroacetimidate donors ( > Scheme 45) [5]. Dodecasaccharide 45a was transformed via 45b into trichloroacetimidate 45c having a 3-hydroxy group at the nonreducing
end. Acid-catalyzed activation led to CD derivative 45d which gave after deacylation target
molecule 45e.
5.9 C-Glycoside Synthesis
The synthesis of “C-glycosides” has been of great interest [320]. C-Glycosylation of electronrich compounds with O-glycosyl trichloroacetimidates continues to be a method of choice.
This work is not discussed here in detail. A few references give an entry to the work in this
field [321,322,323,324].
3
Chemical Glycosylation Reactions
prepared with the help of closely related trichloroacetimidate building blocks and a similar
strategy, such as for instance for woodrosin I [295] and tricolorin F [296]. Structurally related,
highly bioactive antiviral compounds, such as for instance cycloviracin B 1 or glucolipsin A
were isolated and successfully synthesized [297,298,299]. The approach to the synthesis of
glucolipsin A [299] is exhibited in > Scheme 44. Glucosylation of fatty acid derivative 44a
with O-glycosyl trichloroacetimidate 44b gave the desired glycoside 44c in good yield. Ester
hydrolysis (→ 44d) and dimerization to 44e and then hydrogenolysis of the O-benzyl groups
led to target molecule 44f which had physical data in accordance with the natural product
(see [299]).
Saponins are steroid- or triterpenoid-based glycolipids which are found in terrestrial and
marine plants. They possess various biological activities [300]. Several papers reported on
successful glycosylations at the 3-hydroxy group [301,302,303,304,305,306]. When this
hydroxy group is sterically hindered due to disubstitution at C-4, this glycosylation is a difficult task. However, with O-glycosyl trichloroacetimidate donors good yields and anomeric
selectivities have also been obtained for this glycosylation step [307,308,309,310,311,312].
Particularly worth mentioning is the total synthesis of QS 21 A published by Gin et al. [25]
where the decisive step is carried out with a trisaccharide donor.
Most carbohydrates contain ethane-1,2-diol fragments, therefore they were also successfully included into crown ether macrocycles [313]. Polyvalency of carbohydrates was studied
based on dendrimeric structures which were generated on successful glycosylations of highly
branched polyethylene glycol units [314].
5.8 Cyclooligosaccharides
Cyclodextrins (CDs) have gained a lot of interest because they provide useful cavities for the
generation of inclusion complexes. Therefore, branched CDs and CDs with various carbohydrate residues have been synthetic targets. The trichloroacetimidate method was successfully
employed to reach such goals. Mannosyl, galactosyl, lactosyl, and the Galβ(1–4 )-lactosyl
residues were selectively introduced at O-6 of β- and γ -CD, respectively [315,316]. Also
building blocks for oligomannoside attachment to CDs were prepared [317]. Most remarkable is the synthesis of a O-6 branched cyclo(1–3)-glucohexaose and octaose [318,319]. The
ring closure of the prepared nona- and dodecasaccharide worked extremely well based on
O-glycosyl trichloroacetimidate donors ( > Scheme 45) [5]. Dodecasaccharide 45a was transformed via 45b into trichloroacetimidate 45c having a 3-hydroxy group at the nonreducing
end. Acid-catalyzed activation led to CD derivative 45d which gave after deacylation target
molecule 45e.
5.9 C-Glycoside Synthesis
The synthesis of “C-glycosides” has been of great interest [320]. C-Glycosylation of electronrich compounds with O-glycosyl trichloroacetimidates continues to be a method of choice.
This work is not discussed here in detail. A few references give an entry to the work in this
field [321,322,323,324].