Glycal Derivatives
3.6
713
⊡ Scheme 18
Furanoid glycals utilizing elimination of selenoxides were prepared from unusual substrates,
the 4-phenylselenyltetrahydrofurans. These intermediates were obtained with good yields
from D-glyceraldehyde ( > Scheme 21). Their oxidations have led to selenoxides, however,
the elimination reaction required heating. The best yields were noted for reactions refluxed in
1,2-dichloroethane and yields ranged from 62 to 95%.
6 Glycals from 2-Deoxysaccharides
Methods for the preparation of glycals from 2-deoxy sugars, although very efficient, are limited
by the availability of 2-deoxy sugars. Very often the best substrates for 2-deoxy sugars are
glycals themselves. Therefore, such methods are not practical for the preparation of simple
glycals, which can be easily obtained directly from commercially available monosaccharides.
However, such approaches are very useful in the preparation of glycals of rare sugars or glycals
of naturally occurring 2-deoxy sugars. Modified glycals are often needed for the preparation
of biologically important glycosides, oligosaccharides, and sugars modified at C-2.
1-O-esters of 2-deoxyhexopyranoses or respective glycosyl halides appear to be useful substrates in the synthesis of glycals. When added to a silica gel suspended in dry xylene and upon
heating, 1,4-di-O acetyl-N-trifluoroacetyl-α,β-daunosamine 68, formed the respective glycal
69 (4-O-acetyl 1,5-anhydro-2,3,6-trideoxy-3-trifluoroacetamido-L-lyxo-hex-1-enitol) in 60%
yield ( > Scheme 22) [163]. In fact, the whole spectrum of 3-aminoglycals was prepared from
2-deoxy sugars starting from either esters or glycosyl halides. These transformations along
with other approaches to 3-aminoglycals are described in detail by Pelyvas et al. [164].
Biologically interesting unsaturated derivatives of N-acetylneuraminic acid (Neu5Ac), a potent
inhibitors of sialidase, were also obtained from peracetylated substrates. Unsaturated peracetate methyl ester of Neu5Ac2en 71 was obtained in over 90% yield via the CF 3 SO 3 SiMe 3 -
mediated elimination of acetic acid from 70 ( > Scheme 23). Other unsaturated products were
similarly prepared [133].
3.6
713
⊡ Scheme 18
Furanoid glycals utilizing elimination of selenoxides were prepared from unusual substrates,
the 4-phenylselenyltetrahydrofurans. These intermediates were obtained with good yields
from D-glyceraldehyde ( > Scheme 21). Their oxidations have led to selenoxides, however,
the elimination reaction required heating. The best yields were noted for reactions refluxed in
1,2-dichloroethane and yields ranged from 62 to 95%.
6 Glycals from 2-Deoxysaccharides
Methods for the preparation of glycals from 2-deoxy sugars, although very efficient, are limited
by the availability of 2-deoxy sugars. Very often the best substrates for 2-deoxy sugars are
glycals themselves. Therefore, such methods are not practical for the preparation of simple
glycals, which can be easily obtained directly from commercially available monosaccharides.
However, such approaches are very useful in the preparation of glycals of rare sugars or glycals
of naturally occurring 2-deoxy sugars. Modified glycals are often needed for the preparation
of biologically important glycosides, oligosaccharides, and sugars modified at C-2.
1-O-esters of 2-deoxyhexopyranoses or respective glycosyl halides appear to be useful substrates in the synthesis of glycals. When added to a silica gel suspended in dry xylene and upon
heating, 1,4-di-O acetyl-N-trifluoroacetyl-α,β-daunosamine 68, formed the respective glycal
69 (4-O-acetyl 1,5-anhydro-2,3,6-trideoxy-3-trifluoroacetamido-L-lyxo-hex-1-enitol) in 60%
yield ( > Scheme 22) [163]. In fact, the whole spectrum of 3-aminoglycals was prepared from
2-deoxy sugars starting from either esters or glycosyl halides. These transformations along
with other approaches to 3-aminoglycals are described in detail by Pelyvas et al. [164].
Biologically interesting unsaturated derivatives of N-acetylneuraminic acid (Neu5Ac), a potent
inhibitors of sialidase, were also obtained from peracetylated substrates. Unsaturated peracetate methyl ester of Neu5Ac2en 71 was obtained in over 90% yield via the CF 3 SO 3 SiMe 3 -
mediated elimination of acetic acid from 70 ( > Scheme 23). Other unsaturated products were
similarly prepared [133].