348
2
General Synthetic Methods
⊡ Figure 3
Synthesis of unsaturated sugars by a hetero Diels–Alder approach
⊡ Scheme 3
dialdose 4 ( > Scheme 3). The second reaction performed under 20 kbar pressure afforded the
target with almost 100% diastereoselectivity. It was possible to lower the pressure (to 10 kbar)
if the reaction was additionally catalyzed with mild Lewis acid [Eu(fod) 3 ] [6].
This process (hetero Diels–Alder reaction leading to a dihydropyran system) may be also
conducted in an asymmetric version; application of chiral transition-metal catalysts based on
BINOL, BINAP, bisoxazolines, etc. provides adducts in very high optical purity (ee up to
99%) [1,6]. In a series of papers Jurczak reported recently a highly enantioselective cycloaddition of 1-methoxy-1,3-butadiene and butyl glyoxylate catalyzed with chiral salen complexes [21].
The convenient precursors of unsaturated sugars, dihydropyran or dihydrofuran derivatives,
can be also obtained from an appropriately functionalized furan molecule. Two main approaches are used. The first is based on the oxidative rearrangement of furfuryl alcohols (Achmatowicz reaction) [22], the second one involves an acid-catalyzed reaction of aldehydes with
2-trimethyl-silyloxyfuran (Casiraghi reaction) [23] ( > Scheme 4).
In the original Achmatowicz approach, the (racemic) furfuryl carbinol is oxidized with
bromine in the presence of methanol under weakly basic conditions to provide the Clauson-
2
General Synthetic Methods
⊡ Figure 3
Synthesis of unsaturated sugars by a hetero Diels–Alder approach
⊡ Scheme 3
dialdose 4 ( > Scheme 3). The second reaction performed under 20 kbar pressure afforded the
target with almost 100% diastereoselectivity. It was possible to lower the pressure (to 10 kbar)
if the reaction was additionally catalyzed with mild Lewis acid [Eu(fod) 3 ] [6].
This process (hetero Diels–Alder reaction leading to a dihydropyran system) may be also
conducted in an asymmetric version; application of chiral transition-metal catalysts based on
BINOL, BINAP, bisoxazolines, etc. provides adducts in very high optical purity (ee up to
99%) [1,6]. In a series of papers Jurczak reported recently a highly enantioselective cycloaddition of 1-methoxy-1,3-butadiene and butyl glyoxylate catalyzed with chiral salen complexes [21].
The convenient precursors of unsaturated sugars, dihydropyran or dihydrofuran derivatives,
can be also obtained from an appropriately functionalized furan molecule. Two main approaches are used. The first is based on the oxidative rearrangement of furfuryl alcohols (Achmatowicz reaction) [22], the second one involves an acid-catalyzed reaction of aldehydes with
2-trimethyl-silyloxyfuran (Casiraghi reaction) [23] ( > Scheme 4).
In the original Achmatowicz approach, the (racemic) furfuryl carbinol is oxidized with
bromine in the presence of methanol under weakly basic conditions to provide the Clauson-