Glycal Derivatives
3.6
723
⊡ Scheme 40
⊡ Scheme 41
⊡ Scheme 42
Reaction of isomeric dienes 120 with acetaldehyde in the presence of zinc chloride gave a mixture of C-4 epimeric enones 121, 122 ( > Scheme 42) [224], which are convenient substrates
for synthesis of all four racemic 3-amino-2,3,6-trideoxy pyranoses.
Similarly, reaction of 1,4-diacetoxy-1,3-butadiene with methyl glyoxalate afforded glucuronate glycal [228] and a 2,3-unsaturated isomer. Cycloaddition reactions performed on
substrates with inverse electronic properties (e. g., enol ether as a dienofile and unsaturated carbonyl compound as a heterodiene) afford not only the expected products but also
ones having high endo selectivity [229,230] as exemplified below by olivose 126 synthesis
( > Scheme 43).
3.6
723
⊡ Scheme 40
⊡ Scheme 41
⊡ Scheme 42
Reaction of isomeric dienes 120 with acetaldehyde in the presence of zinc chloride gave a mixture of C-4 epimeric enones 121, 122 ( > Scheme 42) [224], which are convenient substrates
for synthesis of all four racemic 3-amino-2,3,6-trideoxy pyranoses.
Similarly, reaction of 1,4-diacetoxy-1,3-butadiene with methyl glyoxalate afforded glucuronate glycal [228] and a 2,3-unsaturated isomer. Cycloaddition reactions performed on
substrates with inverse electronic properties (e. g., enol ether as a dienofile and unsaturated carbonyl compound as a heterodiene) afford not only the expected products but also
ones having high endo selectivity [229,230] as exemplified below by olivose 126 synthesis
( > Scheme 43).