258
2
General Synthetic Methods
⊡ Scheme 63
A modification of the Kiliani–Fischer method for amino functionality introduction involves
the condensation of free sugars with arylamines to give imines, which are treated directly with
hydrogen cyanide to produce aminonitriles. Hydrogenolysis and hydrolysis of the resulting
aminonitriles gives 2-amino-2-deoxyaldoses with one carbon extended [103].
4 Introduction of Sulfur and Selenium
4.1 Nucleophilic Substitutions
Thiosugars discussed here are the carbohydrates with oxygen replaced by sulfur at the
nonanomeric positions. Normally these compounds can be obtained by the displacement
of suitably positioned leaving groups using reagents containing nucleophilic sulfur such as
thiocyanate ion ( − SCN), thiolacetate [Me(C=O)S − ], benzylthiolate (PhCH 2 S − ) and ethyl
xanthate [EtO(C=S)S − ] [104].
As expected, these substitutions occur readily at primary positions, as outlined by the thioacetate displacement applied to the 5-O-tosylxylose derivative 141 giving 5-thio-D-xylose
142 ( > Scheme 64). Similarly, the selenosugar derivative 143 is obtained from compound
141 by substitution using potassium selenocyanate ( > Scheme 64) [104]. And 6-S-phenylgluco/galactopyranosides are readily prepared from the corresponding 6-hydroxy-glycosides
by direct substitution of the hydroxyl group using phenyl disulfide and tri-n-butylphosphine [105].
⊡ Scheme 64
2
General Synthetic Methods
⊡ Scheme 63
A modification of the Kiliani–Fischer method for amino functionality introduction involves
the condensation of free sugars with arylamines to give imines, which are treated directly with
hydrogen cyanide to produce aminonitriles. Hydrogenolysis and hydrolysis of the resulting
aminonitriles gives 2-amino-2-deoxyaldoses with one carbon extended [103].
4 Introduction of Sulfur and Selenium
4.1 Nucleophilic Substitutions
Thiosugars discussed here are the carbohydrates with oxygen replaced by sulfur at the
nonanomeric positions. Normally these compounds can be obtained by the displacement
of suitably positioned leaving groups using reagents containing nucleophilic sulfur such as
thiocyanate ion ( − SCN), thiolacetate [Me(C=O)S − ], benzylthiolate (PhCH 2 S − ) and ethyl
xanthate [EtO(C=S)S − ] [104].
As expected, these substitutions occur readily at primary positions, as outlined by the thioacetate displacement applied to the 5-O-tosylxylose derivative 141 giving 5-thio-D-xylose
142 ( > Scheme 64). Similarly, the selenosugar derivative 143 is obtained from compound
141 by substitution using potassium selenocyanate ( > Scheme 64) [104]. And 6-S-phenylgluco/galactopyranosides are readily prepared from the corresponding 6-hydroxy-glycosides
by direct substitution of the hydroxyl group using phenyl disulfide and tri-n-butylphosphine [105].
⊡ Scheme 64