940 CHAPTER 22 Carbohydrates
Surprisingly,D-glucose forms more of the than the The rea-
son for this is explained in the next section.
Similar to the reaction of a monosaccharide with an alcohol is the reaction of a
monosaccharide with an amine in the presence of a trace amount of acid. The product
of the reaction is an N-glycoside. An N-glycosidehas a nitrogen in place of the oxy-
gen at the glycosidic linkage. The subunits of DNA and RNA are
(Section 27.1).PROBLEM 24Why is only a trace amount of acid used in the formation of an N-glycoside?N-phenyl- -D-ribosylamine
an -N-glycosideN-phenyl- -D-ribosylamine
a -N-glycosideOH
HO HOO
NH 2HOCH 2HOOHOCH 2HOOHOCH 2HO HONHNH+
catalytic
H+b-N-glycosidesa-glycoside b-glycoside.OCH+ 2 CH 3OCH 2 CH 3an oxocarbenium iona -glycoside an -glycoside
major productHB+HB+ BHB+ BHOHCH 2 OHmechanism of glycoside formationHOHOHOOHH
OHCH 2 OHHOOHH 2 O
HOCH 2 OHHOHOO+HCH 2 OHHOO
HCH 2 OHHOOOCH 2 CH 3
H++++ B+HCH 2 OHHOO
HCH 2 OHHOOOCH 2 CH 3+CH 3 CH 2 OH comes
in from the top CHin from the bottom^3 CH^2 OH comesHOHOHOHO
HOHOHOHOHOHOH