Chemistry Reference
In-Depth Information
HO
R
2
HO
OH
R
1
R
2
SiO
2
-SO
3
H (1.7%
)
80 °C
R
1
O
OH
+
O
+
OH
HO
OH
R
1
R
2
10%
90%
OH
R
1
,R
2
: aryl, functionalized alkyl groups
R
R
R
O
OH
(a)
HO
SiO
2
-SO
3
H
O
+
OH
OH
(2.2 mmol%)
R
95 °C, 4 h
(b)
OH
O
OH
OH
R = H, Yield = 92%, a/b = 94/6
R=Et,Yield=93%,a/b=92/8
Scheme 29 Synthesis of MAGEs from glycerol, secondary alcohols and dibenzylether.
the most active.
77
A recent report relevant to the design and economics of
a miniplant in a plug flow reactor using the Amberlyst-15 catalyst gives
more informations about the process.
78
One may also use secondary
alcohols and strained olefins upon using SiO
2
-SO
3
H catalysts at 80 1C, or
even primary benzyl alcohol and, interestingly, dibenzyl ethers, in
which case the reaction leads to mono glyceryl ethers (MAGEs), with a
substitution regioselectivity on the primary hydroxyl groups of ca. 90%
(Scheme 29).
79
3.5 The Fischer glycosidation - synthesis of alkyl polyglycosides
(APGs)
The Fischer glycosidation, also referred to as the Fischer-Helferich
glycosylation allows the one step synthesis of an acetal (alkyl glycoside)
from an aliphatic alcohol and an unprotected carbohydrate.
80
This acid
catalyzed reaction is specific to the anomeric hydroxyl group of
the carbohydrate and involves a transient stabilized oxycarbenium
(Scheme 30). Very simple mineral acids such as sulfuric acid are generally
used; however, noteworthy in the context of this chapter is the
introduction by Lubineau et al. and Plusquellec et al. of Lewis acids to
promote this reaction with high stereoselectivities.
81
Reusable supported
acids have also proven their eciency to promote the Fischer glycosylation
and are advantageously used in view of their easy separation from the
products and potential recyclability.
82
Beyond the variation of the catalysts
employed, ultra sound
83
or microwave activations allow a shortening of
reaction times and can thus be advantageously used in place of a tradi-
tional heating.
84
In most cases, the reaction is performed in the liquid
alcohol used in large excess and it is rather uncommon to employ an
organic co-solvent. In this field, progresses have been obtained from the
use of recyclable ionic liquids as solvent.
85
Owing to the eciency of the
reaction and the wide availability of the reactants, this reaction has found
an important industrial application for the production of APGs.
APGs are non-ionic surfactants derived from glucose.
86
Known since
several decades, they have found large scale applications as green
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