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Scheme 8 Aqueous aldolisation of methyl cyclohexanone with m-nitrophenyl-
benzaldehyde in presence of carbohydrates.
Table 6 Influence of the size of the hydrophobic aglycon of the fructofuranosidic
additive on the selectivity of the aqueous aldolisation.
R
Time (h)
Yield (%)
anti/syn
anti ratio
syn ratio
Methyl
20
50
2.6 : 1
4.9 : 1
1 : 2.6
Ethyl
6
90
4.3 : 1
9 : 1
nd
Propyl
10
70
2.7 : 1
7.3 : 1
nd
Allyl
18
90
1.9 : 1
4 : 1
1 : 2.7
n-Pentenyl
8
84
4 : 1
5.7 : 1
1 : 2.1
Scheme 9 The mono- and bis-addition products of nitromethane and methyl vinyl
ketone.
3.3 Michael addition
In keeping with the effect of carbohydrate additives on aqueous reactions
involving a nucleophilic attack on a carbonyl group, the Michael addition
of nitroalkanes on a,b-unsaturated compounds was investigated. This
reaction, often performed in an organic solvent in the presence of a base,
was also shown to proceed uncatalyzed in water.
32
For example, the re-
action of nitromethane with MVK in water led to the mono- and bis-
addition products as depicted in Scheme 9, at significantly faster rates
than in methanol. Although the full investigation of the origin of the
acceleration in this reaction has not been accomplished, it is however
interesting to note that using a 4M glucose solution in water or a 2M
sucrose solution, a 4-fold rate acceleration was observed, and the same
effect was observed for nitroethane as reactant (18h instead of 48h at
60 1C).
31
Quite recently, Gu and coworkers reported the use of gluconic
acid aqueous solutions (''GAAS'') as medium for Michael addition of
indoles to a,b-unsaturated ketones (Scheme 10).
33
Compared with other
traditional aqueous or organic solvents, gluconic acid has been found
to greatly promote the reaction rate of Michael addition, as shown in
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