Chemistry Reference
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Table 5 Influence of the sugar moiety of the glycoorganic dienes on the rate constant
and activation parameters of the aqueous Diels-Alder reaction with methylvinylketone.
10
5
k
2
(M
1
s
1
)
at 25 1C
DH
z
(kJ mol
1
)
DS
z
(J mol
1
T
1
)
D(DH
z
)
(kJ mol
1
)
D(
TDS
z
)
(kJ mol
1
)
Diene
Solvent
H
2
O
28.5
40.0
0.6
178.8
2.1
6.4
þ
9.6
OH
MeOH/H
2
O
(v/v
=
1/1)
8.5
33.6
0.8
211.1
2.6
O
HO
O
HO
OH
2
H
2
O
22.6
43.5
0.1
168.1
0.5
8.3
þ
11.7
HO
OH
MeOH/H
2
O
(v/v
=
1/1)
5.7
35.2
0.2
207.5
0.6
O
HO
HO
O
26
H
2
O
28.0
34.9
0.8
195.6
2.6
0.2
þ
3.2
HO
OH
MeOH/H
2
O
(v/v
=
1/1)
8.4
34.7
2.0
206.3
6.5
O
HO
O
OH
27
Only small variations were observed among all dienes, though a
relationship could be established between the activation entropy of the
reaction in water and the sensitivity of the reaction to the solvent change in
a few cases, notably, for the a-
D
-galacto 26 and the b-
D
-galacto 27 dienes
residues. The variations were hypothetically discussed by considering how
the carbohydrate moiety fits into the three-dimensional hydrogen-bond
network of water, so to say to the hydration pattern of the dienes.
21,24
3.2 Aldol type reactions
Building on their work reported since 1996 describing investigations on
the use of some sugars, notably fructosides, in reduction,
25
epoxida-
tion,
26
and indium-promoted allylations,
27
Plusquellec and coworkers
recently reported the use of 1M sucrose or ethyl fructofuranoside as
additives in the aqueous aldolisation reaction of methyl 4-methylcyclo-
hexanone with m-nitrophenylbenzaldehyde (and diversely functionalized
analogues) performed in the presence of organocatalysts such as
L
-pro-
line (Scheme 8, Table 6).
28
Significant variations in the anti-syn selectivity
were observed.
The influence of the presence of carbohydrate solutes had been pre-
viously briefly explored by Lubineau and Scherrmann in the case of the
Mukaiyama aldol-type reaction of silylenol ethers with aldehydes, which
had been found to be accelerated with effects on the syn-anti selectivity
similar to what is observed under high pressure.
29,30
Favoring the syn
aldol product was clearly consistent with its smaller transition state
volume as compared to the anti one. When this reaction was studied in
the presence of carbohydrate solutes, a moderate yield increase was ob-
served, in the same syn-directed selectivity. This effect was however
ascribed to slower competitive hydrolysis of the silylenol ether, probably
related to a limitation of water activity in the medium, responsible for
this undesired side reaction.
31
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