Hydrogenation Reactions - Carbohydrates: Tools for Stereoselective Synthesis - page 157

et al . [13], however, observed that diphosphinite 10, derived from d-glucose,
behaved as a pseudo-enantiomer of 11, derived from d-xylose. Indeed, it provides
the opposite enantiomer in the rhodium-catalyzed hydrogenation of (Z)- acetami-
docinnamic acid ( 1a ), although the ee was higher (90%, S ) for the Rh/ 11 than for
the Rh/ 10 catalytic system (63% ee, R ) (entries 4 and 3, respectively, Table 8.1).
In the same way, RajanBabu observed that pyranoside derivatives prepared from
d-glucose and d-glucosamine containing phosphorus bonded to the 3,4-hydroxyl
groups ( 12 - 15 ) could be considered as pseudo-enantiomers of the d-glucose
derived ligands with phosphorus bonded to the 2,3-hydroxyl groups ( 8 ). In fact, in
the hydrogenation of (Z)-

-acetamidocinnamic acids the Rh/ 12-15 catalytic
system provided the (R)- enantiomer, while the Rh/ 8 catalytic system gave the
(S)- enantiomer. Enantioselectivities were higher than 95% when these catalytic
systems were used (entries 9 and 10, Table 8.1) [14]. Scheme 8.4 shows the steps
involved in the synthesis of diphosphinite ligands 12 and 13, derived from d-glu-
cose and d-glucosamine, respectively.

α

OBz OBz

HO
HO
HO

O

O O

(a-b) (c)

HO
HO

Ar 2 PO
Ar 2 PO

HO OMe BzO OMe BzO OMe

12

OTBDMS

AcO

HO

HO
HO
HO

O

(4) O O O

(5-6) (7, 3)

AcO
AcO

HO
HO

Ar 2 PO
Ar 2 PO

OH

OMe

OMe

NHAc

AcHN Cl

AcHN AcHN

13

Scheme 8.4 Synthesis of diphosphinite
ligands 12 and 13: (a) (Bu 3 Sn) 2 O/toluene;
(b) PhCOCl/toluene (65% combined yield of
a and b); (c) Ar 2 PCl/CH 2 Cl 2 /Py/DMAP ( ∼ 91%
yield); (4) MeCOCl/CH 2 Cl 2 (58% yield); (5)

Ag 2 CO 3 /CH 2 Cl 2 /MeOH/mol. sieves 4Å (78%
yield); (6) MeOH/Biorad AGMP(OH - ) resin
(95% yield); (7) TBDMSCl/DMF/imidazole
(yield not reported).

Uemura prepared the rhodium complexes 16a,b, which incorporated diphosphi-
nite ligands derived from

α , α -trehalose ( 16a ) and β , β -trehalose ( 16b ) (Figure 8.2)

[15]. Complex 16b, with a ligand derived from -trehalose, is closely related to 9

(Figure 8.1). In 16b the anomeric phenyl group is replaced by a glucose moiety,
which increases the solubility in water. Complex 16b provided an ee of 88% when
methyl (Z)- α

β , β

-acetamidocinnamate ( 1b ) was hydrogenated in water, but the stere-
oselectivity increased to 99.9% in the presence of sodium dodecyl sulfate (entry
11, Table 8.1). When the reaction was performed in the biphasic system AcOEt/
water, the ee was 87% and the catalyst could be recovered simply by phase separa-
tion at the end of the reaction. The ligands derived from β , β -trehalose gave better

results than those derived from α , α -trehalose, in agreement with what was

observed for α -configured ligand 7 and β -configured 8.