Page 372 - Cascade_Biocatalysis_Integrating_Stereoselective_and_Environmentally_Friendly_Reactions

348 16 Aldolases as Catalyst for the Synthesis of Carbohydrates and Analogs

O
H
O OH O N
Cbz OH OH Cbz 3 H 2 R 1 n OH
N n N n 4
H 1 2 (a) H 1 2 (b) 2 OH
R R R R OH OH R
OH
18 19 20
n = 0 2 1 a b
R R % dr
a H H 91 94:6
b (S)- ; (R)- H 99,95 >98 : 2; 45 : 55
c (S)- ; (R)- H 94,99 >98 : 2; 76 : 24
d (S)- ; (R)- H 94,91 >98 : 2; >2 : 98
e (S)- ; (R)- H 62,64 >98 : 2; >2 : 98
f (S)- ; (R)- H 93,93 >98 : 2, >2 : 98

g (s)- H 89 >98 : 2
h (S)- ; (R)- H 45,52 >98 : 2; >2 : 98
i CH 3 CH 3 99 >98 : 2
n = 1
j H H 94 81:19

O
Cbz O Cbz OH O H OH
* *
N * * OH OH N 4 3 H 2
N OH
(a) OH OH (b)
21 22 23 OH
% a dr b
a (S)- 95 >98 : 2
b (R)- nr nr
a
Percentage of aldol adducts 19 and 22 formed for the S and R enantiomer acceptors
when corresponding.
b
dr = (3R,4S) : (3R,4R) in aldol adducts 19 and 22 for the S and R enantiomer
acceptors when corresponding, nr: no reaction.
Scheme 16.6 Aldol addition of DHA to N-Cbz-aminoaldehydes catalyzed by RhuA wild
type in the presence of borate buffer pH 7.5, 200 mM. (a) RhuA wild type, sodium borate
200 mM, pH 7.5; and (b) Pd/C.

of DHAP to N-Cbz-aminoaldehydes in borate were between 2- and 10-fold faster
than that in triethanolamine buffer. Moreover, the yields of aldol adduct improved
between 1.5- and 4-fold for both FucA wild type and the F131A mutant [30]. It is
likely that the borate inhibits the retroaldol reaction, favoring the formation of the
aldol adduct.
The discovery of a novel FSA isoenzyme [24] and the structurally related transal-
dolase B mutant F178Y (TalB F178Y ; both from E. coli) which readily accepts
unphosphorylated DHA as donor was a significant breakthrough in the field