Albumin-controlled stereoselective reduction of 1,3-diketones to anti-diols

Article abstract of DOI:10.1039/b200474g

An unprecedented combination of high chemo- and stereoselectivity in the NaBH₄ reduction of 1:1 complexes between albumin and aromatic 1,3-diketones results in the formation of anti 1,3-diols with de up to 96%.

Full text of DOI:10.1039/b200474g

Albumin-controlled stereoselective reduction of 1,3-diketones to
anti-diols†
Fabio Benedetti,* Federico Berti,* Ivan Donati and Massimo Fregonese
Dipartimento di Scienze Chimiche, Università di Trieste, Via Giorgieri 1, I-34127 Trieste, Italy.
E-mail: berti@dsch.univ.trieste.it; Fax: 39 040 6763903; Tel: 39 040 6763920
Received (in Cambridge, UK) 15th January 2002, Accepted 5th March 2002
First published as an Advance Article on the web 15th March 2002
An unprecedented combination of high chemo- and ster-
eoselectivity in the NaBH4 reduction of 1+1 complexes
between albumin and aromatic 1,3-diketones results in the
formation of anti 1,3-diols with de up to 96%.
formation of the corresponding anti-diols 4 with de up to
96%.‡
The reduction of the dicarbonyl compounds 1 (Table 1) with
sodium borohydride in 9+1 water–acetonitrile was followed by
HPLC. In the absence of albumins the first reduction is not
chemoselective and at 50% conversion the hydroxyketones 2
and 3 are present in nearly equimolar amounts (Table 1). Little
or no diastereoselectivity is observed in the overall reduction to
the anti and syn diols 4 and 5.
When the reduction of diketones 1 is carried out in the
presence of an equimolar amount of BSA, high levels of chemo-
and stereoselectivity are observed (Table 1).§ With aryl-alkyl
diketones 1a–g the first reduction takes place preferentially at
the aliphatic carbonyl giving b-hydroxyketones 2, that are
almost the sole monoreduction products observed at any
conversion. Further reduction of 2 then yields racemic anti diols
4 with dr up to 98+2. Large aromatic groups such as p-
iodophenyl (1d) or naphthyl (1e) lead to a decrease in selectivity
(anti+syn ratio of 75+25 and 80+20, respectively). The selectiv-
ity also decreases when a bulky aliphatic group is present, as in
1g; the presence of BSA, however, results in a shift of
selectivity from 66+33 in favour of the syn diol 5g to 55+45 in
favour of the anti isomer 4g. A stereoselective reduction is also
observed in the symmetric diketone 1h and in the branched
diketone 1i.
Albumins are well known for their ability to bind a wide range
of small organic molecules.1 While this property is inherent to
their biological role as transport proteins, it can also be
exploited to control the reactivity of bound species. Thus, the
hydrophobic binding site and the presence of a general base in
proximity of the bound substrate are at the basis of albumin’s
ability to behave as enzyme-like catalysts in reactions such as
the Kemp elimination2 and the decomposition of Meisenheimer
complexes.3 The asymmetric environment surrounding the
bound substrate, on the other hand, allows albumins to be used
as chiral auxiliaries and catalysts.4
We report here that albumins induce high levels of ster-
eoselectivity in the borohydride reduction of b-diketones 1 and
b-hydroxyketones 2 (Scheme 1, Table 1), leading to the
The presence of an aromatic ring is an essential requirement
for the BSA-directed anti reduction of 1,3-diketones. This is
clearly demonstrated by the reduction of pentane-1,3-dione 1l
which is unaffected by albumin giving a 1+1 mixture of syn and
anti diols.
The regioisomeric hydroxyketones 2b and 3b (Table 1) were
independently synthesized and reduced with NaBH4 in the
presence of BSA. Reduction of racemic 2b proceeds with high
diastereoselectivity giving the anti/syn pair of diols 4b and 5b,
as racemic mixtures§ in a ratio (95+5) comparable with that
obtained in the direct reduction of the diketone 1b. Under the
same conditions, enantiomerically pure (+)-(S)-2b, obtained by
baker’s yeast reduction of diketone 1b,5 gave (Scheme 1) the
Scheme 1
† Electronic supplementary information (ESI) available: Scatchard and
Lineweaver–Burk plots. See http://www.rsc.org/suppdata/cc/b2/b200474g/
Table 1 Albumin-directed reduction of 1,3-diketones and 3-hydroxyketones
2+3a
anti (4)+syn (5)b
Substrate
R
RA
RB
No albumin
BSA
No albumin
BSA
1a
1b
1c
1d
1e
1f
1g
1h
Ph
Me
Me
(CH2)3OH
Me
Me
Me
t-Bu
Ph
Me
Me
H
H
H
H
H
H
H
H
Me
H
H
H
H
60+40
66+33
60+40
65+35
65+35
60+40
40+60
—
nd
—
—
—
> 98+2
> 98+2
> 95+5
80+20
90+10
90+10
55+45
—
nd
—
—
—
47+53
93+7
p-MeC6H4
p-MeC6H4
p-IC6H4
2-Naphthyl
Ferrocenyl
Ph
Ph
Ph
Me
45+55
98+2
40+60
> 95+5
75+25
80+20
94+6
43+57
45+55
60+40
33+66
56+44
96+4
52+40
1i
1l
33+27+15+25c
50+50
5+45+5+45c
50+50
95+5
( )-2b
(+)-S-2b
3b
p-MeC6H4
p-MeC6H4
p-MeC6H4
Me
Me
Me
45+55
45+55
94+6d
50+50
—
—
50+50
a At 50% conversion. b At 100% conversion. c Reduction gives four diols that have been separated by HPLC but not identified. d (+)-(1R,3S)-4b+ ee >
97%.
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CHEM. COMMUN., 2002, 828–829
This journal is © The Royal Society of Chemistry 2002

anti diol (+)-(1R,3S)-4b with the same 95+5 dr observed for the
racemic mixture and an enantiomeric excess greater than 97%.
This clearly indicates that BSA does not discriminate between
enantiomeric hydroxyketones 2 and that hydride addition takes
place on both enantiomers with the same anti-stereoselectiv-
ity.
of the competition experiments.† Hydroxyketone 3b does not
inhibit binding of the diketone 1b, thus indicating that this
substrate is not recognized by BSA, at least in the same site.
Similarly, neither the anti nor the syn diols 4b and 5b inhibit
diketone binding.
From these findings a mechanistic hypothesis can be
proposed. In the presence of albumin the first reduction of
diketones 4 is highly regioselective favouring the formation of
hydroxyketones 2 (Scheme 1). The UV and CD experiments
indicate that the substrate is converted into enol upon binding,
but do not exclude that a fraction of substrate is bound as the
more reactive diketone tautomer (UV-transparent). While it is
not possible at this stage to identify the reactive species, the
control of chemo-selectivity by binding to albumins is, to our
knowledge, unprecedented. Following the first reduction, both
enantiomers of 2 are recognized by BSA in such a way that the
second addition of hydride is directed toward the formation of
the anti diol. Hydroxyketones 3 are not recognized by the
protein and their reduction takes place without stereoselection.
The anti-stereoselectivity in the reduction of diketones to diols
thus originates from a combination of chemo-selectivity in the
first step and diastereoselectivity in the second.
The reduction of hydroxyketone 3b, on the contrary, is
unaffected by BSA, leading to a 1+1 mixture of syn and anti
diols (Table 1). This is consistent with the observation that a
carbonyl adjacent to an aromatic ring is an essential feature for
recognition by the protein and stereoselective reduction.
In the presence of BSA, a marked change is observed in the
UV spectrum of diketone 1b, the maximum at 316 nm, due to
the enol form, being shifted to 325 nm with a corresponding
increase in absorbance (Fig. 1a). This strong absorption is also
observed in the CD spectrum (Fig. 1b): the absence of
asymmetric centres in the substrate and the non-linear depend-
ence from the substrate concentration (Fig. 1b) indicate that this
band is due to a conjugated species bound to the protein. A
Scatchard analysis† of the CD data reveals a single binding site
for 1b with an association constant of 2.4 3 104 l mol21
.
In order to investigate the nature of the bound species,
diketone 1b and BSA were incubated in H218O for 5 days at
room temperature, after which time no 18O incorporation in the
diketone was detected by ES-MS. Reaction of the diketone with
the lysine residue present in the IIA binding site of BSA (Lys
199)6 can thus be excluded as the reversible formation of an
enaminone should lead to a fast isotope exchange in the
diketone.7 The characteristic UV and CD spectra of Fig. 1 can
thus be attributed to the enol form of diketone 4b non-
covalently bound to albumin. The value of the binding constant
obtained by the Scatchard analysis is consistent with literature
data on the formation of non-covalent complexes between
albumin and small aromatic molecules.6
Work is in progress to verify the proposed mechanism and the
structure of the complexes. The synthetic utility of the albumin-
directed reduction is also being explored under catalytic
conditions.
We are grateful to CNR for financial suport and to Polytech
for the gift of a chiral HPLC column.
Notes and references
‡ The stereochemistry of the diols was assigned from the 13C shifts of the
corresponding acetonides: S. D. Rychnovsky and D. J. Skalitzky,
Tetrahedron Lett., 1990, 31, 945.
§ In a typical experiment, 6 eq. of NaBH4 were added to a solution of BSA
(100 mg, 1.5 mmol) and the diketone in 2 ml of H2O–CH3CN 9+1. The
reaction mixture was sampled by drawing 200 ml aliquots to which 200 ml
of ethanol and 5 ml TFA were added. The denaturated protein was removed
by centrifugation and the solution was analyzed by HPLC with an Alltech
Alltima C18 column. The enantiomeric excess of diol 4b was determined by
HPLC using a Polytech CHIRAL-PS1 column. The diol was obtained as a
racemic mixture (ee @ 5%), except in the reduction of (+)-2b giving (+)-4b
(ee > 97%). Hydroxyketone 2a and diols 4a and 4b were also obtained
from a 100 mg scale reduction of 1a and 1b, respectively. After extraction
of the reaction mixture with ether and purification by preparative TLC, they
resulted optically inactive.
Binding of hydroxyketones 2 and 3 to albumins can not be
studied directly, since no spectral changes are observed.
However, recognition of diketone 1b by BSA is competitively
and reversibly inhibited by the hydroxyketone 2b. The
inhibition effect is rather large, indicating that the binding
constants of 4b and 2b are similar, and a value of 9 3 103 l
mol21 for the association constant of this hydroxyketone with
BSA was obtained by a preliminary Lineweaver–Burk analysis
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Fig. 1 (a) UV spectra of BSA (44444), diketone 1b (---), and a 1+1
mixture of BSA and 1b (——). All substrates are 160 mM in water. (b) CD
spectra of 60 mM BSA (——) and of BSA/1b mixtures. [BSA] = 60 mM;
[1b] = 30mM (44444), 60 mM (---), 90 mM (-4-4-) and 120 mM (––
––).
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