Single nucleotide-catalyzed biomimetic reductive amination

Article abstract of DOI:10.1002/adsc.201000178

We have successfully developed a single nucleotide (adenosine 5'-diphosphate)-catalyzed enantioselective direct reductive amination of aldehydes and ketones using a Hantzsch ester as reducing agent. The process is a simple, efficient and a real mimic of the NADH reduction approach for the synthesis of structurally diverse amines. This reaction is the first report demonstrating the ability of a single nucleotide as catalyst and one of the most genuine biomimetic reactions of organic chemistry.

Full text of DOI:10.1002/adsc.201000178

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DOI: 10.1002/adsc.201000178
Single Nucleotide-Catalyzed Biomimetic Reductive Amination
Atul Kumar,a,* Siddharth Sharma,a and Ram Awatar Mauryaa
a
Medicinal and Process Chemistry Division, Central Drug Research Institute, CSIR, Lucknow – 226001, India
Fax : (+91)-522-2623-3405; e-mail: dratulsax@gmail.com
Received: March 8, 2010; Revised: June 18, 2010; Published online: September 8, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000178.
Abstract: We have successfully developed a single is the first report demonstrating the ability of a
nucleotide (adenosine 5’-diphosphate)-catalyzed single nucleotide as catalyst and one of the most gen-
enantioselective direct reductive amination of alde- uine biomimetic reactions of organic chemistry.
hydes and ketones using a Hantzsch ester as reduc-
ing agent. The process is a simple, efficient and a
real mimic of the NADH reduction approach for the Keywords: biomimetic process; Hantzsch esters; nu-
synthesis of structurally diverse amines. This reaction cleotides; reductive amination
Introduction
studies, we have selected reductive amination, which
is a distinguished and vital reaction in biosynthesis.
It has long been established that nature has perfect-
Nucleic acids as catalyst were merely a theoretical
possibility until the discovery of catalytic RNAs (ribo- ed the reduction of ketimines as an in vivo chemical
zymes) in the early 1980s.[1] Today, a variety of natural tool for the synthesis of essential biomonomers. As a
ribozymes,[2] many artificial ribozymes as well as de- preeminent example, transferase enzymes in combina-
oxyribozymes have been identified through in-vitro tion with a dinucleotide (NADH), selectively reduce
selection from pools of random sequences[3] and were pyruvate-derived ketimines, thereby ensuring the for-
found to catalyze many chemical reactions such as, mation of naturally occurring amino acids.[11] This
phosphodiester cleavage, ligation,[4] RNA polymeri- finding prompted us to explore whether the conceptu-
zation,[5] redox reactions,[6] carbon-carbon bond for- al blueprints of biochemical amination could be trans-
mation (Diels–Alder reaction),[7,8] enantioselective lated to a reduction of ketimines wherein enzymes
Michael reactions.[9] The catalytic activity of nucleic and cofactors (NADH) are replaced by single nucleo-
acid place DNA and RNA[10] as the primordial macro- tide catalysts analogues.
molecules in the emergence of life at the earth. Thus,
This single nucleotide-catalyzed transfer hydroge-
“RNA world” and “DNA world” hypothesis for the nation approach is one of the closest biomimetic reac-
origin of life was postulated. Since the basic units of tions of organic chemistry. It is also of great interest
these nucleic acids are the nucleotides, we argued to investigate the catalytic efficiency of nucleotides in
whether a single nucleotide could be an efficient cata- organic synthesis.
lyst in organic reactions. The literature survey reveals
that there are no reports on the application of single bination with a Hantzsch ester is the most genuine
nucleotides as catalyst in organic reactions. catalytic model mimicking the naturally occurring re-
Nucleotide adenosine diphosphate (ADP) in com-
A variety of enzyme cofactors such as coenzyme A, ducing agent NAD(P)H (Figure 1). In this paper, for
nicotinamide adenine dinucleotide (NAD+) and flavin the first time, we are reporting the use of a single nu-
adenine dinucleotide (FAD) serving a wide range of cleotide (ADP) as a catalyst for direct asymmetric re-
biochemical functions, include adenosine as part of ductive amination.
their structure. The adenosine portionꢀs participation
Imine reduction is one of the most useful methods
in the primary function is not yet very clear but the for the preparation of amines and related functional
removal of adenosine generally results in a drastic re- compounds.[12–17] Recently some fascinating reports on
duction of cofactor activities. Based on these finding asymmetric imine reduction using substituted chiral
we selected adenosine nucleotides as catalysts to dem- binaphthyl hydrogen phosphate as catalyst were re-
onstrate the catalysis by single nucleotides. For these ported by List (5h),[18] and Rueping (5g).[19] Extensive
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Figure 1. Biomimetic catalyst design.
studies on enantioselective imine reduction have been
done by Sun,[20] and Kocovsky using trichlorosilane
and a Lewis base organocatalyst. However, only Mac-
Millan et al. (5i)[22] have reported the synthesis of
enantioselective amines via reductive amination using
an ortho-triphenylsilyl variant of the Terada–Akiyama
catalyst. This prompted us to develop a nucleotide-
catalyzed reductive amination instead of a conven-
tional imine reduction.
[21]
ˇ
´
Scheme 1. Nucleotide-catalyzed biomimetic reductive ami-
nation.
Results and Discussion
for achieving both the high yields and enantioselectiv-
ity. Introduction of 4ꢂ molecular sieve increased the
Initial evaluation of the proposed reductive amination yield of 3a to a significant extent. This reveals that
was performed with acetophenone 1a, aniline 2a, water, generated in the initial condensation step, has
ethyl Hantzsch ester 4b, and a number of organocata- a deleterious impact on both iminium formation and
lysts (Scheme 1, Table 1). While adenine 5a and ade- the hydride reduction step. As such, the introduction
nosine 5b did not induce reductive amination to any of 4ꢂ molecular sieves was found to be critical to
significant extent, adenosine 3’-monophosphate 5c, achieve a useful reaction rate and selectivity.
adenosine 5’-monophosphate 5d, ADP 5e and ATP 5f
We observed that 2 mol% of ADP was sufficient to
did provide the desired amine product in excellent push the reaction forward and further increasing the
yields (Figure 2). ADP 5e was found to be the best amount of catalyst (2 to 5 mol%) did not improve the
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Single Nucleotide-Catalyzed Biomimetic Reductive Amination
Table 1. Screening of organocatalysts for the reductive ami-
nation of acetophenone with aniline and ethyl Hantzsch
ester.[a]
Table 2. Screening of solvents.
Entry Catalyst Additive Time
[h]
ee
Yield of 3a
[%][b] [%][c]1
1
2
3
4
5
6
7
8
9
none
5a
none
none
none
none
none
none
none
72
72
72
48
48
48
48
–
–
–
40
44
60
57
81
62
0
<5
Entry[a]
X
Solvent Time
[h]
Conversion
[%][b]
ee
5b
<5
60
55
78
70
95
95
[mol%]
[%][c]
5c[d]
5d[e]
5e[f]
5f[e]
5e
1
2
3
4
5
6
7
8
5
5
5
5
5
5
5
5
5
2
2
1
CHCl3 72
47
51
62
63
85
69
79
76
97
96
59
89
56
59
72
74
79
66
75
71
64
82
66
63
DCM
dioxane 72
THF 48
72
4ꢂ MS 48
4ꢂ MS 48
5e[f]
xylene 48
AcOEt 48
benzene 48
[a]
Reaction conditions: acetophenone (1 mmol), aniline
(1 mmol), ethyl Hantzsch ester (1.1 mmol), catalyst
2 mol%, toluene (5 mL), 308C, N2 atomosphere.
Enantiomeric exess was determined by chiral HPLC.
Isolated yield.
Free acids.
Sodium salt.
5 mol% catalyst.
Et2O
48
[b]
[c]
[d]
[e]
[f]
9
toluene 48
toluene 48
toluene 48
toluene 48
10
11[d]
12
[a]
Reaction conditions: acetophenone (1 mmol), aniline
(1 mmol), 1.10 mmol of 4b.
Determined by H NMR.
Determined by chiral HPLC analysis.
Reaction was carried out at 08C.
[b]
[c]
[d]
1
yield or selectivity to any significant extent. Yield and
ee were significantly decrease by reducing the amount
of the catalyst to 1 mol% (Table 2, entry 12). With
2 mol% of 5e as the catalyst at room temperature,
various solvents have been examined for this reaction,
as summarized in Table 2. The asymmetric transfer in the ADP-catalyzed reductive amination of aceto-
hydrogenations in several solvents such as benzene, phenone with aniline. Methyl Hantszsch ester 4a, as
dichloromethane (DCM), ethyl acetate, THF, MeOH, reducing agent resulted in a poor yield of 3a. This
CHCl3, dioxane, Et2O, toluene and xylene have been could be attributed to the poor solubility of methyl
evaluated. Toluene was chosen as the optimal solvent Hantzsch ester 4a. Butyl Hantzsch ester 4c resulted in
since the reaction in toluene led to complete conver- reasonably good yields and enantioselectivity but the
sion in 48 h (Table 2, entry 10). The ee has been de- reaction took a longer time to complete (3 days).
creased by lowering the temperature to 08C (Table 2, Ethyl Hantzsch ester 4b was found to be the best for
entry 11).
achieving both the high yields and enantioselectivity;
We further investigated the suitability of methyl, thus, further experiments were carried out using 4b as
ethyl and tert-butyl Hantzsch esters as reducing agent reducing agent (Table 3).
Figure 2. Nucleotides and precursors screened for catalytic activity.
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Atul Kumar et al.
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Table 3. Investigation of Hantzsch esters.
sis of imine salts of ADP but we could not succeed in
getting a good crystal of imine salt of ADP to date.
Having established the optimal conditions for the
reductive amination of ketones, we next examined the
scope of the ketone and amine components in this nu-
cleotide-catalyzed asymmetric reductive amination.
However, presently this reductive amination is re-
stricted to a smaller substrate spectrum. As revealed
in Figure 4, a variety of substituted acetophenone de-
rivatives can be successfully coupled with a number
of primary amines including electron-rich as well as
electron-deficient aryl ketone systems. The products
were obtained generally in good enantioselectivity
and excellent yields. However, reductive aminations
Entry[a] Hantzsch
Ester
Time
[h]
Conversion
[%][b]
ee
[%][c]
1
2
3
4a
4b
4c
68
48
74
65
95
77
63
81
69
[a]
Reaction conditions: acetophenone (1 mmol), aniline
(1 mmol), 2 mol% of 5e in toluene at room temperature.
Determined by H NMR.
[b]
[c]
1
Determined by chiral HPLC analysis.
The structure-selectivity relationship suggests that with secondary amines were unsuccessful.
the 3’-nucleotide 5c should induce greater enantiose- In order to examine the versatility of the single nu-
lectivity than the remaining 5’-nucleotides 5d, 5e and cleotide-catalyzed reductive amination, we carried out
5f as the phosphoric acid group is directly attached to the ADP-catalyzed reductive amination of aldehydes
the chiral carbon in 3’-nucleotide 5c but the experi- (Scheme 2, Table 4). The reactions were carried out in
mental results are against it. The nucleotides with di-/ dry toluene under a nitrogen atomosphere. Ethyl
triphosphate groups (5e and 5f) are showing some- Hantzsch ester 4b was taken as reducing agent.
what better enantioselectvity than nucleotides with
monophosphate groups (5c and 5d).
Recently Goodman and co-worker have reported
on computational studies[23] of the Hantzsch ester hy-
drogenation of imines catalyzed by chiral BINOL-
phosphoric acid; based on this report we have pro-
posed a transition state of ADP-bound imine and
Hantzsch ester structure, in which the phosphate
group is attached with both the Schiff base and the
Hantzsch ester, respectively, and the transition state
might be further stabilized via pi–pi interactions be-
tween adenine and theSchiff base. The Re face of the
imine is sterically more crowed in comparison to the
Si face. Thus, the transfer hydrogenation with
Hantzsch ester occurs predominantly from the less
hindered Si face of the imine. We have achieved good
enantiomeric excess in the reductive amination of ke-
tones (ee 75–89%). These experimental results are
well explained by the transition state (Figure 3). In
order to find an exact picture of the transition state,
we attempted to perform a single crystal X-ray analy-
Figure 4. Scope of ADP-catalyzed reductive amination of
ketones.
Figure 3. Plausible transition state for the catalyst-bound
imine and Hantzsch ester structure.
Scheme 2. Biomimetic reductive amination of aldehydes.
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Single Nucleotide-Catalyzed Biomimetic Reductive Amination
Table 4. ADP-catalyzed reductive amination of aldehydes.[a]
liquid chromatography (HPLC) was performed on Hewlett–
Packard 1100 Series chromatographs using a Daicel Chiracel
OD-H column (25 cm) and equivalent guard column (5 cm)
and HPLC data for compounds 3a–3i are provided in the
Supporting Information. The absolute configurations were
determined by comparing the sign of optical rotation with
values form the literature. Optical rotations were measured
on a Rudolf Autopol III polarimeter, and [a]tD values are re-
ported in 10À1 dgcm2 gÀ1; concentration (c) is in g/100 mL.
Entry R4
R5
Product Yield [%][b]
1
2
3
4
5
6
7
8
9
10
C6H5
C6H4CH=CH C6H5
4-NO2-C6H4 8a
91
93
94
95
92
92
94
93
91
96
8b
8c
8d
C6H5
2-furyl
C6H5
C6H5
4-OCH3-C6H4 4-NO2-C6H4 8e
C6H5
C6H5CH2
8f
8g
8h
8i
4-Cl-C6H4
C6H5
Typical Experimental Procedure for the synthesis of
Hantzsch Ester (4b)
3-OCH3-C6H4 C6H5
n-pentyl C6H5
3-methylbutyl 4-F-C6H4
8j
[a]
Reaction conditions: aldehyde (1 mmol), amine
(1 mmol), ethyl Hantzsch ester (1.1 mmol), ADP
(2 mol%), toluene (5 m), 308C, N2 atomosphere.
Isolated yield.
[b]
In a 50-mL round-bottom flask, 37% aqueous formaldehyde
(0.81 mL, 10 mmol), ethyl acetoacetate (2.60 g, 20 mmol),
ammonium acetate (0.77 g, 10 mmol), PTSA (0.038 g, 20
mol%) and 15 mL methanol were charged. The resulting
mixture was stirred at room temperature (4 h) until the re-
action was complete (TLC). After the completion of reac-
tion, the solvent was evaporated to yield a crude product.
Pure ethyl Hantzsch ester 4b was obtained by crystallization
of the crude product from methanol; yield: 2.38 g (94%).
Conclusions
In conclusion, for the first time we have demonstrated
that a single nucleotide (adenosine 5’-diphosphate) is
able to catalyze the direct asymmetric reductive ami-
nation of carbonyl compounds using a Hantzsch ester
as transfer hydrogenating agent, resulting in structur-
ally diverse amines. The process is a real mimic of
NADH reduction in biological system in terms of se-
lection of the hydrogen transfer agent as well as cata-
lyst. Besides this, one major advantage of using nucle-
otides as catalyst is that they are commercially avail-
able. This fascinating single nucleotide catalysis have
an immense impact in many fields of science such as
chemistry, biochemistry and even in prebiotic studies,
especially the RNA world and DNA world hypothesis
for understanding the origin of life.
1
ESI-MS: m/z=254 (M+H)+; H NMR (CDCl3, 200 MHz):
d=1.25 (t, J=7.1 Hz, 6H), 2.12 (s, 6H), 4.15 (q, J=7.1 Hz,
4H), 3.19 (s, 2H), 5.20 (bs, 1H).
Typical Experimental Procedure for Reductive
Amination of Aldehydes/Ketones
A solution of acetophenone 1a (0.120 g, 1.00 mmol) and ani-
line 2a (0.093 g, 1.00 mmol) in 5 mL of dry toluene was
treated with the Hantzsch ester 4b (0.280 g, 1.10 mmol), ade-
nosine 5’-diphosphate 5e (0.0086 g, 0.02 mmol) and 4ꢂ mo-
lecular sieve (1.0 g). The mixture was stirred 2 days at 308C
under nitrogen. After filtration over celite, the solvent was
evaporated and the residue purified by flash chromatogra-
phy on silica gel (hexane/ethyl acetate=95:5) to give 3a;
yield: 96%. The enantiomeric excess was measured by using
HPLC with a chiral column. The absolute configurations de-
termined by comparing the sign of optical rotation with
values from the literature.
Experimental Section
General Information
Unless otherwise specified all the reagents and catalysts
were purchased from Sigma–Aldrich and were used without
further any purification. The common and HPLC grade sol-
vents were purchased from Ranbaxy. Organic solutions were
concentrated under reduced pressure on a Bꢃchi rotary
evaporator. Molecular sieves (4 ꢂ) were activated by flame
under vacuum and stored at 1808C. Chromatographic purifi-
cation of products was accomplished using flash chromatog-
raphy on 230–400 mesh silica gel. Reactions were monitored
by thin-layer chromatography (TLC) on 0.25 mm silica gel
plates visualized under UV light, iodine or KMnO4 staining.
1H and 13C NMR spectra were recorded on a Bruker DRX-
300 spectrometer. Chemical shifts (d) are given in ppm rela-
tive to TMS and coupling constants (J) are in Hz. IR spectra
Acknowledgements
The authors SS and RAM are thankful to CSIR New Delhi
for the award of senior research fellowships. The authors also
acknowledge the SAIF-CDRI for providing the spectral and
analytical data. CDRI communication No. 7519.
were recorded on an FT-IR spectrophotometer type Shi- References
madzu 8201 PC and are reported in terms of frequency of
absorption (cmÀ1). Mass spectra (ESI MS) were obtained on
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