Phosphothreonine as a Catalytic Residue in Peptide-Mediated Asymmetric Transfer Hydrogenations of 8-Aminoquinolines

Article abstract of DOI:10.1002/anie.201505898

Phosphothreonine (pThr) was found to constitute a new class of chiral phosphoric acid (CPA) catalyst upon insertion into peptides. To demonstrate the potential of these phosphopeptides as asymmetric catalysts, enantioselective transfer hydrogenations of a previously underexplored substrate class for CPA-catalyzed reductions were carried out. pThr-containing peptides lead to the observation of enantioselectivities of up to 94:6 e.r. with 2-substituted quinolines containing C8-amino functionality. NMR studies indicate that hydrogen-bonding interactions promote strong complexation between substrates and a rigid β-turn catalyst.

Full text of DOI:10.1002/anie.201505898

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201505898
German Edition: DOI: 10.1002/ange.201505898
Asymmetric Catalysis
Phosphothreonine as a Catalytic Residue in Peptide-Mediated
Asymmetric Transfer Hydrogenations of 8-Aminoquinolines
Christopher R. Shugrue and Scott J. Miller*
Abstract: Phosphothreonine (pThr) was found to constitute
a new class of chiral phosphoric acid (CPA) catalyst upon
insertion into peptides. To demonstrate the potential of these
phosphopeptides as asymmetric catalysts, enantioselective
transfer hydrogenations of a previously underexplored sub-
strate class for CPA-catalyzed reductions were carried out.
pThr-containing peptides lead to the observation of enantio-
selectivities of up to 94:6 e.r. with 2-substituted quinolines
containing C8-amino functionality. NMR studies indicate that
hydrogen-bonding interactions promote strong complexation
between substrates and a rigid b-turn catalyst.
mentally interesting stereoselective reactions.[4] In this study,
we report the enantioselective transfer hydrogenation of 8-
aminoquinolines[5,6] mediated by phosphothreonine (pThr)-
containing peptides with the NADH-like dihydropyridine
reductant Hantzsch ester hydride (HEH).
Among the myriad of reported asymmetric chiral phos-
phoric acid (CPA) catalysts, the pioneering C2-symmetric
BINOL-derived catalytic scaffold (2), independently
reported by Akiyama and Terada in 2004, has dominated
the field.[7] Important variations on this substructure (3–4)
complement this impressive record (Figure 1a),[8] but C2-
symmetry is a pervasive feature. Indeed, a radical departure
from these established design principles has remained elusive.
A catalyst framework that is not limited to C2-symmetry could
be particularly important, especially if catalysts in this family
are to be applied to the selective derivatization of complex
molecules.[9]
F
ew organocatalytic systems have been demonstrated to be
as versatile in the realm of asymmetric synthesis as chiral
phosphoric acids (Figure 1). Their prowess as chiral Brønsted
acids and chiral counterions has been demonstrated over
a remarkable range of enantioselective reactions of interest to
synthetic chemists.[1] In parallel, but seemingly unrelated, the
phosphorylation of proteinogenic amino acids, including
threonine (1), has emerged as a post-translational modifica-
tion of proteins with an ever-expanding list of functions of
biochemical significance.[2,3] In our laboratory, we continue to
study the interfaces of biochemically important building
blocks and structures for their capacity to influence funda-
In constructing a novel pThr-containing CPA, we faced
two primary challenges. First, the lack of C2-symmetry of pThr
À
precludes the equivalency of the two free P O bonds,
between which the acidic proton can rapidly tautomerize
(Figure 1b). Second, without the two phenolic hydroxy
groups tied into a BINOL-like ring, free rotation about the
À
pThr P O bond is enabled (Figure 1c). Both of these factors
can produce numerous chemically distinct acid catalyst
conformations, each exhibiting different intrinsic reaction
selectivities. Nonetheless, we wondered whether pThr embed-
ded within a well-defined peptide framework might lead to
the observation of selective reactions, perhaps through
bifunctional catalysis.[10]
We chose to test these concepts with pThr-containing
peptides (5, Scheme 1) as catalysts for the transfer hydro-
genation of quinolines, since CPAs have been shown to
mediate the reduction of variously functionalized quinolines
with high yields and selectivity.[5] Initial reaction development
commenced with 2-substituted quinolines with C8-amino
functionality owing to 1) their underexplored nature in the
field of asymmetric heterocycle reduction,[6] 2) their presence
in many bioactive agents,[11] and 3) our previously observed
Figure 1. a) Previously developed chiral CPAs: BINOL (2), SPINOL (3),
and VAPOL (4). b) A new pThr-based CPA (1). Lack of C2-symmetry
À
results in diastereotopic P OH environments. c) Free rotation of the
À
P OR bond produces non-equivalent rotamers.
[*] C. R. Shugrue, Prof. S. J. Miller
Department of Chemistry, Yale University
225 Prospect Street, New Haven, CT 06511 (USA)
E-mail: scott.miller@yale.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201505898.
Scheme 1. pThr-containing peptides (5) as catalysts for the reduction
of 6a to 7a.
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results wherein appropriately functionalized amines often
lead to effective catalyst–substrate interactions in peptide-
mediated asymmetric reactions.[12]
cyclopropane carboxylic acid (Acpc; 5c) resulted in an
increase in enantioselectivity (79:21 e.r.; Table 1, entry 3),
which could potentially be the result of Acpc affecting the
overall conformation or rigidity of the b-turn scaffold.[16]
Exchanging the C-terminal methyl ester for a dimethyl
amide (5d) led to a drop in e.r. (70:30 e.r.; Table 1, entry 4).
While extensive variation of the the i + 3 residue did not
drastically affect selectivity, it did reveal that Met (5e) was the
most effective, resulting in 82:18 e.r. (Table 1, entry 5). The
sulfur atom of Met is not critical since the carbon isostere of
Met, norleucine (Nle), was synthesized (5 f) and behaves
similarly to 5e (80:20 e.r.; Table 1, entry 6). The stereochem-
istry of the i + 3 residue exerts a small effect on selectivity
since catalyst 5g (with d-Leu in place of l-Leu at i + 3) leads
to the formation of product with 82:18 e.r. (Table 1, entry 7),
although this effect was not explored further. Critically, the
phosphoric acid itself is essential for
The synthesis of catalysts of type 5 followed conventional
peptide synthesis techniques, with a notable BOP-mediated
coupling of the free side-chain monobasic phosphoric acid
(Fmoc-pThr(Bn)-OH) to peptide sequences of interest.[13] As
shown in Table 1, a variety of pThr-containing peptides were
found to catalyze the reduction of 6a to 7a with high
conversion and good enantiomeric ratio (e.r.). Starting with
a b-turn-promoting sequence of l- or d-Pro-Aib-XXX-
OMe,[14] we were intrigued to find that catalysts with either
l-Pro (5a) or d-Pro (5b) at the i + 1 position produced the
same enantiomer of 7a, albeit with higher selectivity for d-
Pro (Table 1, entries 1 and 2).[15] We next turned to the i + 2
position, where we observed that utilization of 1-amino-
mediating the reduction since
Table 1: Optimization of the peptide sequence for transfer hydrogenation of 6a.[a,b,e]
exchanging pThr for Thr(Bn) abol-
ishes catalysis (5h; Table 1,
entry 8). Similarly, a study of trun-
cated peptides revealed that the full
tetrameric sequence was essential
for the highest selectivity with this
Entry
Cat.
i
i+1
i+2
i+3
Conv.
[%][c]
e.r.[d]
1
2
3
4
5
6
7
8
5a
5b
5c
5d
5e
5 f
5g
5h
5i
Fmoc-pThr(Bn)
Fmoc-pThr(Bn)
Fmoc-pThr(Bn)
Fmoc-pThr(Bn)
Fmoc-pThr(Bn)
Fmoc-pThr(Bn)
Fmoc-pThr(Bn)
Fmoc-Thr(Bn)
Fmoc-pThr(Bn)
Fmoc-pThr(Bn)
Fmoc-pThr(Bn)-NMe2
–
Pro
Aib
Aib
Leu-OMe
Leu-OMe
Leu-OMe
Leu-NMe2
Met-OMe
Nle-OMe
d-Leu-OMe
Met-OMe
–
88
83
98
92
99
98
98
0
77
78
92
99
60:40
72:28
79:21
70:30
82:18
80:20
82:18
n/a
60:40
60:40
45:55
14:86
d-Pro
d-Pro
d-Pro
d-Pro
d-Pro
d-Pro
d-Pro
d-Pro
d-Pro-OMe
–
Acpc
Acpc
Acpc
Acpc
Acpc
Acpc
Acpc-OMe
–
series
of
catalysts
(Table 1,
entries 9–11).
Since the pThr-containing cata-
lysts represent a departure from C2-
symmetric CPAs, we wished to
carry out a direct comparison to an
established catalyst. We observed
that (R)-TRIP (2a), a catalyst pre-
viously reported for 3-aminoquino-
line reductions, affords 7a with
14:86 e.r., thus revealing that
a pThr-containing catalyst offers
a slightly lower but still comparable
e.r. in this case (entry 12).[5h] Inter-
9
10
11
12
5j
5k
2a
–
–
–
–
–
–
[a] Reported results are the average of two trials. [b] Conditions: 0.06 mmol wrt 6a, peptide 5
(10 mol%), HEH (2.5 equiv), dichloromethane (0.05m with respect to 6a). [c] Conversion (conv.) was
determined by comparing the 1H NMR integrations of the aromatic peaks of 6a and 7a.
[d] Enantiomeric ratios were determined by HPLC using an OD-H column. [e] Abbreviations:
Fmoc=fluorenylmethyloxycarbonyl; pThr(Bn)=benzylphosphothreonine; Aib=alpha-aminoisobutyric
acid; Acpc=1-aminocyclopropane carboxylic acid; Nle, norleucine (butylglycine).
Figure 2. a) 1H NMR spectrum of 5e in CD2Cl2; b) 6b in CD2Cl2; and c) 5e and 6b (1:1 stoichiometry) in CD2Cl2. i) For 5e, the NHAcpc signal
(orange) shifts downfield and the NHpThr signal (yellow) broadens into the baseline, thus suggesting the formation of H-bonds. ii) The chemical
=
shift of the NHMet signal (red) remains constant, which is characteristic of a b-turn H-bond with the pThr C O. iii) For 6b, large downfield shifts
À
are observed for the C7 (blue) and C4 (purple) protons, thus suggesting the formation of a 5e–6b salt. iv) A large downfield shift of the C8 N H
À
(green) is further evidence for the protonation of 6b, as well as potential complexation between this N H and 5e. d) 5e and HEH (1:9
stoichiometry) in CD2Cl2. The same trends are observed for 5e as above.[18] e) Speculative ensemble of potential interactions between substrates
and catalyst. Protons in purple are believed to be engaged in hydrogen-bonding interactions.
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estingly, in other cases, the pThr-peptides deliver a slightly
higher e.r. (substrate 6 f; see below).
substitution at the quinoline C2-position (6j-6k) led to
products with lower e.r. for reasons that are not fully
understood (Table 2, entries 19–20). Finally, exchange of the
urea for acetamide (6l) and carbamate (6m) resulted in
slower reaction rates but similar and even higher e.r.
compared to isostere 6a (Table 2, entries 21–22).The absolute
stereochemistry of the major product was determined explic-
itly for compound 7 f by X-ray crystallography.[17]
With catalyst 5e in hand, we turned our attention to
substrate scope, and the catalyst was found to function well
for substrates with variations at the 8-position (Table 2). For
example, benzylic ureas (6b–6 f) led to products with up to
92:8 e.r. at RT (Table 2, entries 3, 5, 7, 9, and 11), while
aliphatic ureas (6g–6i) are also tolerated (Table 2, entries 13,
15, 17). Lowering the temperature to 48C led to increases in
e.r. in many cases (e.g., 94:6 e.r.; Table 2, entries 4, 6, 8, and
10), although extended reaction times were generally
required. Parenthetically, in the case of 6 f, we observed that
the pThr peptide delivers a slightly higher e.r. than the (R)-
TRIP catalyst 2a (13:87 e.r. for 6 f over 24 h). Aliphatic
We have begun to study the possible mechanism of action
1
of the pThr-containing peptides by examining the H NMR
chemical shifts upon mixing peptide 5e with substrate 6b or
HEH (Figure 2a–d). On the one hand, these additives have
little effect on the shift of the i + 3 NHMet signal (Figure 2,
red). Since this resonance is characteristic of a 10-membered
ring intramolecular H-bond with
=
the pThr C O (a b-turn), the
Table 2: Substrate Scope.[a,b]
addition of the substrates may not
perturb this key catalyst secondary
structural feature. On the other
hand, the NHpThr signal (Figure 2,
yellow) appears to broaden signifi-
cantly and that of i + 2 NHAcpc
(Figure 2, orange) shifts downfield
in the presence of either 6b or
HEH, thus suggesting the forma-
tion of intermolecular interactions
(e.g, through H-bonding) between
the catalyst and substrates. Down-
field shifts are also observed for the
quinoline C7 (blue) and C4
(purple) proton signals, as well as
Entry
Substrate
R1
R2
t [h]
T
Conv.[c]
[%]
Yield[d]
[%]
e.r.[e]
1
2
24
51
RT
48C
99
92
92
86
82:18
82:18
6a
Me
3
4
24
51
RT
48C
97
86
84
82
91:9
94:6
6b
6c
6d
Me
Me
Me
5
6
24
51
RT
48C
99
86
76
79
90:10
94:6
7
8
24
51
RT
48C
96
88
74
81
92:8
94:6
À
for a quinoline urea N H signal.
These chemical shift perturbations
are consistent with salt formation
between 5e and 6b.[19] The down-
9
10
24
51
RT
48C
96
86
74
79
92:8
94:6
6e
Me
À
field shift of a quinoline C8 N H
11
12
24
51
RT
48C
99
86
79
70
90:10
93:7
6 f
Me
Me
signal (Figure 2, green) could also
be ascribed to an H-bond with 5e.
Further evidence for this H-bond
was revealed through the reduction
of 2-methylquinoline with 5e, since
the lack of directing group yields
product with only 58:42 e.r.. Taken
together, these observations sup-
port a reaction for which enantio-
selectivity derives from the forma-
tion of noncovalent interactions
between 6 and the b-turn peptide
catalyst 5e (Figure 2e).
13
14
30
51
RT
48C
92
92
79
86
86:14
90:10
6g
15
16
36
51
RT
48C
97
86
73
70
88:12
90:10
6h
6i
Me
Me
17
18
30
51
RT
48C
91
83
86
74
83:17
87:13
19
20
6j
iPr
24
24
RT
RT
94
97
71
84
77:23
6k
Cy
75:25[g]
21
22
6l
Me
Me
24
24
RT
RT
52
68
n/a[f]
n/a[f]
89:11
79:21
In conclusion, we report the
development of pThr-containing
peptides as a CPA scaffold. Strik-
ingly, this framework, which lacks
the C2-symmetry of better known
CPA scaffolds, is able to overcome
the existence of both non-equiva-
lent tautomeric states and the
many rotatable bonds extant
within the catalyst. It achieves this
6m
[a] Reported results are the average of two trials. [b] Conditions: 0.06 mmol 6, peptide 5e (10 mol%),
HEH (2.5 equiv), dichloromethane (0.05m with respect to 5e). [c] Conversion (conv.) was determined by
comparing the 1H NMR integrations of the quinoline aromatic peaks for the substrates and products.
[d] Yield of isolated product after column chromotography. [e] Enantiomeric ratios were determined by
HPLC using an OD-H column. [f] 6l and 6n co-eluted with oxidized Hantzsch ester pyridine (HEox) and
could not be isolated. [g] Enantiomeric ratios were determined by chiral HPLC using an AD-H column.
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Retailleau, A. Voituriez, J. F. Betzer, A. Marinetti, Adv. Synth.
Catal. 2013, 355, 3613 – 3624; g) A. Aillerie, V. Lemau de Ta-
lancØ, A. Moncomble, T. Bousquet, L. PØlinski, Org. Lett. 2014,
16, 2982 – 2985; h) X. F. Cai, R. N. Guo, G. S. Feng, B. Wu, Y. G.
Zhou, Org. Lett. 2014, 16, 2680 – 2683.
selectivity through interactions with the substrates in the
context of the peptide secondary structure. The pThr catalyst
tolerates a range of functionality at the C8-urea and the C2-
position of the quinoline, and is able to deliver substrates with
e.r. up to 94:6 e.r.. Given the remarkable range of chemical
reactions that may be catalyzed with chiral CPAs, we are
hopeful that these new pThr-based catalysts will have
significant utility as asymmetric catalysts, perhaps especially
in the realm of chemical operations on complex substrates.
[6] Selected reports of asymmetric reductions of 1,10-disubstituted
phenanthrolines: a) K. Skupinska, E. J. McEachern, R. T. Skerlj,
G. J. Bridgr, J. Org. Chem. 2002, 67, 7890 – 7893; b) C. Metal-
linos, F. B. Barrett, S. Xu, Synlett 2008, 720 – 724; c) T. Wang, F.
Chen, J. Qin, Y.-M. He, Q.-H. Fan, Angew. Chem. Int. Ed. 2013,
52, 7172 – 7176; Angew. Chem. 2013, 125, 7313 – 7317.
[7] Seminal reports: a) D. Uraguchi, M. Terada, J. Am. Chem. Soc.
2004, 126, 5356 – 5357; b) T. Akiyama, J. Itoh, K. Yokota, K.
Fuchibe, Angew. Chem. Int. Ed. 2004, 43, 1566 – 1568; Angew.
Chem. 2004, 116, 1592 – 1594.
Acknowledgements
[8] a) F. Xu, D. Huang, C. Han, W. Shen, X. Lin, Y. Wang, J. Org.
Chem. 2010, 75, 8677 – 8680; b) G. B. Rowland, H. Zhang, E. B.
Rowland, S. Chennamadhavuni, Y. Wang, J. C. Antilla, J. Am.
Chem. Soc. 2005, 127, 15696 – 15697.
[9] a) J. Clardy, C. Walsh, Nature 2004, 432, 829 – 837; b) S. Han, S. J.
Miller, J. Am. Chem. Soc. 2013, 135, 12414 – 12421; c) S.
Yoganathan, S. J. Miller, J. Med. Chem. 2015, 58, 2367 – 2377.
[10] A. G. Doyle, E. N. Jacobsen, Chem. Rev. 2007, 107, 5713 – 5743.
[11] a) N. Vale, R. Moreira, P. Gomes, Eur. J. Med. Chem. 2009, 44,
937 – 953; b) K. Kaur, M. Jain, R. P. Reddy, R. Jain, Eur. J. Med.
Chem. 2010, 45, 3245 – 3264; c) M. Delves, D. Plouffe, C.
Scheurer, S. Meister, S. Wittlin, E. A. Winzeler, R. E. Sinden,
D. Lerory, PLOS Med. 2012, 9, e1001169.
[12] a) M. M. Vasbinder, E. R. Jarvo, S. J. Miller, Angew. Chem. Int.
Ed. 2001, 40, 2824 – 2827; Angew. Chem. 2001, 113, 2906 – 2909;
b) D. K. Romney, S. M. Colvin, S. J. Miller, J. Am. Chem. Soc.
2014, 136, 14019 – 14022.
[13] Catalyst identities for screening were verified on the basis of
mass spectrometry and NMR analysis, with 31P as particularly
diagnostic. The identity of the optimized catalyst (5e) for this
work was determined by additional characterization and also
confirmed as the monobasic phosphoric acid by combustion
analysis. See the Supporting Information for details.
C.R.S. thanks the National Science Foundation Graduate
Research Fellowship Program. This work was supported by
National Institutes of Health (NIH R01-GM096403). We
would like to thank Dr. C. L. Allen for helpful discussions,
and Dr. B. Q. Mercado for crystallographic structural deter-
minations for 7 f.
Keywords: asymmetric catalysis · peptides · phosphothreonine ·
quinoline · transfer hydrogenation
How to cite: Angew. Chem. Int. Ed. 2015, 54, 11173–11176
Angew. Chem. 2015, 127, 11325–11328
[1] Selected reviews: a) M. Terada, Synthesis 2010, 1929 – 1982;
b) A. Zamfir, S. Schenker, M. Freund, B. S. Tsogoeva, Org.
Biomol. Chem. 2010, 8, 5262 – 5276; c) M. Rueping, A. Kuenkel,
I. Atodiresei, Chem. Soc. Rev. 2011, 40, 4539 – 4549; d) D.
Parmar, E. Sugiono, S. Raja, M. Rueping, Chem. Rev. 2014, 114,
9047 – 9153.
[2] For selected examples, see: a) T. Pawson, D. J. Scott, Science
1997, 278, 2075 – 2080; b) P. J. Lu, X. Z. Zhou, M. H. Shen, K. P.
Lu, Science 1999, 283, 1325 – 1328.
[3] Selected reviews: a) T. Hunter, Cell 2000, 100, 113 – 127; b) M. B.
Yaffe, A. E. H. Elia, Curr. Opin. Cell Biol. 2001, 13, 131 – 138.
[4] Selected examples: a) A. Dermenci, P. S. Selig, R. A. Domaoal,
K. A. Spasov, K. S. Anderson, S. J. Miller, Chem. Sci. 2011, 2,
1568 – 1572; b) P. A. Lichtor, S. J. Miller, Nat. Chem. 2012, 4,
990 – 995.
[5] Selected reports: a) M. Rueping, A. P. Antonchick, T. Theiss-
mann, Angew. Chem. Int. Ed. 2006, 45, 3683 – 3686; Angew.
Chem. 2006, 118, 3765 – 3768; b) Q. S. Guo, D. M. Du, J. T. Xu,
Angew. Chem. Int. Ed. 2008, 47, 759 – 762; Angew. Chem. 2008,
120, 771 – 774; c) M. Rueping, M. Stoechel, E. Sugiono, T.
Theissmann, Tetrahedron 2010, 66, 6565 – 6568; d) M. Rueping,
T. Theissmann, M. Stoeckel, A. P. Antonchick, Org. Biomol.
Chem. 2011, 9, 6844 – 6850; e) X. F. Cai, M. W. Chen, Z. S. Ye,
R. N. Guo, L. Shi, Y. Q. Li, Y. G. Zhou, Chem. Asian J. 2013, 8,
1381 – 1385; f) J. Stemper, K. Isaac, J. Pastor, G. Frison, P.
[14] E. A. Colby Davie, S. M. Mennen, Y. Xu, S. J. Miller, Chem. Rev.
2007, 107, 5759 – 5812.
[15] G. T. Copeland, E. R. Jarvo, S. J. Miller, J. Org. Chem. 1998, 63,
6784 – 6785.
[16] A. J. Metrano, S. J. Miller, J. Org. Chem. 2014, 79, 1542 – 1554.
[17] See the Supporting Information for structure and details.
CCDC 1412999 (7 f) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre.
À
[18] The singlet at 8.7 ppm corresponds to the aromatic C H of
HEox, thus showing that 5e converts HEH to HEox.
[19] G. Barbieri, R. Benassi, P. Lazzeretti, L. Schenetti, F. Taddei,
Org. Magn. Resonance 1975, 7, 451 – 454.
Received: June 26, 2015
Published online: August 5, 2015
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Angew. Chem. Int. Ed. 2015, 54, 11173 –11176