Phosphothreonine (pThr)-Based Multifunctional Peptide Catalysis for Asymmetric Baeyer-Villiger Oxidations of Cyclobutanones

Article abstract of DOI:10.1021/acscatal.8b04132

Biologically inspired phosphothreonine (pThr)-embedded peptides that function as chiral Br?nsted acid catalysts for enantioselective Baeyer-Villiger oxidations (BV) of cyclobutanones with aqueous H₂O₂ are reported herein. Complementa

Full text of DOI:10.1021/acscatal.8b04132

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Article
Phosphothreonine (pThr)–Based Multifunctional Peptide Catalysis
for Asymmetric Baeyer–Villiger Oxidations of Cyclobutanones
Aaron L. Featherston, Christopher Ryan Shugrue, Brandon Q. Mercado, and Scott J. Miller
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04132 • Publication Date (Web): 20 Nov 2018
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ACS Catalysis
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Phosphothreonine (pThr)–Based Multifunctional Peptide Catalysis for
Asymmetric Baeyer–Villiger Oxidations of Cyclobutanones
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Aaron L. Featherston, Christopher R. Shugrue, Brandon Q. Mercado, and Scott J. Miller*
Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520-8107, United States
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ABSTRACT: Biologically inspired phosphothreonine (pThr)-embedded peptides that function as chiral
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Brønsted acid catalysts for enantioselective Baeyer–Villiger oxidations (BV) of cyclobutanones with
aqueous H
2
O are reported herein. Complementary to traditional BINOL-derived chiral phosphoric acids
2
8
9
0
1
2
1
1
1
0
1
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(CPAs), the functional diversity of the peptidic scaffold provides the opportunity for multiple points of
contact with substrates via hydrogen bonding, and the ease of peptide synthesis facilitates rapid
diversification of the catalyst structure, such that numerous unique peptide-based CPA catalysts have
been prepared. Utilizing a hypothesis-driven design, we identified a pThr-based catalyst that contains
an N-acylated diaminopropionic acid (Dap) residue, which achieves high enantioselectivity with catalyst
loadings as low as 0.5 mol%. The power of peptide-based multi-site binding is further exemplified
through reversal in the absolute stereochemical outcome upon repositioning of the substrate–directing
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L
L
group (ortho- to meta). Modifications to the i+3 residue ( Dap to Phe) lead to an observed
enantiodivergence without inversion of any stereogenic center on the peptide catalyst, due to
1
1
noncovalent interactions. Structure–selectivity and H– H-ROESY studies revealed that the proposed
hydrogen bonding interactions are essential for high levels of enantioinduction. The ability for the
phosphopeptides to operate as multifunctional oxidation catalysts expands the scope of pThr catalysts
and provides a framework for the future selective diversification of more complex substrates, including
natural products.
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KEYWORDS: asymmetric catalysis, Baeyer–Villiger, chiral phosphoric acid, peptides, phosphothreonine
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INTRODUCTION:
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Phosphate esters are ubiquitous in biochemistry. Phosphothreonine (pThr), among other
proteinogenic amino acids (e.g. pSer, pTyr), is the product of site-selective post-translational
2
,3
modifications of proteins, which both induces global conformational changes and binds signaling
4
molecules, as part of cellular regulation (Figure 1a). Yet, no known catalytic site of an enzyme
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engages pThr as part of the bond-forming or bond-breaking mechanistic machinery. However, chiral
phosphoric acids (CPAs) have emerged as powerful catalysts for an ever-expanding list of asymmetric
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transformations. So far, BINOL- or SPINOL-derived CPAs have dominated the expansive literature of
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CPA-catalyzed reactions (Figure 1b). As a result of their C
2
-symmetry and rigidity, BINOL CPAs have
1
a well-defined binding pocket that facilitate high levels of enantioinduction, predominately arising from
6e,7
8 11
the steric bulk on the 3,3’-positions.
While further modifications to the scaffold can attenuate the
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2
electronics and enhance enantioselectivities in select transformations, the ability to construct diverse
6,8
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CPAs with new opportunities for secondary interactions, especially hydrogen bonding, is limited.
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(
a)
(b)
ATP
ADP
Kinase
serine,
R
threonine,
or tyrosine
O
O
O
P
O
P
OH
OH
O
OH
OH
R
Protein
Pi
Phosphorylated
Protein
"
BINOL"
1
a: R = 1-pyrenyl
1b: R = 2,4,6-(i-Pr)3C6H2
Phosphatase
(c)
O
O
P
Me
N
HN
BnO
O
O
HO
O
NH
HN
Fmoc
SMe
O
R2
2
N
H
R2
N
P1 (10 mol%)
OMe
O
NH
NH
O
NH
NH
2
3
Hantzsch Ester (2.5 equiv)
R1
CH2Cl2, 0 °C
R1
13 examples;
up to 94:6 er,
up to 92% yield
=
directing group
(d)
(e)
O
O
[H8]-1a (10 mol%)
via
O
H
R2
R1
O
O
O
O
H2O2 (1.5 equiv)
O
OH
R1 R2
CHCl3, −40 °C
P
4
5
H
R1
R2
1
9
2 examples
1−99% yield
5−93% ee
R1 = H, CH3
R2 = aryl, benzyl
O
5
1
5
8
9 16
0 17
1 18
Figure 1. (a) Phosphorylation event as a post-translational modification. (b) BINOL-derived chiral phosphoric
acids. (c) Peptide-mediated transfer hydrogenation of 8-aminoquinolines using pThr as a catalytic residue. (d)
Ding and co-workers’ pioneering Brønsted acid catalyzed Baeyer–Villiger oxidation of 3-substituted
1
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cyclobutanones. (e) Proposed mechanism for the CPA catalyzed BV reaction utilizing H
2
O as the oxidant.
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2
3
As a primary focus of our group is to functionalize complex molecules, we sought to develop an
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analogous strategy with added opportunities for non-covalent interactions (NCIs). Chiral catalysts that
operate on highly functionalized substrates often require less rigid catalysts that possess an increased
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capacity to interact with substrates through multivalent NCIs, though the explicit, rational design of a
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suitable catalyst is not always straightforward.
Additionally, the choice of functionalized model
substrates provides an opportunity to examine substrate–catalyst interactions with drug-like
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molecules. Accordingly, we have initiated a program that explores pThr as a biologically inspired
catalytic residue in peptide-based Brønsted acid catalysts for a broad array of synthetically interesting
reactions. Our first proof-of-principle study established that pThr-based catalysts could indeed mediate
enantioselective reactions, including archetypal transfer hydrogenations of functionalized quinolines
0
1
2
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(Figure 1c) and reductive aminations of functionalized ketones. More recently, we have also found
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that these catalysts facilitate atroposelective cyclodehydrations in complex molecular settings.
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Reported herein are applications of pThr-based catalysts to an entirely orthogonal mode of reactivity,
the oxidative ring-expansion of cyclobutanones.
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1
2
O
Ar
4
H2O2
H
OH
4
2
OH
3
1
Ar
Ar
O
O
O
C2-migration
syn-6
C4-migration
O
O
OH
OH
Ar
H
O
Ar
(S)-5
(R)-5
4
2
3
1
anti-6
Criegee intermediates
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1
1
4
5
Scheme 1. Syn/anti Criegee intermediates.
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1
2
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1
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Discovered in 1899, the Baeyer–Villiger (BV) reaction remains an indispensable tool for
accessing chiral esters and lactones from the corresponding ketone precursors through the use of
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enzymes, organocatalysts, and transition metal complexes. Inspired by the pioneering work of
17b
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Ding and co-workers on CPA-catalyzed BV oxidation , we endeavored to learn whether pThr-based
CPAs could facilitate oxidation chemistry with comparable or complementary selectivities and achieve a
benchmark for peptide catalysts for the diversification of more complex substrates (Figure 1d). The
enantioselective BV reaction of symmetrical, cyclic ketones is particularly appealing for elucidating
catalyst control given the mechanistic ramifications. In general, the migratory aptitude is determined by
the substituent best able to stabilize the partial positive charge (e.g. 3° alkyl > 2° alkyl > phenyl > 1°
2
2
2
2
0
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2
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2
5
alkyl > methyl). However, for symmetrical ketones, the migratory aptitude of the substituents is
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identical. Furthermore, upon addition of H
O , two Criegee intermediates may be formed (syn-6 and
2
2
5
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2
2
7
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anti-6), both of which can go on to form either enantiomer of product. Therefore the stereochemical
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outcome is dictated by the ~90° bond rotation of the O–O peroxyl group to obtain the necessary σC–C to
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σ*O–O orbital overlaps for migration (Scheme 1). The consequence of possible syn- and anti- Criegee
intermediates is that the catalysts must posses the ability to control delivery of H , through the
2
O
2
differentiation of the diastereotopic faces of the ketone and/or position the dihedral angle of the peroxyl
O–O bond with high fidelity. As we show below, the application of pThr-based CPAs in the BV reaction
resulted in a remarkable level of catalyst-control through the explicit design of peptide sequences that
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exploit secondary NCIs, similar to those often associated with enzymes. Taken together, the results
herein expand the specific capabilities of CPAs for both enantioselective BV and pThr-based catalysts
to the previously unreported realm of oxidation catalysis. Yet, it could be that the most exciting
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ramification of this work is the generalization of CPA catalysis to include scaffolds beyond C -symmetry,
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including those that contain catalyst functionality that enables specific interactions with functionalized
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substrate chemotypes. Shown below are dramatic examples of stereodivergence that highlight this
point.
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Results and Discussion
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Preliminary studies in our laboratory on pThr-based peptide catalysts for the enantioselective
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BV oxidation commenced with 3-phenylcyclobutanone (4a) and peptide catalyst P2 (Scheme 2).
17b
8 17
Employing similarly established reaction conditions , the Baeyer–Villiger oxidation of 4a proceeded to
give 5a with high conversion, albeit with low enantioselectivity (55:45 er). With 4b, bearing a prochiral
quaternary carbon, γ-butyrolactone 5b was obtained in 95% conversion with a slight increase in
enantioselectivity (57:43 er). Given the modularity of peptide catalysts, we focused on sequential
residue optimization first. We chose peptide sequences that are predicted to adopt well-defined
secondary structures, namely β-turns, and identified aspects of the peptide catalyst responsible for
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enantioselectivity (Table 1).
O
O
P
Me
N
HN
BnO
HO
O
O
O
NH
HN
Fmoc
i-Pr
O
O
O
P2 (20 mol%)
OMe
O
H2O2 (30% w/w aq. 1.5 equiv)
R
R
Ph
CHCl3 (0.10 M), 4 °C, 24 h
Ph
5
4
4
4
a, R = H
5a, R = H, 97%, 55:45 er (R)
5b, R = CH3, 95%, 57:43 er
b, R = CH3
2
2
4
5
Scheme 2. Preliminary pThr-catalyzed Baeyer–Villiger oxidation of 4a–b.
2
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Given the significant influence of the C-terminal group on enantioselectivity in prior studies, the
dimethyl amide analog P3 was prepared first, which afforded 5b in 87% conversion and 64:36 er (Table
4
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2
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9
5
6
7
8
9
0
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1, entry 1) with 10 mol% catalyst loading. While other reactions investigated from our group have
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demonstrated that the i+2 residue has a pronounced effect on the enantioselectivity, modifications to
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the i+2 or i+3 positions (P4–P7) provided little influence on the enantioselectivity in the BV oxidation of
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4b (entries 2–5) (see Table 1 for “i+n” nomenclature). Investigation into the diastereomeric series of
the tetrameric peptides revealed the L-stereochemical configuration of the i+3, and D-stereochemical
configuration of Pro i+1, was crucial (entry 6–7). While a number of reaction parameters were
evaluated, permutations from the aforementioned conditions did not offer any advantages (see SI).
0
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However, when H
2
O is substituted with t-BuOOH, 5b is obtained in only 19% conversion and poor
2
3 8
enantioselectivity (entry 8). This observation suggests that a similar mechanism proposed by Ding and
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co-workers is operative, which invokes the phosphoric acid as a bifunctional catalyst. Furthermore,
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when the phosphate moiety is removed, as in P10, catalysis is lost, highlighting the catalytic role of the
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1
2
3
phosphorylated amino acid (entry 9).
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3
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9
0
1
2
3
4
5
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7
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0
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9
0
a,b
Table 1. Initial Peptide sequence optimization.
i + 2
O
i + 1
R3
R4
O
O
N
H
N
O
catalyst (10 mol%)
R2
O
O
HN
O
O
i
R5
O
P
H2O2 (30%, 1.5 equiv) Me
i + 3
Me Ph
CHCl , 24 h
Ph
NH
3
1
R O
R6
OH PG
4
b
5b
catalyst
catalyst
conv
(%)c
entry
erd
PG
i
i + 1 i + 2 i + 3
1
2
3
4
5
6
7
P3
P4
P5
P6
P7
P8
P9
Fmoc
Fmoc
Fmoc
Fmoc
Fmoc
Fmoc
Fmoc
Fmoc
Fmoc
Fmoc
Fmoc
Fmoc
Fmoc
pThr(Bn)
pThr(Bn)
pThr(Bn)
pThr(Bn)
pThr(Bn)
pThr(Bn)
pThr(Bn)
pThr(Bn)
Thr(Bn)
pThr(H)
D-Pro Acpc Leu-NMe2
87
64:36
66:34
64:36
62:38
69:31
52:48
44:56
51:49
D-Pro Acpc Leu-D-Phe-NMe2 67
D-Pro Acpc Phe-NMe2
D-Pro Acpc Ala-NMe2
D-Pro Aib Phe-NMe2
D-Pro Aib D-Phe-NMe2
88
85
92
69
54
19
<5
44
47
98
73
96
88
88
93
Pro
Aib Phe-NMe2
Aib Phe-NMe2
8e P8
P10
D-Pro
9
D-Pro Aib Phe-NMe2
D-Pro Aib Phe-NMe2
n/a
1
0
P11
P12
P13
P14
P15
55:45
51:49
67:33
58:42
70:30
67:33
72:28
73:27
1
1
1
1
1
allo-pThr(Bn) D-Pro Aib Phe-NMe2
2
3
4
pPhSer(Bn) D-Pro Aib Phe-NMe2
pSer(Bn)
D-Pro Aib Phe-NMe2
D-Pro Aib Phe-NMe2
D-Pro Aib Phe-NMe2
D-Pro Aib Phe-NMe2
D-Pro Aib Phe-NMe2
Fmoc-Phe pThr(Bn)
Fmoc-D-Phe pThr(Bn)
15 P16
1
1
1
1
6
7
P17
P18
Fmoc-Pro
pThr(Bn)
Fmoc-D-Pro pThr(Bn)
Fmoc-D-Pro pThr(Bn)
Fmoc-D-Pro pThr(Bn)
8f P18
9h P18
D-Pro
D-Pro
Aib Phe-NMe2
Aib Phe-NMe2
73 (75)g 78:22
50
97
77:23
64:36
2
0h (R)-1b
1
4
a
1
1
1
1
1
2
2
5
6
7
8
9
0
1
Reaction conditions: Cyclobutanone 4b (0.05 mmol, 1.0 equiv) at 0 ® 4 °C (ambient temperature) with [4b] =
b
c
1
d
0.10 M CHCl
3
.
Reported results are the average of two trials. Determined by H NMR relative integrations.
Enantiomeric ratios were determined by CSP-HPLC analysis (Chiralpak IA, 210 nm). Absolute configuration was
e
f
not determined. Reaction performed with t-BuOOH (70% w/w aq. 1.5 equiv). Reaction was carried out at –
g
1
h
15 °C. Yield in parentheses determined by H NMR internal standard (1,3,5-trimethoxybenzene). Reaction was
carried out at –25 °C. Abbreviations: pThr, phosphothreonine; Bn, benzyl; Acpc, 1-aminocyclopropane carboxylic
acid; Aib, 2-aminoisobutryic acid; pPhSer, β-threo-phosphophenylserine; pSer, phosphoserine, Bz, benzoyl.
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Given the relatively subtle effects on enantioselectivity observed thus far, we sought to make
alterations to the catalytically active residue, as well as modifications nearest to pThr, that might
2
enhance enantioselectivity. In contrast to rigid C -symmetric CPAs, non-equivalent rotamers about the
β-C–OpThr bond, in addition to the stereogenic P(V) and rapid tautomerization of the acidic proton,
25
provide numerous, chemically distinct catalyst conformers. When the phosphate benzyl group is
removed, thereby producing a non-stereogenic phosphorous atom, the enantioselectivity is diminished
(55:45 er, entry 10). Inversion of the Thr β-stereochemistry to allo-pThr ablates catalyst activity and
selectivity, resulting in 47% conversion and 51:49 er (entry 11). Subsitution of the β-methyl group with a
more bulky β-phenyl substituent (P13) affords the product in 97% conversion, although surprisingly the
enantioselectivity remains unaffected (67:33, entry 12) given the close proximity of the β-substituent to
the phosphate moiety. The importance of the β-substituent is further highlighted as replacement of pThr
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with pSer resulted in 78% conversion and 58:42 er (entry 13). While elongation of the C-terminus did
not attenuate enantioselectivity (entry 2), an additional D-Pro at the N-terminus resulted in increased
selectivity (73:27 er), (entries 14–17). At low temperatures (–15, –25 °C) 5b could be obtained in up to
78:22 er with a slight decrease in yield (entries 18–19). Notably, P18 provided higher enantioselectivity
than commercially available BINOL derived CPA, (R)-TRIP 1b (64:36 er), and comparable selectivities
4 15
5
1
1
6
7
6
7
8
17b
(81:19 er) to those previously reported.
9 18
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
(a)
(b)
2.97 Å
3.0 Å
3.08 Å
3
.11 Å
2
.92 Å
2
.78 Å
P4
7
Fmoc-pThr(Bn)-D-Pro-
Acpc-Leu-D-Phe-NMe2
Boc-Dmaa-D-Pro-Acpc-
Leu-D-Phe-NMe2
(c)
O
P
Me
O
O
H
N
Ph
O
O
OH
Fmoc
Fmoc-pThr(Bn)-OH
Boc
OH
HO
HN
NMe2
Boc-Dmaa-OH
1
9
7
8 20
9 21
0 22
Figure 2. Comparison of X-ray crystal structures of pThr- and Dmaa-embedded peptides. (a) X-ray structure of
P4. Two 1,4-dioxane solvent molecules omitted for clarity (see SI for details). (b) X-ray structure of 7. Two distinct
packing polymorphs were observed in the unit cell (7a, 7b), only one is shown for clarity (See SI for details). (c)
Catalytically active residues (pThr and Dmaa) embedded in i position of conserved peptide sequence.
2
2
3
4
1
2
3
4 25
In order to gain additional information about the catalyst structure and develop hypotheses
about NCIs, we determined the solid-state structure of P4 using X-ray crystallography (Figure 2a).
Interestingly, there were three intramolecular hydrogen bonds, including the predicted 10-membered β-
5
2
2
6
7
6
7
8
9
0
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9
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1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
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5
5
5
5
5
5
6
1
2
3
4
5
6
7
turn hydrogen-bond between N–H(i+3)×××O(i) and N–H(i)×××O(i+4). Recently, the structures of various
dimethylaminoalanine (Dmaa)-embedded β-turn catalysts were extensively studied, and we wondered
27
whether the principles learned would be applicable in ongoing catalyst design. In order to make a
28
direct comparison, we obtained a crystal structure of the Dmaa analog (7) (Figure 2b). Indeed, both
catalysts showed nearly identical secondary structures in the solid-state, despite the rather large
change to the catalytically active residue (Fmoc-pThr(Bn) vs. Boc-Dmaa) as depicted by the similar
hydrogen bond lengths and structural overlays (see SI for details). Moreover, both peptides exhibit
similar φ and ψ dihedral angles that are consistent with a type II′ β-turn secondary structure, and only
0
1
2
3 8
4
5
2
2
9
show a significant difference in the conformation of the i+3 Phe side chain orientation. One
observation from the solid-state structure of P4 was the relatively exposed nature of the phosphate
moiety, which may position substrates in such a way as to prevent favorable NCIs needed to achieve a
highly enantioselective BV reaction.
6
7
8
9
0
1
2
3
4
5
1
1
1
0
1
2
1
1
1
3
4
5
Given the numerous examples of peptide catalyst-substrate interactions involving hydrogen
bonding networks and the diversity of functionalization in drug-like molecules, we hypothesized an
additional hydrogen bond donor or acceptor on the substrate might engage in further secondary
23,29
6 16
contacts with the catalyst and perturb enantioselectivity.
Using substrate 4c, bearing a carbamate
7
8
1
7
ortho-substituent and 10 mol% of P7, product 5c was obtained in 97% yield and 66:34 er (Scheme 3).
Initially, this result suggested additional functionality on the substrate had no influence on the
enantioselectivity of the reaction, and that perhaps the selectivities observed thus far are due largely in
part to steric hindrance. Based on the relative positions of the i pThr and i+3 Leu side chains in the
solid state of P4, we postulated that an i+3 side chain-directing group (e.g. amide functionality) would
afford the opportunity for substrate 4c to engage in attractive secondary interactions with the amino
acid side chains synergistically.
9 18
0
1
2
2
2
9
0
1
2
1
2
3
4
5
6
7 23
8
2
4
9
0
1
2
3
4
5
6
7
8
9
O
O
O
P7 (10 mol%)
H
N
O
H2O2 (30% w/w aq. 1.5 equiv)
CHCl3 (0.10 M), 4 °C, 24 h
Ph
O
O
NH
Ph
O
5
c
4
c
9
7%, 66:34 er
2
2
5
6
Scheme 3. Baeyer–Villiger oxidation of carbamate 4c.
0 27
While more than half of proteinogenic amino acids possess side-chain functionality that can
engage in hydrogen bonding interactions, for our purposes, we envisioned an ideal amino acid residue
to be neutral, easily diversifiable, and sufficiently short as to reduce the rotational degrees of freedom.
We imagined that the readily available 2,3-diaminoproprionic acid (Dap) residue, a biologically
1
2
2
3
3
8
9
0
1
2
3
4
5
6
7
8
9
0
30
significant non-proteinogenic amino acid, would fulfill all these requirements. Indeed, when Dap(Cbz)
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8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
2
3
(P19) was incorporated in the peptidic scaffold, the reaction delivered product 5c in 97% yield and
78:22 er, lending credence to our hypothesis (Table 2a). Encouraged by these results, we began to
further investigate the role of the i+3 Dap residue on the enantioselectivity of the BV reaction.
4
5
Table 2. Peptide i+3 and N-terminus optimization for directed BV reaction.a.b.c
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
O
O
O
catalyst (10 mol%)
O
NHPh
H O (30% w/w aq. 1.5 equiv)
2
2
CHCl
3
, 4 °C, 24 h
O
O
NHPh
O
4
c
5c
(a)
O
O
P
Me
N
Me
Me
O
HN
O
NH
2
BnO
O
1
N
H
OBn
P19
HO
O
R
R =
NH
HN
N
H
Fmoc
1
O
O
97%, 78:22 er
"
Dap"
side-chain
modification
NMe
2
O
O
O
O
O
Me
N
CF
3
N
Me
N
H
P21
Ph
R1
=
H
H
P22
P23
P20
00%, 82:18 er
1
100%, 80:20 er
95%, 69:31 er
O
97%, 72:28 er
O
O
O
CF
3
N
H
Me
N
H
N
H
N
H
Ph
Ph
OMe
CF
3
P24
9%, 75:25 er
P25
99%, 80:20 er
P26
100%, 79:21 er
P27
96%, 80:20 er
9
O
O
O
O
H
N
S
Ph
N
H
Ph
N
H
N
H
N
H
OPh
O
Me
P28
00%, 82:18 er
P29
99%, 74:26 er
P30
97%, 73:27 er
P31
88%, 67:33 er
1
(b)
O
O
Fmoc
N
O
O
P
Me
N
Me
Me
R2
=
HN
O
BnO
O
P32
100%, 76:24 er
P33
100%, 85:15 er
HO
O
NH
HN
R2
NHAc
O
O
O
N-terminus
modification
Me
t-Bu
NMe
2
P36
00%, 85:15 er
P37
100%, 82:18 er
1
O
O
O
O
Ph
Me
Ph
Ph
OMe
NO
2
Ph
P34
100%, 86:14 er
P35
100%, 84:16 er
P38
100%, 81:19 er
P39
100%, 78:22 er
6
a
6 7
7 8
8 9
9 10
0
Reaction conditions: Cyclobutanone 4c (0.025 mmol, 1.0 equiv), catalyst (10 mol%), and H
2
O (30% w/w aq. 1.5
2
b
1
equiv) at 0 ® 4 °C (ambient temperature) with [4c] = 0.10 M CHCl . Conversion determined by H NMR relative
integrations and enantiomeric ratios determined by CSP-HPLC analysis (Chiralpak IA, 230 nm). Reported results
are the average of two trials.
3
c
1
1
1
2 12
Hydrogenation of the Cbz-protecting group allowed access to a variety of side chain analogs,
following amino functionalization, and in comparing catalysts P19–P31, a number of differences were
realized (Table 2a). Initially, acetamide P20 and benzamide P21 were prepared, which both resulted in
increased enantioselectivity (82:18 er and 80:20 er, respectively), whereas the serine analog (P22)
3
1
1
1
3
4
5
4
5
6
7
8
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afforded 5c in decreased selectivity (69:31 er). Replacement of the acetamide (P20) with electron-
deficient amides (P23–P24) resulted in generally lower selectivity, while the electron-rich amide P25
gave similar results (80:20 er). There was little difference observed in enantioselectivity among bulkier
amides (P26, P27), signifying that the increased selectivity of the catalyst is likely not attributable to
steric factors alone. Catalysts containing a phenyl urea performed equally well (P28), although
carbamate P29 and sulfonamide P30 afforded the product with reduced enantioselectivity. The
directionality of the amide group was shown to be crucial, as the Asn analog P31 resulted in a
substantial decrease in selectivity (67:33 er). Together, these results suggest that while the N–H of the
side chain might not be directly involved in the putative secondary interactions, an amido group on the
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3 8
4
9
5
3
1
6 10
side chain is necessary for achieving high selectivities. The enantioselectivity could then be further
increased through N-terminal modification (Table 2b, P32–P39). It was found that an electron-rich N-
terminal amide was advantageous (P34, 86:14 er), while bulkier amides afforded decreased
selectivities. Although a steric effect is possible, the increased enantioselectivity is thought to be
associated with changes in the electronic nature of the N-terminal amide that alters the intramolecular
7
1
1
1
1
1
2
3
4
8
9
0
1
2
3
13
4 15
hydrogen bonding of the peptide.
5
1
1
6
7
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
a
Table 3. In depth optimization of peptide and reaction conditions.
O
Me
N
R
i+2
O
P
HN
O
R
BnO
O
O
HO
O
NH
HN
NHAc
O
O
O
X
O
catalyst
OMe
O
NHPh
H2O2 (30% w/w aq. 1.5 equiv)
O
O
conditions below, 24 h
NHPh
O
4c
5c
erc
entry
cat.
i + 2
X
cat.
conc.
T
(°C)
conv.
(%)b
(mol%) [M]
1
P34
P34
P34
P34
P34
P34
P40
P41
P41
Aib
Aib
Aib
Aib
Aib
Aib
Aic
Aic
Aic
NMe2
NMe2
NMe2
NMe2
NMe2
10
0.10
0.10
0.10
0.05
0.05
0.05
0.05
0.05
0.05
0.05
4
100
100
100
100
99
85:15
86:14
86:14
88:12
89:11
88:12
92:8
2
5
4
3
2.5
2.5
2.5
2.5
2.5
2.5
0.5
2.5
4
4
4
5
−15
−25
−15
−15
−15
−15
6d
7
NMe2
NMe2
100
99
8
NEt2
NEt2
99
94:6
9e
88
92:8
1
0
(R)-1b
74
77:23
1
8
a
1
2
2
9
0
1
Reaction conditions: Cyclobutanone 4c (0.05 mmol, 1.0 equiv) and H
2
O
2
(30% w/w aq. 1.5 equiv). Reported
b
1
c
results are the average of two trials (0.05 mmol). Conversion determined by H NMR relative integrations.
Enantiomeric ratios were determined by CSP-HPLC analysis (Chiralpak IA, 230 nm). Reaction performed for 48
hours. Reaction performed for 96 hours. Abbreviations: Aic, 2-aminoindane-2-carboxylic acid.
d
e
3 22
4
5
6
7
8
9
0
2
3
2
4
With P34 identified as a more selective catalyst, we set out to perform an optimization of the
reaction conditions. A catalyst loading screen with 10, 5, and 2.5 mol% peptide P34 resulted in a slight
2
5
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1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
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5
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5
5
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6
7
increase in enantioselectivity with no loss in conversion at 4 °C in 24 hours (Table 3, entries 1–3). By
reducing the reaction concentration from 0.10 M to 0.05 M in CHCl , 5c was obtained in 99% yield and
3
88:12 er (entry 4). The enantioselectivity was further increased by lowering the reaction temperature to
–15 °C (89:11 er), although at –25 °C the selectivity was not improved (entries 5–6). We chose to
briefly re-examine the i+2 position and found that when Aib was replaced with Aic (P40), the product
was obtained in 99% conversion and 92:8 er under the optimized reaction conditions. Additionally, by
switching the dimethylamide C-terminal group with a diethylamide (P41), the lactone could be obtained
with 99% yield and 94:6 er. With 0.5 mol% of P41, 5c could be obtained in 88% yield and nearly the
same enantioselectivity (92:8 er) with prolonged reaction time, which demonstrates both the robustness
0
1
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3 8
4
9
5
32
6 10
and efficiency of the pThr-embedded catalysts. It should be noted that when (R)-1b is used as the
catalyst under the reaction conditions optimized for the peptide catalyst, 5c is obtained in 74% yield and
77:23 er (entry 10). Although the mode of catalysis is likely the same, the ability for the peptide catalyst
to engage in additional hydrogen bonding interactions offers an advantage in achieving highly selective
transformations of more functionalized substrates. With the optimized conditions in hand, peptide P41
was used to evaluate the substrate scope of the enantioselective BV of cyclobutanones.
7
1
1
1
1
1
2
3
4
8
9
0
1
2
3
4 15
5
1
1
6
7
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
a
Table 4. Carbamate substrate scope.
O
O
O
P41 (2.5 mol%)
R1
R1
H
N
O
H2O2 (1.5 equiv)
CHCl3 (0.05 M), −15 °C
R2
O
O
NH
R2
O
X-ray (R)-5f
4
5
entry
R1
H
R2
time (h)
yield (%)b
erc
1
2
3
4
C6H5 (c)
24
24
24
24
99
99
98
94
94:6
93:7
93:7
94:6
H
4-OMeC6H4 (d)
4-CF3C6H4 (e)
4-BrC6H4 (f)
H
H
5
6
7
H
H
H
Me (g)
O
24
24
24
95
97
96
92:8
94:6
93:7
4-t-BuC6H4 (h)
O
(i)
O
8
9
H
H
CH3CH2 (j)
C6H5CH2 (k)
C6H5 (l)
84
72
72
96
95
94
85:15
82:18
80:20
1
0
CH3
8 18
9
a
1
2
2
2
2
2
2
9
0
1
2
3
4
5
Reaction conditions: Cyclobutanone 4 (0.10 mmol, 1.0 equiv), P41 (2.5 mol%), and H
2
O
2
b
(30% w/w aq. 1.5
0
1
2
3
4
5
6
7
8
9
0
equiv) at –15 °C with [4c] = 0.05 M CHCl
3
. Reported results are the average of two trials. Isolated yield after
c
chromatography. Enantiomeric ratios were determined by CSP-HPLC analysis. See supporting information for
details on assignment of absolute configuration. The configuration of products 5c-d, 5g–l were assigned in
analogy to 5e and 5f.
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As shown in Table 4, P41 provides a number of N-substituted substrates with high
enantioselectivities. In general, N-aryl carbamates are processed with high enantioselectivity, and
electron-donating or electron-withdrawing groups in the para-position do not lead to significant
perturbations in the enantioselectivity, such that substrates 4d–4g were smoothly converted to the
corresponding lactones in nearly quantitative yield and good enantioselectivities within 24 hours (93:7 –
94:6 er) (Table 4, entries 2–5). When the reaction was carried out with a cyclobutanone containing a p-
t-butyl phenyl carbamate, the product was obtained in 97% yield and 94:6 er (entry 6). Additionally, the
dioxolane product can be isolated in 96% yield and 93:7 er (entry 7). Benzyl or alkyl substituted
carbamates are generally less well tolerated, requiring longer reaction times (72–84 hours) and
decreased enantioselectivity (82:18 – 85:15 er) (entries 8–9). Using the carbamate-functionalized
analog of 4b, which contains a methyl group at quaternary position C3, lactone 5l can be isolated in
94% yield and 80:20 er, albeit with increased reaction time (entry 10). Lastly, we were able to establish
the absolute configuration of the lactone products 5e and 5f by X-ray diffraction (see SI for details).
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3 8
4
9
5
6 10
7
1
1
1
1
2
3
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
1
4
1
5
1
1
1
6
7
8
Figure 3. (a) General reaction scheme for BV oxidation with peptide catalyst and (b) structure–selectivity study
with P41 and Phe analog (P42). Reaction conditions: Cyclobutanone 4 (0.05 mmol, 1.0 equiv), catalyst (2.5
1
mol%), and H
2
O
2
(30% w/w aq. 1.5 equiv) at –15 °C with [4c] = 0.05 M CHCl
3
. Yield was determined by H NMR
2 19
3 20
4 21
5 22
6 23
7
analysis of the crude reaction mixture using an internal standard (1,3,5-trimethoxybenzene). Enantiomeric ratios
were by CSP-HPLC analysis. Reported results are the average of two trials. The absolute configuration of
products 5m–r were assigned in analogy to 5e and 5f. The absolute configuration of 5a was assigned as (R)-(–)-
17a
5a by comparison of the measured optical rotation to literature values.
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1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
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6
7
Further studies were then undertaken to gain a better understanding of the apparent directing
group effects and key substrate–catalyst interactions that are required to achieve a highly
enantioselective transformation. To probe the putative hydrogen bonding with the i+3 Dap residue, the
phenylalanine analog P42 was prepared, containing only a single-point mutation (Phe) at the i+3
position (Figure 3a). Various substrates were then subjected to the optimized reaction conditions using
2.5 mol% of P41 or P42 for 24 hours. In agreement with our earlier observations, catalyst P42 (Phe)
0
1
2
‡
afforded product 5c in 78% yield and 79:21 er, which corresponds to a decrease in ∆(∆∆G ) of 0.73
–1
3 8
4
9
5
kcal·mol compared to P41 (Dap) (Figure 3). When the carbamate directionality is inverted, 5m was
obtained with excellent enantioselectivity using P41 (Dap, 91:9 er) and again diminished yield and
enantioselectivity with catalyst P42 (Phe). The same trend is observed for products 5n–5p, wherein
6 10
7
32
1
1
1
1
1
2
3
4
urea and amide groups are better matched for the Dap-containing peptide P41, which contains more
functionality for attractive secondary interactions, than for P42 containing Phe. The presence of a
hydrogen bond donor is necessary for achieving high selectivities, which we believe arises in part to
8
9
0
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2
3
4
5
6
7
8
29
essential hydrogen bonding interactions occurring in the transition structure of the reaction.
1
1
1
5
6
7
Altogether, these results demonstrate a synergistic effect between the nature of the appended
substrate directing group and the multifunctional nature of the peptide catalysts. It is notable that in all
of the substrates tested thus far, the yields are consistently better with P41 (Dap), which suggests
associative interactions that lead to a rate acceleration for oxidations compared with P42 (Phe). Using
catalyst P41 (Dap), with carbonate substrate 4q lacking an N–H, 5q was obtained in only 37% yield and
66:34 er. In contrast, catalyst P42 (Phe) performs better, generating the product in 49% yield and 78:22
er. The difference in selectivities achieved with catalyst P41 when the N–H of 4c is replaced with an
9 18
0
1
2
2
2
9
0
1
2
1
2
3
4
5
6
‡
–1
oxygen atom (4q) corresponds to a ∆(∆∆G ) of 1.07 kcal·mol (94:6 er for 5c and 66:34 er for 5q).
Substrates 4r and 4a are again obtained in better enantioselectivity with the Phe catalyst P42, although
the phenol is obtained in higher yield with P41 (Dap). Tuning of the i+3 position provides a
complementary pair of catalysts that can obtain good-to-high enantioselectivities for a range of
substrates through secondary NCIs.
7 23
8
9
2
4
0 25
1
2
2
6
7
2
3
4
5
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7
8
9
0
1
2
O
O
O
P41 (2.5 mol%)
H
2
O
2
(30 %, 1.5 equiv)
CHCl (0.05 M)
−15 °C, 24 h
O
3
O
Ph
O
N
H
O
NHPh
4s
(S)-(+)-5s
97%, 13:87 er
X-ray (S)-5s
3 28
4
2
9
Scheme 4. Observed reversal in absolute stereochemistry in the BV of meta-4s.
5
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7
Having demonstrated that ortho- directing groups lead to high enantioselectivity, we were
interested in the question, whether alternative placement of the identical directing group could allow for
switching of catalysis trajectories. To examine whether the peptides could recognize more distal
functionality we prepared substrate 4s, which repositions the N-aryl carbamate from the ortho- to the
meta- position. Under the optimized conditions, lactone 5s was obtained in 97% yield and 13:87 er with
2.5 mol% P41 (Scheme 4). Moreover, the X-ray structure of 5s revealed that the opposite absolute
configuration was obtained, compared to substrates containing an ortho- directing group, now favoring
2
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1
1
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4
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5
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5
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0
1
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34
3 8
the S-enantiomer (see SI for X-ray analysis). The results of the divergence in the absolute
4
9
5
stereochemistry can be explained by either (1) delivery of H to the diastereotopic face of the ketone
2
O
2
6 10
with the same C–C σ-bond migration or (2) the substrate–catalyst complex repositioning the peroxyl
group in the Criegee intermediate ~90° to achieve the opposite, but equivalent C–C bond migration
(Scheme 1). It is possible that both explanations can account for the divergence, as the addition of
7
1
1
1
1
1
2
3
4
8
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2
3
H
2
O to either face of the prochiral ketone leads to syn and anti isomers, which can both afford either
2
enantiomer of product. The ability for P41 to recognize both substrates in a manner that achieves the
opposite absolute stereochemistry of the lactone products (R-5c and S-5s), under otherwise identical
4 15
5
1
6
reaction conditions, is notable.
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1
(a)
O
O
O
P42 (2.5 mol%)
or
(R)-1b (2.5 mol%)
O
O
P41 (2.5 mol%)
H
2
O
2
(1.5 equiv)
CHCl (0.05 M)
15 °C, 24 h
H
2
O
2
(1.5 equiv)
CHCl (0.05 M)
−15 °C, 24 h
O
3
O
3
O
−
O
O
PhHN
O
NH
Ph
NHPh
(S)-(+)-5s
97% yield, 13:87 er
ΔΔG‡ = 0.98 kcal⋅mol−1
4s
(R)-(−)-5s
Reversal in selectivity without
inversion of a stereogenic
center in peptide.
74% yield, 66:34 er
ΔΔG‡ = 0.34 kcal⋅mol−1
(w/ 1b: 86% yield, 73:27 er)
(b)
O
O
O
P42 (2.5 mol%)
or
(R)-1b (2.5 mol%)
O
O
P41 (2.5 mol%)
H
2
O
2
(1.5 equiv)
H
2
O
2
(1.5 equiv)
CHCl (0.05 M)
−15 °C, 24 h
CHCl (0.05 M)
3
3
−15 °C, 24 h
R
R
R
4t
(
S)-(+)-5t
(R)-(−)-5t
60% yield, 79:21 erc
O
81% yield, 39:61 erc
R =
Ph
O
N
H
(w/ 1b: 74% yield, 76:24 er)
(c)
O
O
Me
N
Me
N
O
P
O
P
HN
HN
BnO
O
O
O
BnO
O
O
O
HO
O
HO
O
NH
HN
NH
HN
NHAc
Ph
O
O
L
-Dap
L
-Phe
NEt
2
NEt
2
OMe
P41
OMe P42
1
7
2 18
Figure 4. Divergences in enantioselectivity in meta- and para-N-aryl carbamate without inversion of a
3
a,b
1
9
catalyst stereogenic center.
4
5
6
7
8
9
0
a
2
2
0
1
Conditions: Cyclobutanone 4 (0.05 mmol, 1.0 equiv), H
2
1
O
2
(30% w/w aq. 1.5 equiv), catalyst (2.5 mol%) at –15
b
°C with [4] = 0.05 M CHCl .
3
Yield was determined by H NMR analysis of the crude reaction mixture using an
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5
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internal standard (1,3,5-trimethoxybenzene), and enantiomeric ratios determined by CSP-HPLC analysis.
Reported results are the average of two trials. Absolute configuration of 5t assigned in analogy to 5s.
c
3
4
5
6
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9
0
In continuing our analysis of the effect of Dap(Ac) and Phe substitution at the i+3 position on
enantioselectivity, we applied catalyst P42, bearing the Phe i+3 residue, to the oxidation of substrate 4s
(Figure 4a). With 2.5 mol% P42, the opposite enantiomer lactone (R)-5s is obtained in 74% yield and
0
1
2
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4
5
6
35
66:34 er under the optimal reaction conditions, relative to P41 (Dap). However, it should be noted that
the absolute configuration of product (R)-5s is the same as the γ-butyrolactone products 5 bearing an
ortho- directing group, previously obtained from the BV reaction with P42 (Figure 3). This observed
inversion in enantioselectivity is all the more impressive given the constituent amino acids in both
peptides (P41 and P42) have the same stereochemical configuration, yet afford opposite enantiomers
1
7 11
8
9
23d
‡
–1
1
2
of product 5s. The ∆(∆∆G ) of 1.3 kcal·mol is a consequence of how the two catalysts are able to
recognize the substrate upon binding to form different substrate–catalyst complexes. Moreover, when
the reaction is carried out with (R)-1b as the catalyst, (R)-5s is obtained in 86% yield and 73:27 er,
which matches the absolute stereochemistry and selectivities obtained in the oxidation of 4c with (R)-
1b (Table 3, entry 10). This divergence in enantioselectivity could stem from either (1) a conformational
change in the catalyst structure upon mutation of the i+3 position or (2) a perturbation in the secondary,
non-covalent interactions between the side-chains of the peptide and the functionalized substrate.
Once again, it appears that the catalyst P42 with an i+3 Phe residue and (R)-TRIP (1b) achieve
selectivity through steric hindrance, whereas P41 bearing the Dap residue operates through multivalent
NCIs, resulting in the products bearing opposite absolute configurations.
0 13
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8
9
0
2
2
2
2
2
3
4
5
After realizing this intriguing divergence in the absolute sense of enantioinduction by catalysts
P41 and P42 with 4s, we prepared substrate 4t, which contains a remote para-N-aryl carbamate
directing group (Figure 4b). When we applied optimized catalyst P41 bearing the i+3 Dap(Ac) residue
to the oxidation of 4t, lactone 5t was obtained in 81% yield and diminished selectivity (39:61 er).
Alternatively, phenylalanine analog P42 afforded the product in 56% yield and 79:21 er, similar to the
results obtained with (R)-1b (74% yield, 76:24 er). Once again, P42 and (R)-1b afford the opposite
absolute stereochemistry of lactone product relative to P41. Based on the results presented hitherto,
catalyst P42 appears to provide enantioinduction through steric repulsive interactions, whereas catalyst
P41, containing an i+3 Dap residue can engage the substrate in additional hydrogen-bonding
interactions. These differences in mechanism can lead to a divergence in the absolute stereochemistry
of product without inversion of a catalyst stereocenter, and seem to suggest that even a remote
directing group can engage in NCIs with the peptidic scaffold and perturb the enantioselectivity.
1 26
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(b)
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5
5
6
9.0
8.8
8.6
8.4
7.8
7.6
7.4
7.2
7.0
O
O
i NH
MeO
MeO
N
HN
N
HN
P41-NHpThr
P41-NHAic
P41-NHDap
H
N
H
N
O
Me
O
O
O
O
HN HN
HN
O
O
O
P
O
Me
O
P
O
Me
i+2 NH
O
O
HO
HO
P42-NHpThr
P42-NHAic
P42-NHPhe
O
N
O
N
Me
Me
Me
Me
0
1
2
3
4
P41
P42
i+3 NH
6
.8
ROE contacts shared between P41 and P42
ROE contacts unique to P41
ROE contacts unique to P42
0
5
10
15
20
25
%
DMSO-d6 in CDCl3 (v/v)
1
1
1
5 2
Figure 5.(a) H− H ROESY NMR of peptide catalysts P41 and P42 show both unique and conserved interresidue
nOe correlations (800 MHz, 5.0 mM in CDCl
solvent exposed and hydrogen bonded amides for peptides P41 and P42 (5.0 mM concentration in CDCl
(referenced to 7.26 ppm) at 25 °C).
1
6 3
7
8
9 6
0
1
2
3
4
3
at 25 °C). (b) H NMR solvent titration curve of DMSO-d to identify
6
4
5
3
7
To explain the emerging trends from the structure–selectivity studies and intriguing divergence
8
9
in observed enantiomeric products 5s–t, we sought to understand the effect of i+3 substitution on
36
peptide secondary structure using 2D NMR spectroscopy (Figure 5a) (see SI for details). In the case
5 10
6
of P41, a number of nOe contacts were present between the i+3 Dap residue and the i+1 D-Pro residue
that indicated the Dap side chain may be orientated on the top face of the peptide turn. However, for
P42 no nOe contacts were observed between the i+3 Phe side chain and the peptide backbone, which
suggests the phenyl ring is positioned away from the peptide turn. Yet, nOe contacts are still observed
between the βC–H of the i pThr residue and the βC–H of the i+3 residues for both peptides, indicating
some similarity between the positioning of the Phe and Dap residues in the peptide secondary
structure. Furthermore, the orientation of the pThr(Bn) residue is likely the same in each catalyst as
indicated by five additional conserved interresidue contacts present at the i catalyst position. DMSO
titration studies were then conducted to elucidate the hydrogen bonding environments of each N-H
1
1
1
1
1
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3
4
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1
2
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4
5
6 17
7
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2
3
1
6
1
1
2
2
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9
0
1
37
proton, which further showed structural similarities between P41 and P42 (Figure 5b). While the N–
H(i) exhibits a subtle downfield shift, the N–H(i+2) and N–H(i+3) peaks remain constant and shift upfield
respectively, suggesting both N–H(i+2) and N–H(i+3) being previously engaged in intramolecular
hydrogen bonding. Together with the observed nOe contacts, these observations are suggestive of
4 22
5
18,22
2
2
2
3
4
5
both peptides adopting a type I’ double β-turn.
It is worth noting that P41 and P42 appear to adopt a
6
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2
3
4
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type I’ β-turn secondary structure in solution, whereas previously P4 adopted a type II’ β-turn in the
3
8
solid-state.
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5
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5
5
5
5
5
5
6
(
a)
R
R
H
O
H
O
H
H
R' Me
R'
Me
O
Me
N
Me
N
N
H
N
H
O
O
H
H
N
H
N
H
N
H
N
H
O
H
O
O
O
O
N
O
H
H
H
P
O
P
N
O
H
O
O
O
O
H
H
H
N
H
O
H
O
O
O
Ph
O
O
R''
Ph
O
R''
O
N
O
H
H
O
O
Ar
O
Ar
H
H
O
P41 + anti-6c
P41 + syn-6c
0
1
2
3
4
5
6
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0
1
2
3
4
5
6
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(b)
R
H
O
H
R' Me
Me
N
N
H
O
H
H
N
H
N
H
O
O
O
H
P
O
H
(
S)-5s
O
O
N
H
O
N
O
H
Ph
R''
H
2
H O
O
O
O
Ar
O
H
O
P41 + anti-6s
R
H
O
Me
N
H
R'
N
O
H
N
H
O
H
O
P
O
sterics
O
H
H
O
N
H
O
O
O
O
H
(R)-5s
Ph
H
R''
O
N
Ar
H
2
H O
H
O
O
P42 + anti-6s
1
9 2
0 3
1 4
2 5
3
4
5 7
6
7
8
9
0
1
2
Figure 6. (a) Speculative transition structures showing putative substrate-catalyst hydrogen bonding interactions
with an antiperiplanar orientation of the peroxyl group in the Criegee intermediate, which is consistent with the
absolute configuration of the product. (b) Speculative transition structures for the switching in stereoselection in
substrate 4s, which is meta-substituted.
6
With these considerations in mind, a mechanistic hypothesis for the formation of the
enantioenriched products could be assembled (Figure 6a). In our thinking, we preserve the type I’ β-
turn secondary structure in accord with our experimental observations. Given the similarity between the
two structures, as determined by 2D NMR and DMSO titration studies, we believe that the incorporation
of the Dap(Ac) residue does not result in a different conformation of the peptide. Rather, the i+3 Dap
side chain of P41 is orientated on the top face of the catalyst with the C=O of the i+3 side chain
acetamide free to engage in a hydrogen bond with the substrate, whereas the i+3 Phe side chain in
P42 does not provide additional hydrogen bonding. We envision that the bifunctional phosphate moiety
8
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24
of pThr activates the ketone for addition of H
2
O
2
.
Although speculative, the peroxyl group of the
Criegee intermediate could engage in further hydrogen bonding with the peptide backbone amides. In
the instances of switching in stereoselection, it is plausible to consider a similar binding model (Figure
6b). However, upon substrate binding with the catalyst side-chain amide, a slight change in the
conformation of the peroxyl group results in the opposite stereochemical outcome. Since the i+3 Phe
side chain in P42 is unable to engage in further hydrogen bonding, steric hindrance is the
predominating factor in stereoselectivity, and thus the absolute stereochemistry of product is
1 17
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unchanged when the carbamate directing group is placed in the o-, m-, or p- positions. The multi-point
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5
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5
5
5
5
5
5
5
6
binding of the substrate-catalyst complex allows for stabilization of the Criegee intermediate and
16c
facilitates a highly selective rearrangement, akin to those interactions found in BVO-monoxygenases.
4
5
6
Conclusion:
0
1
In summary, we have reported a multifunctional Brønsted-acid pThr-based peptide catalyst for
the enantioselective BV reaction of functionalized cyclobutanones with low catalyst loadings (0.5–2.5
mol%). By utilizing hypothesis-driven design, aided by detailed NMR and crystallographic studies, we
have identified the inclusion of an i+3 Dap residue to be advantageous for achieving high selectivities
through secondary non-covalent interactions with a variety of directing groups. Moreover, the ability of
the peptide to interact with a carbamate directing group in multiple positions of an aryl ring leads to an
observed divergence in the absolute stereochemistry (94:6 er to 13:87 er R/S) of the γ-butyrolactone
products (5) with a pThr-embedded peptide catalyst containing Dap. Permutations to the i+3 residue
provide a complementary pair of catalysts for a further range of cyclobutanone substrates, which
affords the products 5 in high yield and stereoselectivities, and in select cases opposite absolute
2 7
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39
configuration. This work provides a framework for the further development of pThr-based catalysts for
8 17
applications in asymmetric catalysis and modifications of complex molecules, where multiple sites of
engagement are necessary to achieve selective functionalization.
9
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ASSOCIATED CONTENT
AUTHOR INFORMATION
2
2
Corresponding Author
8 23
* E-mail: scott.miller@yale.edu
9
0 24
1
2
5
Author Contributions
2
3
4
5
6
2
2
6
7
The manuscript was written through contributions of all authors. All authors have given approval to the
final version of the manuscript.
7 28
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2
3
ORCID
9
0
1
2
3
4
5
6
7
8
9
0
Aaron L. Featherston: 0000-0003-0918-067X
Christopher R. Shugrue: 0000-0002-3504-9475
Scott J. Miller: 0000-0001-7817-1318
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Notes
The authors declare no competing financial interest.
3
4
5
Crystallographic data are deposited with the Cambridge Crystallographic Data Center under the
accession numbers CCDC 1865894 (4d), 1867752 (5c), 1865895 (5e), 1865896 (5f), 1865897 (5s),
1865899 (P4), and 1865898 (7).
0
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Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: ##
6 9
Additional figures, synthetic procedures, characterization data for all compounds, and crystallographic
information (PDF).
7
1
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X-ray crystallographic data for compounds 4d, 5c, 5e, 5f, 5s, P4, 7 (CIF).
3 13
ACKNOWLEDGMENTS
4
5 14
This work was supported by the National Institute of Health (NIH R01-GM096403). C.R.S. gratefully
acknowledges the National Science Foundation Graduate Research Fellowship program for funding
(DGE1122492). We also acknowledge Newlyn N. Joseph for the preparation of intermediates towards
4t. The crystallographic data for 4d was acquired at the Advanced Light Source at Lawrence Berkeley
National Laboratory. The Advanced Light Source is supported by the Director, Office of Science, Office
of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-
05CH11231.
6
7
1
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(28) Peptide 7 exhibited two packing polymorphs in the solid state that were similar to one another. Only one
polymorph is shown for clarity as a point of comparison to P4. See SI for detailed analysis of the
polymorphic structures.
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(33) Due to poor solubility of 4n in CHCl , the conversion of 4n to 5n was generally low, and therefore the
3
reaction with 4n was conducted for 48 hours.
(34) Lactone (S)-5s crystallized as two packing polymorphs with a minor component of the minor enantiomer
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stereochemical assignment and full details of the crystal structure.
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Peptide‐Catalyzed Acylation Utilizing Advanced NMR Techniques. Angew. Chem. Int. Ed. 2016, 55,
15754–15759. (b) Rigling, C.; Kisunzu, J. K.; Duschmalé, J.; Häussinger, D.; Wiesner, M.; Ebert, M.-C.;
Wennemers, H. Conformational Properties of a Peptidic Catalyst: Insights from NMR Spectroscopic
Studies. J. Am. Chem. Soc. 2018, 140, 10829–10838.
0
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(37) For effects of increasing Lewis base concentration: (a) Venkatachalapathi, Y. V.; Venkataram Prasad, B.
V.; Balaram, P. Conformational Analysis of Small Disulfide Loops. Spectroscopic and Theoretical Studies
on a Synthetic Cyclic Tetrapeptide Containing Cysteine. Biochemistry 1982, 21, 5502–5509. (b) Raj, P. A.;
Balaram, P. Conformational Effects on Peptide Aggregation in Organic Solvents: Spectroscopic Studies of
Two Chemotactic Tripeptide Analogs. Biopolymers 1985, 24, 1131–1146. (c) Rai, R.; Raghothama, S.;
Balaram, P. Design of a Peptide Hairpin Containing a Central Three-Residue Loop. J. Am. Chem. Soc.
2006, 128, 2675–2681.
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(38) Attempts to obtain crystal structures of either P41 or P42 or analogs thereof, were unsuccessful.
(39) Tripp, A. E.; Burdette, D. S.; Zeikus, J. G.; Phillips, R. S. Mutation of Serine-39 to Threonine in
Thermostable Secondary Alcohol Dehydrogenase from Thermoanaerobacter ethanolicus Changes
Enantiospecificity. J. Am. Chem. Soc. 1998, 120, 5137–5141.
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ACS Catalysis
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O
O
O
O
P
Me
N
HN
O
H
BnO
H
O
O
N
O
O
O
Ph
O
NH
HN i+3
O
Ph
PG
O
N
O
H
N
H
Me
NEt
2
H
2
O
2
catalyst
Dap
H
2 2
O
O
(
0.5−2.5 mol%)
O
O
•
•
Low catalyst loading
Directed catalysis
PhHN
O
O
O
O
O
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NHPh
O
• i+3 Dap → Phe =
stereodivergence
R'
O
2
0 Examples
up to 100% yield and 94:6 er
(
S)-enantiomer
3:87 er
(R)-enantiomer
R
1
94:6 er
2
2
3
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