Mimicking Enzymes: Asymmetric Induction inside a Carbamate-Based Steroidal Cleft

Article abstract of DOI:10.1021/acs.orglett.9b01170

Cholic acid has been elaborated into a carbamate-based tripodal architecture, which is able to promote an asymmetric organic transformation inside its chiral cavity. The nature of this steroidal catalyst has been disclosed by quantum-chemical calculations. It comprises the preorganization and confinement of the reagents within the cavity of the steroid to form a supramolecular complex held together by means of cooperative H-bond contacts. This operational mode resembles that of some enzymes.

Full text of DOI:10.1021/acs.orglett.9b01170

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Mimicking Enzymes: Asymmetric Induction inside a Carbamate−
Based Steroidal Cleft
,†
†
‡
†
‡
́
́
Carmen Concellon,* Judith Martín, Miguel Gallegos, Noe Fanjul-Mosteirín, Aurora Costales,
‡
,†
́
́
Angel Martín Pendas, and Vicente del Amo*
†
́
́
́
Departamento de Química Organica e Inorganica, Universidad de Oviedo, C/Julian Clavería 8, 33006 Oviedo, Spain
‡
́
Departamento de Química Física y Analítica, Universidad de Oviedo, C/Julian Clavería 8, 33006 Oviedo, Spain
S
* Supporting Information
ABSTRACT: Cholic acid has been elaborated into a carbamate-
based tripodal architecture, which is able to promote an
asymmetric organic transformation inside its chiral cavity. The
nature of this steroidal catalyst has been disclosed by quantum-
chemical calculations. It comprises the preorganization and
confinement of the reagents within the cavity of the steroid to
form a supramolecular complex held together by means of
cooperative H-bond contacts. This operational mode resembles
that of some enzymes.
holic acid, 1, the archetypal bile acid scaffold, is a cheap,
naturally occurring molecule, being one of the major bile
acids produced by the liver of mammals (Figure 1). Equipped
corresponding adducts 4. This particular reaction is recognized
as a general playground and a first test for such purposes.
Catalysts previously employed on this transformation have
been traditionally constructed from backbones such as BINOL,
chiral 1,2-diamines, proline (or other amino acids), and
cinchona alkaloids. These frameworks have been equipped
with guanidinium,4 urea/thiourea,5 squaramide moieties,6 or 2-
aminobenzimidazole.7 Such motifs are capable of activating
nitroalkenes 3 through the formation of an array of two parallel
and contiguous H-bond contacts. Figure 2A represents this
activation mode exemplifying an urea/thiourea motif.
C
Staying away from the classical approach represented in
Figure 2A, we envisioned the possibility of activating
nitroalkenes 3 within the cavity of cholate derivatives
Figure 1. General structure of cholic acid, 1, with its numbering (in
red).
with three codirected hydroxyl groups, it possesses an
inherently chiral cavity that can be functionalized with ease.
Making use of these properties, the groups of Davis1 and
others2 have demonstrated ways in which this steroid can be
converted into supramolecular receptors for anions and
carbohydrates. Yet, to our knowledge, steroidal derivatives
have never been elaborated into hosts capable of promoting
asymmetric transformations within their architectures.3 Herein,
we describe the ability of a tripodal cholate, decorated with
carbamate units, to work as a synthetic enzyme, promoting an
enantioselective reaction inside its chiral cavity by making use
of a novel activation mode based on the action of spatially
segregated but cooperative H-bonding interactions.
Figure 2. Classical activation mode of nitroalkenes 3 by H-bonding
with catalysts containing, for example, a urea or thiourea moiety.
Malonate anion 2′ attacks on the activated alkene (A, left). Our work:
activation of nitroalkenes 3 by multiple cooperative H-bond contacts
established from the cavity of a steroidal scaffold (B, right).
Following the standardized protocol for assessing the
abilities of novel catalytic structures, we have first examined
the Michael-type addition of dimethyl malonate, 2, to
nitroalkenes, 3, in the presence of a base to render the
Received: April 3, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01170
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
decorated with several single-H-bond donating points (Figure
2B). If these groups are arranged with an optimum topology
and distance, forced by the preorganization of the steroidal
backbone, they could activate nitroalkanes 3 through the
establishment of simultaneous and cooperative, but spatially
segregated, H-bond contacts. Therefore, as a consequence of
their inherently chiral cavity, these supramolecular hosts might
act as three-dimensional compartments capable of confining
substrates and forcing them to react in an asymmetrical
manner. Such an approach resembles the activation mode of
some enzymes, such as chorismite mutase, which can fit and
activate substrates within their active sites by means of multiple
enzyme−substrate contacts, normally far apart in space.8
To pursue our concept we decided to prepare a collection of
steroidal polyols and carbamates. Triol 5 (methyl cholate) and
diols 6−10 were readily prepared from commercially available
cholic acid, 1, making use of previously described method-
ologies, or by conventional syntheses. Diol 11 corresponds to
methyl deoxycholate, and 12 to methyl chenodeoxycholate.
Phenyl carbamates 13−17 were easily afforded from some of
these polyols (Figure 3). The Supporting Information (SI)
contains the experimental details.
Table 1. Preparation of Michael Adducts 4a−j Employing
a
Cholic Acid-Derived Steroidal Triscarbamate 13
b
c
entry
1
2
Ar
yield (%)
ee (%)
3a: C6H5
3a: C6H5
94
3
91
d
3
4
3b: 4-FC6H4
3c: 4-ClC6H4
3d: 4-Br-C6H4
3e: 4-OMeC6H4
3f: 4-MeC6H4
3g: 4-NO2C6H4
3h: 4-PhC6H4
3i: 3-Cl-C6H4
3j: 2-furyl
84
99
80
76
65
98
99
96
71
73
90
89
90
80
85
82
85
86
73
90
e
5
e
6
e
7
8
e
e
9
10
11
12
e
f
3a: C6H5
a
General reaction conditions: aromatic nitroalkene 3a−j (0.2 mmol),
and steroid 13 (0.03 mmol) were dissolved in dry toluene (1.6 mL),
at −78 °C, before dimethyl malonate 2 (0.6 mmol) and dry NEt3 (0.2
mmol) were added. The reaction mixtures were stirred for 72 h and
b
then quenched with NH4Cl (aq, satd). Isolated yield of analytically
c
pure products 4a−j. Enantiomeric excess of analytically pure
d
products 4a−j as determined by chiral HPLC. Reaction carried
out without steroid 13, affording 3% conversion of 3a into product 4a.
e
f
Reaction stirred for 7 days. MeOH (0.06 mmol, 30 mol %) was
added at the start of the reaction.
derivatives 5−17, triscarbamate 13 possesses a unique
topology of H-bond donors, laid out with the right pattern
and optimum distances, which qualify this framework for
promoting the reaction. Remarkably, these H-bond donors are
separated one from each other by a minimum of 11 covalent
bonds.
Figure 3. Structures of potential steroid-based catalysts 5−17,
prepared by modifying the functionalization vectors R1, R2, and R3.
To our delight, after a profound screening of parameters
(see the SI for details, Tables SI_1−SI_7) we found optimal
conditions for the tripodal triscarbamate 13. Thus, in the
presence of steroid 13, when a solution of nitrostyrene 3a in
dry toluene, at −78 °C, was treated with 2 and dry NEt3, the
desired product 4a was rendered in quantitative conversion
and 91% ee (Table 1, entry 1). The stereochemical
configuration of 4a was confirmed as (S)-(+) by comparison
with previously published HPLC data.9 In the absence of
steroid 13, the reaction between 2, 3a, and NEt3 is inefficient
(Table 1, entry 2). Catalyst 13 can be prepared in multigram
scale from cholic acid 1 in two synthetic steps, with an overall
yield of 71%.
To establish the scope of our protocol, a selection of
aromatic nitroalkenes 3b−j, bearing diverse functional groups
and substitution patterns, were reacted with dimethyl malonate
2 under our best reaction conditions (Table 1, entries 3−11).
The corresponding products 4b−j could be isolated in good to
high yield with reliable ee figures, ranging from 73 to 90%.
Substrates such as ortho-substituted nitrostyrenes or larger
malonate derivatives (i.e., diethyl malonate) are not tolerated
by steroid 13, which points to the existence of a tight and size-
restrictive reaction pocket.
We postulate that the reaction between 2′ and nitroalkenes
3 occurs inside the cavity of tripod 13. To support this
hypothesis, we considered carrying out a series of quantum
chemical-calculations employing both Density Functional
(DFT) and Hartree−Fock (HF) levels of theory. Initially, a
study was undertaken to validate a set of functionals (B3LYP,
M06-2X) and basis sets (cc-pVDZ, 6-31G*) in our system,
employing the Gaussian09 program.10 The M06-2X11/cc-
pVDZ12 combination afforded the best results; thus, it was
chosen for the rest of the study (see the SI for details). Next,
the structure of steroid 13 was computed. It displays an
architecture resembling that of cholic acid, in which the
steroidal backbone is barely altered. As anticipated, the
carbamate moieties are oriented toward the inner part of the
steroid giving rise to the formation of a well-defined chiral
cavity, similar to that of other tripodal cholates.13 This cavity is
decorated with three H-bond donating groups that constitute
the active site of triscarbamate 13 (Figure SI_8). Isothermal
NMR titration experiments proved that nitrostyrene 3a docks
within catalysts 13 by means of H-bonding interactions and
allowed the quantification of a binding constant for the
complex [3a·13] (Figures SI_1−SI_5). It was confirmed
further by quantum-chemical calculations (Figure SI_9).
Importantly, upon complexation, the reactivity of 3a toward
a nucleophilic attack, as revealed through its electrophilicity
index (Table SI_9), is markedly enhanced as a consequence of
It is important to remark that, to our knowledge, it is the first
occasion in which carbamate units have been used solely as
supramolecular catalytic sites. In addition, amid steroidal
B
DOI: 10.1021/acs.orglett.9b01170
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
H-bond-mediated polarization.14 Pointing toward the same
direction, adding at the start of the reaction substoichiometric
amounts of MeOH, which competes for H-bonding, lowers the
conversion of 3a to the reaction product 4a, without altering
its ee (Table 1, entry 12). Quantum-chemical calculations
reveal that MeOH inserts within the cavity of triscarbamate 13
by forming an extended network of H-bond interactions (for
details, see Figures SI_14−SI_16). The so occupied steroidal
cleft cannot operate as catalyst, and consequently, the reaction
rate is affected and the conversion diminished. In addition, the
background process between 2, 3a, and NEt3 does not take
place outside the cavity of 13, as it would render racemic
product 4a, and as a consequence, its ee would be severely
impacted.
the TS (Figure 4B), which can be regarded as a near-attack-
conformation.16 According to the computational study the
entropic cost associated with this preorganization is likely to be
compensated by the stabilization of the supramolecular
complex [13·2′·3a] through cooperative H-bond contacts
(Table SI_11). In either case, all the interconverting
supramolecular equilibria are funneled through the reactive
complex [13·2′·3a], which evolves in an asymmetric manner
toward product 4a.
Last, NMR experiments and in silico calculations showed
that product 4a is not a suitable substrate for the cavity of 13,
so it frees the catalyst which can be reinserted into a new
catalytic cycle. Considering all of the previous findings, a
reaction mechanism is proposed (Figure 5).
Continuing with our study, the influence of steroid 13 in the
activation energy of the C−C bond-forming step, which is
liable for the spatial configuration of the reaction product, was
evaluated. For the noncatalyzed reaction between 2′ and 3a a
mean activation energy of +4.4 kcal/mol (ΔG⧧) was computed
for the formation of either (R)- or (S)-4a in the gas phase
(SI).15 For the catalyzed reaction, the catalytic effect as well as
the stereopreference of triscarbamate 13 toward the formation
of product (S)-4a were also studied by theoretical means. In
this case, energy profile calculations showed a large decrease in
the activation energy in the presence of the catalyst, leading to
a virtually barrierless process (Figure SI_11 and SI_12), which
supports the catalytic effect of our steroidal triscarbamate.
Figure 4B presents the minimum energy transition state
(TS) found for the attack of dimethyl malonate anion 2′ to
Figure 4. Representation (rendered with Jmol)17 of (A) the
calculated geometry for the preoganized reagents 2′ and 3a inside
the cavity of steroidal triscarbamate 13; (B) the calculated TS for the
reaction between dimethyl malonate anion 2′ and nitrostyrene (3a)
catalyzed by steroid 13. The steroidal backbone is colored green (H
atoms are omitted for clarity). C atoms are colored gray, O atoms are
colored red, N atoms are colored blue, and H atoms are colored
white. Carbamates’ H atoms are colored orange. H-bonds are
represented by pink dashed lines.
Figure 5. Reaction mechanism suggested for the addition of reaction
between dimethyl malonate, 2, and nitrostyrene, 3a, catalyzed by the
cavity of steroid 13. H-bonds are represented by green dashed lines.
To conclude, for the first time a steroidal scaffold has been
elaborated into a supramolecular host that offers an optimum
three-dimensional setting for promoting an asymmetric
transformation inside its cavity, thus mimicking the action of
some enzymes. It consists of a cholic acid derivative decorated
with three carbamate appendages. Remarkably, this is also the
first occasion in which carbamate units have been disclosed as
active sites in catalysis. The operational mode of our steroidal
tripod has been elucidated with the aid of quantum-chemical
calculations. It implies the preorganization and confinement of
the reagents within the chiral cavity of the steroid in a
conformation close to that of the TS, held up by three
cooperative H-bond contacts.
Considering that cholic acid is rather inexpensive and that it
can be functionalized with ease, we anticipate the use of this
platform in the design, synthesis, and implementation of other
structures, both for organocatalysis and homogeneous metal-
based catalysis. We are currently working in this direction, and
the results will be reported in due course.
nitrostyrene 3a in the presence of the steroidal catalyst 13.
According to the calculations, the asymmetric reaction takes
place inside the cavity of the steroid orchestrated by two H-
bond contacts, established between the NO2 moiety of
nitrostyrene 3a and the N−H motifs of the carbamates
borne on C7 and C12, and a third interaction, between the N−
H of the C3 carbamate and one carbonyl O atom of anion 2′.
This set of supramolecular interactions constrains anion 2′ to
confront the si-face of nitrostyrene 3a, thus rendering product
(S)-4a, in agreement with the experimental observations.
Moreover, the nature of the catalytic phenomenon can be
characterized by considering the initial placement of the
reagents within the steroidal cleft. Anion 2′ and 3a can be
preorganized inside the cavity of 13 prior to the C−C bond-
forming process (Figure 4A) in a disposition strikingly close to
C
DOI: 10.1021/acs.orglett.9b01170
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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H.; Sirit, A. Tetrahedron: Asymmetry 2016, 27, 148. (n) Jimenez, E. I.;
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01170.
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Eexperimental procedures, characterization of the
products, NMR titration experiments, and quantum-
chemical calculations (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
́
́
*E-mail: ccf@uniovi.es.
*E-mail: vdelamo@uniovi.es.
ORCID
Vicente del Amo: 0000-0003-4237-2817
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank CTQ2016-76829-R, CTQ2015-65790-P, AEI/
FEDER, and UE for financial support. C.C. expresses her
gratitude to MINECO for the award of a “Ramon y Cajal”
■
́
contract (RYC-2014-16021). We gratefully acknowledge Dr.
́
Dimas Suarez (Universidad de Oviedo) for his advice during
the preparation of the manuscript.
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DOI: 10.1021/acs.orglett.9b01170
Org. Lett. XXXX, XXX, XXX−XXX