Rhodium-Catalyzed Asymmetric Transfer Hydrogenation/Dynamic Kinetic Resolution of 3-Benzylidene-Chromanones

Article abstract of DOI:10.1021/acs.orglett.1c00047

Straightforward access to enantiomerically enriched cis-3-benzyl-chromanols from (E)-3-benzylidene-chromanones was developed through Rh-catalyzed asymmetric transfer hydrogenation. This transformation allowed the reduction of both the CaC and CaO bonds and the formation of two stereocenters in high yields with excellent levels of diastereo- A nd enantioselectivities (up to >99:1 dr, up to >99% ee) in a single step through a dynamic kinetic resolution process using a low catalyst loading and HCO2H/DABCO as the hydrogen source.

Full text of DOI:10.1021/acs.orglett.1c00047

pubs.acs.org/OrgLett
Letter
Rhodium-Catalyzed Asymmetric Transfer Hydrogenation/Dynamic
Kinetic Resolution of 3‑Benzylidene-Chromanones
Ricardo Molina Betancourt, Phannarath Phansavath,* and Virginie Ratovelomanana-Vidal*
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ABSTRACT: Straightforward access to enantiomerically enriched
cis-3-benzyl-chromanols from (E)-3-benzylidene-chromanones was
developed through Rh-catalyzed asymmetric transfer hydrogena-
tion. This transformation allowed the reduction of both the CC
and CO bonds and the formation of two stereocenters in high
yields with excellent levels of diastereo- and enantioselectivities (up
to >99:1 dr, up to >99% ee) in a single step through a dynamic
kinetic resolution process using a low catalyst loading and
HCO2H/DABCO as the hydrogen source.
Scheme 1. Catalytic Asymmetric Reduction of Three-
Substituted Chromanones
omoisoflavonoids are a widespread family of molecules,
naturally occurring in plants, that possess a promising set
H
of biological activities.1 Among them, antioxidant, anti-
inflammatory, antitumoral, antiviral, antibacterial, and protec-
tive vascular actions can be cited as potential therapeutic
indications.2 The 3-benzyl-chromanol substructure is present
in several molecules, for example, CP-105,696, that possess a
selective and potent LTB4 receptor-inhibiting ability.3 LTB4 is
a chemoattractant for granulocytes and is involved in several
inflammatory diseases such as rheumatoid arthritis and asthma.
An efficient and straightforward route to access enantiomeri-
cally enriched 3-benzyl-chromanols is thus highly desirable. In
this context, Koch et al. reported the synthesis of an
enantiomerically pure 3-benzyl-chromanol starting from the
ketone precursor using a chemical resolution, after NaBH4
reduction, esterification with t-Boc-L-tryptophan, and hydrol-
ysis of the resulting ester.4 Seo et al. later devised an
asymmetric synthesis of cremastranone by using an asymmetric
transfer hydrogenation (ATH) coupled to a dynamic kinetic
resolution (DKR) of a 5,6,7-substituted homoisoflavanone
catalyzed by Noyori’s ruthenium complex [RuCl(p-cymene)-
{(S,S)-Ts-DPEN}] or [RuCl(p-cymene){(R,R)-Ts-DPEN}],
followed by tetrapropylammonium perruthenate (TPAP)
oxidation of the resulting enantiomerically enriched alcohol
(Scheme 1).5 The ATH reaction proceeded using a 3:1
mixture of DBU/HCO2H as the hydrogen source and a
catalyst loading of 30 mol % to achieve full conversion.
Subsequent oxidation of the alcohol with TPAP gave the
desired (R) and (S)-cremastranone without racemization,
allowing confirmation of the absolute configuration of the
natural product as (R) and showing that the antiangiogenic
effect of the (S) isomer was superior to the natural (R) form.
The authors used the same strategy for the total synthesis of
several 5,7,8-trioxygenated chroman-4-ones and homoisoflavo-
noids.6
In the context of our ongoing studies directed toward the
development of efficient methods for the asymmetric reduction
of functionalized ketones7 and to access a wide range of 3-
Received: January 6, 2021
Published: February 18, 2021
https://dx.doi.org/10.1021/acs.orglett.1c00047
© 2021 American Chemical Society
Org. Lett. 2021, 23, 1621−1625
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Letter
a
benzyl-chromanol derivatives, we report herein the first
rhodium-catalyzed ATH of 3-benzylidene-chromanones that
efficiently reduces both the CC and CO bonds in a single
synthetic step and provides in good yields the targeted
molecules with excellent levels of diastereo- and enantiose-
lectivity through a DKR process.8
To investigate the proposed ATH/DKR, racemic (E)-3-
benzylidene-chromanone 1a9,10 was subjected to asymmetric
reduction using several organometallic catalysts in acetonitrile
at 50 °C for 24 h (Table 1).
Table 2. Optimization of the Reaction Conditions
yield
ee
(%)
b
c
d
entry
solvent
hydrogen donor
(%)
dr
1
2
3
4
5
6
7
CH3CN
CH2Cl2
THF
HCO2H/DBU (2:1)
HCO2H/DBU (2:1)
HCO2H/DBU (2:1)
HCO2H/DBU (2:1)
HCO2H/DBU (2:1)
HCO2H/DBU (2:1)
HCO2H/DBU (2:1)
HCO2H/DBU (2:1)
HCO2H/DBU (2:1)
HCO2NH4
95
94
92
84
87
88
95
94
43
49
63
97:3
92:8
96:4
96:4
97:3
90:10
93:7
96:4
97:3
92:8
74:26
>99
99
>99
>99
>99
>99
99
>99
>99
98
toluene
AcOEt
i-PrOH
MeOH
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
e
8
9
a
f
Table 1. Catalyst Screening for the ATH of 1a
10
11
12
13
g
(HCO2)2Ca
i-PrOH/KOH
HCO2H/DABCO (2:1)
89
h
96
99:1
>99
a
Conditions: 1a (0.79 mmol), (R,R)-C (0.5 mol %), hydrogen donor
b
(5 equiv), solvent (1.5 mL), 50 °C, 24 h. Isolated yield of 2a.
c
Determined by 1H NMR of the crude product after the ATH
d
e
reaction. ee for the cis product determined by SFC analysis. 0.25
mol % of (R,R)-C was used. 0.1 mol % of (R,R)-C was used. 0.1 mL
of water was added. 3 equiv of i-PrOH/KOH was used.
f
g
h
b
c
d
entry
cat.
HCO2H/base
yield of 2a (%)
dr
ee (%)
CH2Cl2, THF, toluene, AcOEt, i-PrOH, and MeOH,
performed well (Table 2, entries 1−7). High yields of 84−
95% were obtained in these solvents with diastereomeric ratios
ranging from 90:10 to 97:3 and enantioselectivities of 99% to
>99% ee, with acetonitrile giving the best results.
1
2
3
4
5
(R,R)-A
(R,R)-B
(R,R)-C
(R,R)-B
(R,R)-C
HCO2H/Et3N
HCO2H/Et3N
HCO2H/Et3N
HCO2H/DBU
HCO2H/DBU
84
92
91
96
95
77:23
92:8
97:3
93:7
97:3
>99
95
99
98
>99
Next, the S/C ratio was progressively increased (Table 2,
entries 8 and 9). Increasing the S/C to 400 did not affect the
outcome of the ATH reaction. However, using an S/C of 1000
had a detrimental effect on the yield, which dropped to 43%.
To complete the optimization of the reaction parameters, other
hydrogen sources were examined. Formate salts such as
HCO2NH4 and (HCO2)2Ca led to lower yields, with a
significant unfavorable impact on the stereoselectivity in the
latter case (Table 2, entries 10 and 11). Whereas using
potassium hydroxide in isopropanol failed to afford any
conversion (Table 2, entry 12), the hindered 1,4-
diazabicyclo[2.2.2]octane (DABCO) gave excellent results,
allowing the diastereoselectivity to reach 99:1 dr while
maintaining the enantioselectivity at >99% ee (Table 2, entry
13). From this survey, the optimized conditions were set as
follows: (R,R)-C (0.5 mol %) as the precatalyst and HCO2H/
DABCO (2:1) (5 equiv) as the hydrogen source in CH3CN
solvent at 50 °C.
Having identified an effective stereoselective method to set
the vicinal stereocenters and being amenable to a DKR
process, we explored the scope and limitations of the
asymmetric reduction on a series of 3-benzylidene-chroma-
none derivatives that could be utilized in this novel DKR
transformation (Table 3). Good results were obtained with a
wide range of arene substitution by varying the position (ortho,
meta, or para) of the methoxy group on the aryl ring of the
benzylidene moiety, and compounds 2b−2d were formed in
92−95% yields with 99:1 dr and enantioselectivities up to
>99% ee (Table 3, entries 2−4). Other 3-benzylidene-
chromanone derivatives bearing either electron-donating or
electron-withdrawing groups were efficiently reduced to the
corresponding cis alcohols in good yields up to 93% with high
levels of diastereo- and enantioselectivities (Table 3, entries 5−
10, up to 99:1 dr, up to >99% ee).
a
Conditions: 1a (0.79 mmol), cat. (0.5 mol %), 5 equiv of HCO2H/
Et3N (5:2) or HCO2H/DBU (2:1), MeCN (1.5 mL), 50 °C.
b
c
1
Isolated yield; complete conversion in all cases. Determined by H
d
NMR of the crude product after the ATH reaction. ee for the cis
product determined by supercritical fluid chromatography (SFC)
analysis.
The HCO2H/Et3N (5:2) azeotropic mixture (5 equiv) was
first used as the hydrogen source in the presence of 0.5 mol %
of rhodium or ruthenium complexes (Table 1, entries 1−3).
These conditions led to a full conversion with all of the tested
complexes. The ATH using an oxo-tethered ruthenium catalyst
(R,R)-A11 occurred with a modest diastereomeric ratio of
77:23 in favor of the cis alcohol 2a, which was obtained in 84%
yield with >99% ee (Table 1, entry 1). With the [RuCl(p-
cymene){(R,R)-Ts-DPEN}] complex (R,R)-B,12 a high yield
(92%) and high levels of diastereo- and enantioinductions were
observed (Table 1, entry 2, 92:8 dr, 95% ee). We were
delighted to find that the homemade (R,R)-C13 containing an
(R,R)-TsDPEN ligand tethered to the ancillary η5-arene ligand
outperformed the previous catalysts by yielding a diastereo-
meric ratio of 97:3 with 99% ee (Table 1, entry 3). We next
chose to screen a variety of bases and replaced triethylamine
with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in the hydro-
gen source mixture. The use of a HCO2H/DBU (2:1)
combination pleasingly allowed a slight increase in the yield
of 2a with both (R,R)-B and (R,R)-C, the latter still giving the
best stereoselectivities (Table 1, entries 4 and 5). On the basis
of this encouraging series of results, the rhodium complex
(R,R)-C was chosen as the catalyst for this study.
The investigations continued with the screening of the
solvent, the catalyst loading (S/C), and the nature of the
hydrogen donor (Table 2). Several solvents, such as CH3CN,
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Letter
a
Table 3. Substrate Scope of the ATH/DKR of 1a−1n
a
X-ray crystallography of 2g. Displacement ellipsoids are shown at the 30% probability level. Conditions: 1a−1n (0.79 mmol), (R,R)-C (0.5 mol
b
c
%), 5 equiv of HCO2H/DABCO (2:1), MeCN (1.5 mL). Isolated yield; complete conversion in all cases. Determined by 1H NMR of the crude
d
e
product after the ATH reaction. ee for the cis product determined by SFC analysis. ATH reaction was performed with complex (S,S)-C under
otherwise identical conditions.
Interestingly, a bulky substituent such as a naphthyl group
(Table 3, entry 11) and a heteroaryl substituent such as a furyl
subunit or biphenyl substituents (Table 3, entries 12 and 13,
respectively) were well tolerated. Importantly, the reaction was
not limited to arylchromanone derivatives, and an alkenyl-
substituted chromanone was also accommodated in this
transformation (Table 3, entry 14). In this case, the reduction
proved to be chemoselective of the CC bond located next to
the carbonyl group, yielding the corresponding cis-chromanol
in good yield (69%) with good diastereoinduction (94:6 dr)
and excellent enantiocontrol (>99% ee).
and by analogy. we conjectured that the remainder of the ATH
products followed the same trend. In addition, the (S,S)-
alcohols 2o and 2p could be readily prepared as well by using
the (S,S)-isomer of the rhodium complex C instead of the
(R,R)-enantiomer. In both cases, the reduced compounds were
obtained with results comparable to those obtained for the
parent alcohols 2a and 2b, respectively (Table 3, entries 15
and 16 vs entries 1 and 2).
The utility of the developed reaction was illustrated by its
performance on the gram scale. Compound 1n was subjected
to the ATH/DKR under the same reaction conditions to
provide 2n in 89% yield with 99:1 dr and >99% ee (Scheme 2).
Furthermore, compound 2h was postfunctionalized into 4 in
The absolute configuration of compound 2g was unambig-
uously determined as (R,R) by X-ray crystallographic analysis,
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Letter
0000-0003-1167-1195; Email: virginie.vidal@
chimieparistech.psl.eu
Scheme 2. Post-Functionalization and Scale-Up
Author
Ricardo Molina Betancourt − PSL University, Chimie
ParisTech, CNRS, Institute of Chemistry for Life and Health
Sciences, CSB2D Team, 75005 Paris, France
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00047
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the Ministere de l’Enseignement
■
̀
Supérieur, de la Recherche et de l’Innovation (MESRI) and
the Centre National de la Recherche Scientifique (CNRS). We
gratefully acknowledge the MESRI for a grant to R.M.B. We
thank J. Forté (Sorbonne Université, Paris) for solving the X-
ray structure of compound 2g and Dr C. Fosse (Chimie
ParisTech) for the mass spectroscopy analysis.
80% yield with no loss of diastereo- or enantioselectivity by
protecting the alcohol group with a tert-butyl-di-methyl silane
group followed by a Suzuki−Miyaura cross-coupling by using
Pd(OAc)2, cataCXium A as the ligand, K2CO3 as a base, 4-
methoxyphenylboronic acid, and dimethylformamide (DMF)
as the solvent.
In summary, the practical rhodium-catalyzed ATH of 3-
benzylidene chromanones allows the reduction of two double
bonds in a single step in a stereocontrolled manner. The
unique combination of the Rh(III) complex developed in the
group and formic acid/DABCO (2:1) as a hydrogen source
enables at low catalyst loading under mild conditions the facile
reductive DKR of 3-benzylidene chromanones to access the
corresponding cis-3-benzyl chromanols in high yields with
excellent stereoselectivities (up to >99:1 dr, up to >99% ee).
This efficient and straightforward catalytic route provides
access to synthetically useful chromanol derivatives and
valuable chroman pharmacophores as well and tolerates a
broad range of functionalities.
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ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00047.
Experimental procedures, compound characterization
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CCDC 2048735 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Phannarath Phansavath − PSL University, Chimie ParisTech,
CNRS, Institute of Chemistry for Life and Health Sciences,
CSB2D Team, 75005 Paris, France;
Email: phannarath.phansavath@chimieparistech.psl.eu
Virginie Ratovelomanana-Vidal − PSL University, Chimie
ParisTech, CNRS, Institute of Chemistry for Life and Health
Sciences, CSB2D Team, 75005 Paris, France; orcid.org/
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