Catalytic asymmetric oxo-Diels-Alder reactions with chiral atropisomeric biphenyl diols

Article abstract of DOI:10.3762/bjoc.15.92

New chiral atropisomeric biphenyl diols 3, 4 and 6 containing additional peripheral chiral centers with different steric bulkiness and/or electronic properties were synthesized. The X-ray crystal structure of 3 shows the formation of a supramolecular stru

Full text of DOI:10.3762/bjoc.15.92

Catalytic asymmetric oxo-Diels–Alder reactions with chiral
atropisomeric biphenyl diols
Chi-Tung Yeung1,2, Wesley Ting Kwok Chan2, Wai-Sum Lo1,2, Ga-Lai Law1,2
and Wing-Tak Wong*1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 955–962.
1The Hong Kong Polytechnic University Shenzhen Research Institute,
Shenzhen, PR China and 2State Key Laboratory of Chemical Biology
and Drug Discovery, Department of Applied Biology and Chemical
Technology, The Hong Kong Polytechnic University, Hung Hom,
Kowloon, Hong Kong
doi:10.3762/bjoc.15.92
Received: 11 December 2018
Accepted: 30 March 2019
Published: 18 April 2019
Email:
Associate Editor: M. Rueping
Wing-Tak Wong* - w.t.wong@polyu.edu.hk
© 2019 Yeung et al.; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
asymmetric organocatalysis; axial chirality; biaryls; hydrogen bond;
oxo-Diels–Alder reaction
Abstract
New chiral atropisomeric biphenyl diols 3, 4 and 6 containing additional peripheral chiral centers with different steric bulkiness
and/or electronic properties were synthesized. The X-ray crystal structure of 3 shows the formation of a supramolecular structure
whereas that of 6, containing additional CF3 substituents, shows the formation of a monomeric structure. Diols 1–6 were found to
be active organocatalysts in oxo-Diels–Alder reactions in which 2 recorded a 72% ee with trimethylacetaldehyde as a substrate.
Introduction
The Diels–Alder (DA) reaction is a useful and easy-to-perform subunits of natural products with biological activities (e.g.,
method for the synthesis of six-membered rings through the carbohydrates, antibiotics, toxins etc.) [9,10]. Alternate synthe-
direct formation of C–C bonds between a diene and a dieno- tic pathways include ring formations of open-chained precur-
phile (a substituted alkene); it is called a hetero-Diels–Alder sors [11,12], reactions of dicarbonyl compounds with ketene
(HDA) reaction when one or more heteroatoms (most often diethylacetal followed by hydrolysis [13] or total syntheses
oxygen or nitrogen) are present among the reactants, such as the [14]; however, none of these alternatives could rival the combi-
use of carbonyl compounds or imines as dienophiles [1-5]. An nation of ease and cost-effectiveness of HDA reactions.
asymmetric HDA reaction is capable of introducing up to four
stereogenic centers in a one-step [4 + 2] cycloaddition or cycli- Oxo-Diels–Alder (oxo-DA) reactions between electron-poor
zation reaction [6-8] and it has become hugely popular in pre- aldehydes and electron-rich dienes such as Danishefsky’s
paring vital intermediates for the syntheses of key structural dienes or Brassard’s dienes are efficient ways to construct
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Beilstein J. Org. Chem. 2019, 15, 955–962.
oxygen-containing six-membered heterocycles via [4 + 2] cycli- lecular hydrogen bonding of two axially chiral biphenyl hybrid
zations, and have been dominated by metal-based chiral Lewis diols (1 and 2 in Scheme 1) which contain point chirality at the
acid catalysts for over three decades [15-26]. Comparatively, side arms and axial chirality at the biphenyl backbone [37]. We
interest in the utilization of metal-free organocatalytic oxo-DA envisage the structural similarity and the ability of our scaffold
reactions began to grow only after Rawal’s group reported a to form strong hydrogen bonds could perform the same catalyt-
ground-breaking contribution in using a diol molecule, ic role in oxo-DA reactions as reported in the literature. Inspired
TADDOL, as a hydrogen bonding organocatalyst for the reac- by Rawal’s work on TADDOL and BAMOL organocatalysts,
tion between 1-dimethylamino-3-tert-butyldimethylsilyloxy- we, in this work, have adopted a similar reaction of Rawal’s
1,3-butadiene (Rawal’s diene) and aldehydes with excellent en- diene with benzaldehyde as a starting point of our study and
antioselectivities (aromatic aldehyde: up to 86–98% ee) in 2003 other biphenyl hybrid diols with different steric bulky substitu-
[27]. Activation via a single-point hydrogen bond between one ents were incorporated (3 and 4). Since it is known that the
of the hydroxy groups of the TADDOL and the carbonyl catalyst acidity has significant influence on hydrogen-bond-cat-
oxygen of the aldehyde was proposed to be a crucial factor for alyzed reactions [32], we also incorporated a CF3 group in our
the success of this organocatalyst in the reaction. Two years molecular scaffold to investigate how it would affect the reac-
later, they reported another efficient diol-based hydrogen bond- tivity and selectivity (5 and 6). Recently, we found that com-
ing organocatalyst, BAMOL, for catalyzing the same oxo-DA pound 5, which can form a pair of atropisomer (P)-(R,R)-5 and
reactions with a library of aldehydes (aromatic: 97–99% ee; ali- (M)-(R,R)-5, gave different results in N-nitroso aldol reactions
phatic: 84–98% ee) [28]. Thereafter, many different kinds of compared to 1 [38].
hydrogen bonding-based organocatalysts have been developed
Results and Discussion
for oxo-DA reactions [29-35]. One kind of organocatalyst in
particular, which is based on an oxazoline template with Synthetic procedures
hydroxy and NH units for hydrogen bonding activation, % ee Synthetic steps of catalyst 3 were similar to that for 1 and 2
and yields in oxo-DA reactions were found to be enhanced with [37]. Asymmetric reduction of the carbonyl group of
increasing NH acidity, leading to stronger hydrogen bonds [32]. 2’-bromobenzophenone (a) with borane dimethyl sulfide in the
So, there is much room for further investigation and improve- presence of (S)-(−)-2-methyl-CBS-oxazaborolidine catalyst
ment with other hydrogen bonding organocatalysts.
gave (S)-(2-bromophenyl)(phenyl)methanol (S)-b in 93% yield
and 94% ee (Scheme 2 and Figure S1 in Supporting Informa-
An earlier work by Goldfuss and his co-worker on an atropiso- tion File 1). Homo-coupling of (S)-b with Ni(COD)2 was
meric biphenyl compound showed that atropisomerism can be achieved with very high diastereoselectivity and only a single
induced and stabilized with hydrogen bonding from fenchyl atropisomer was formed. The X-ray crystal structure showed
alcoholic units [36]. Regarding to atropisomeric properties of that it is (M)-(S,S)-3 (Figure 1). This high atropstereoselectivity
biphenyl compounds, our group have previously reported the is believed to be due to the energetically unfavorable formation
formation of supramolecular helices or dimers through intermo- of the other atropisomer (P)-(S,S)-3 as a higher steric repulsion
Scheme 1: Chiral biphenyl diol organocatalysts 1–6.
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Beilstein J. Org. Chem. 2019, 15, 955–962.
Scheme 2: Synthesis of 3.
Figure 1: (a) Single crystal X-ray structure of 3: showing intra- and intermolecular hydrogen bonds (green dashed line). Torsion angles of biphenyl
rings is 76.25(35)–78.27(35)°; axial configuration is M. (b) Crystal packing of 3 shows one strand of supramolecular left-handed helixes (hydrogens
that are not involved in interactions are omitted for clarity).
is generated during the close approach of two bulky phenyl For catalyst 4, different from the synthesis of catalyst 3, asym-
peripheral substituents. The X-ray crystal structure also reveals metric reduction of the corresponding ketone, (2-bromo-
that an alternative formation of intra- and intermolecular hydro- phenyl)(mesityl)methanone c, using (S)-(–)-2-methyl-CBS-
gen bonds [intra- D(OH···O): 1.951(3) Å; inter- D(OH···O): oxazaborolidine as the catalyst resulted in a very low enantiose-
1.822(3) Å] led to an enantiomerically pure infinite helical lectivity (<15% ee) of the product, (2-bromophenyl)(mesityl)-
supramolecular structure. In contrast, only a dimeric structure methanol (d, Scheme 3). For this reason, another strategy of ob-
was observed for the corresponding racemic mixture [39]. In taining high % ee of d was employed. Chiral resolution of
our case, atropisomerization from (M)-(S,S) to (P)-(S,S) was not racemic-d with (R)-menthyl chloroformate gave a high % ee of
observed in solution.
this key intermediate d. After column chromatographic purifica-
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Beilstein J. Org. Chem. 2019, 15, 955–962.
Scheme 3: Synthesis of 4.
tion of the crude reaction mixture, the corresponding diastereo- tion of (S)-h was checked with its X-ray crystal structure (Sup-
mers (R)-e or (S)-e can be partly separated. Their diastereose- porting Information File 1, Figure S4). The optically pure (S)-g
lectivities can be further enhanced by slow evaporation of the was then obtained by removing the chiral camphanic substitu-
corresponding diastereomers from acetonitrile and the absolute ent (Supporting Information File 1, Figure S5) and (S)-g was
configuration of one diastereomer was confirmed with X-ray then utilized for homo-coupling with Ni(COD)2 to give catalyst
crystal structure (Supporting Information File 1, Figure S2). 6. X-ray crystal structure of 6 showed that it is an (P)-(S,S) atro-
After removing the chiral-resolving menthyl substituent with pisomer (Figure 2). From the structure, no extensive intermolec-
lithium aluminium hydride (LAH), the enantioselectivity of ular OH···O hydrogen bonds can be found for the formation of
(R)-d was higher than 99% ee when checked with HPLC (Sup- the supramolecular structure. Instead, intramolecular hydrogen
porting Information File 1, Figure S3). Catalyst 4 was then ob- bonds [D(O–H···O): 2.072(31) Å; (O–H∙∙∙O): 161.39(31)°]
tained by homo-coupling of (R)-d with Ni(PPh3)3Br2/Zn with were found to keep the conformation of the biphenyl ring intact.
37% yield. The low yield of catalyst 4 may be due to steric Relative spatial arrangement of the larger phenyl substituent is
bulkiness of the mesityl group that hampers close approach of d located at the side where it is relatively far away from the intra-
during coupling.
molecular hydrogen bond unit.
For the synthesis of catalyst 6, the CF3 substituent was intro- Catalytic studies
duced to f by trifluoromethylation (Scheme 4). A racemic mix- With the organocatalyst 1 in hand, we firstly examined the oxo-
ture of g was obtained after the TMS group was removed with DA reaction of benzaldehyde and Rawal’s diene in standard
TBAF. This racemate then underwent chiral resolution by conditions. The reaction was found to be very sluggish when
reacting with (1S)-(−)-camphanic chloride to give a diastereo- performed at −40 °C. However, at −20 °C, better yields were
meric mixture of (R)-h or (S)-h. After separating these dia- obtained. The reactions with organocatalysts 1–4 showed mod-
stereomers by column chromatography, the absolute configura- erate yields (41–64%) of the catalytic product (2,3-dihydro-2-
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Scheme 4: Synthesis of 6.
Figure 2: X-ray crystal structure of (P)-(S,S)-6 at two different orientations to show (a) P atropselectivities; (b) different relative spatial arrangement of
the CF3- and phenyl-substituents. Torsional angle of biphenyl rings is 104.55(22).
phenyl-4H-pyran-4-one). Unfortunately, benzaldehyde was have been reported previously to have a significant effect on
found to be a challenging substrate and low enantioselectivities controlling catalytic enantioselectivites [41,42]. For organocata-
(2–8% ee) (Table 1, entries 1–4) were obtained. The enantiose- lyst 6, it gave the highest enantioselectivity (59% ee) but a low
lectivities can be improved slightly (32% ee and 11% ee) when chemical yield.
(P)-(R,R)-5 and (M)-(R,R)-5 were employed as catalysts, al-
though the yields decreased sharply (Table 1, entries 5 and 6). It When the bulkier trimethylacetaldehyde was used as substrate,
should be noted that the absolution configuration of the catalyt- the enantioselectivities were improved when 1–3 were used as
ic product seems to be determined by the axial chirality (P or catalysts (Table 1, entries 8–11). With 1, the yield and enantio-
M) of 5 rather than its point chirality at the side arms (both have selectivity of the catalytic product (2,3-dihydro-2-tert-butyl-4H-
the same R-chirality) [40]. The axial chirality of other catalysts pyran-4-one) were 50% and 12% ee (Table 1, entry 8). The re-
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Table 1: Catalytic asymmetric oxo-DA reactions with stereolabile chiral biphenyl diols 1–6.a
Entry
Catalyst
Aldehyde
Yield (%)b
ee (%) (configuration)c
1
2
3
4
5
6
7
1
46
41
52
64
15
16
15
7 (S)
2 (S)
2
3
3 (S)
4
8 (R)
(P)-(R,R)-5
(M)-(R,R)-5
6
32 (S)
11 (R)
59 (R)
8
9
1
50
66
26
28
16
14
15
16
12e
72e
74e
16e
6e
2
10d
11
12
13
14
15
2
3
4
(P)-(R,R)-5
(M)-(R,R)-5
6
30e
10e
10e
16
17
2
2
42
32
56 (R)
52f
18
19
20
2
2
2
33
30
25
43(R)
36f
2f
aReactions were run with 0.1 mmol of catalyst, 1.0 mmol aldehyde and 0.5 mmol diene in 0.5 mL dried toluene under nitrogen at −20 °C for 1 day.
Then the reactions were worked up with 1 mmol of AcCl at −78 °C. Products were isolated by column chromatography with silica gel. bIsolated yields.
cDetermined with HPLC with chiral columns, and the absolute configuration assigned by comparison with the order of elution of known compounds
[28,29,43]. dReaction was performed at −40 °C for 3 days. eAbsolute configuration undetermined. OD-H column, Hex:IPA = 98:2, 1 mL/min, (10.7 min
and 11.6 min) latter peak is a major peak for entries 8–12, 14 and 15. Previous peak is a major peak for entry 13. fAbsolute configuration undeter-
mined.
activity and enantioselectivity were significantly increased to yields and enantioselectivities of the catalytic product de-
66% yield and 72% ee when 2 was employed as the catalyst creased dramatically from 2 (Table 1, entries 11 and 12). For 5,
(Table 1, entry 9). The enantioselectivity can be improved similar to the case of using benzaldehyde as substrate,
slightly to 74% when the temperature was decreased to −40 °C (P)-(R,R)-5 resulted in a higher enantioselectivity than
(Table 1, entry 10). For 3 and 4, with aromatic substituents, the (M)-(R,R)-5 and their products have opposite absolute configu-
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Beilstein J. Org. Chem. 2019, 15, 955–962.
rations (Table 1, entries 13 and 14). For organocatalyst 6, the University [University Research Facility for Chemical and
enantioselectivity was not as good as when using benzaldehyde Environmental Analysis (UCEA)]. G.-L. Law gratefully
as substrate (Table 1, entry 15).
acknowledges financial support from Natural Science Founda-
tion of China (NSFC, 21875201)
In order to explore the scope of the present oxo-DA reaction,
we next examined the reactions of other aldehydes with 2 as the
catalyst. With less bulky aliphatic aldehydes, which only have
hydrogens at the alpha positions such as isobutyraldehyde, 3,3-
dimethylbutyraldehyde, cyclohexylaldehyde and 2-phenylacet-
aldehyde, decrease in the yields and enantioselectivities were
observed (Table 1, entries 16–19). With another aromatic alde-
hyde, 2-naphthaldehyde, a racemic product was obtained.
ORCID® iDs
Wesley Ting Kwok Chan - https://orcid.org/0000-0002-5933-0181
Wai-Sum Lo - https://orcid.org/0000-0001-5329-4581
Ga-Lai Law - https://orcid.org/0000-0002-2192-6887
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