Planar chiral [2.2]paracyclophane-based bisoxazoline ligands: Design, synthesis, and use in cu-catalyzed inter- and intramolecular asymmetric O-H insertion reactions

Article abstract of DOI:10.1248/cpb.c18-00519

Centrally chiral bisoxazolines connected directly to a planar chiral [2.2]paracyclophane backbone were synthesized and evaluated as asymmetric ligands in Cu-catalyzed intermolecular ethanolic O-H insertion reactions of α-diazo esters. The reactivities and enantioselectivities of Cu complexes of the synthesized bisoxazoline ligands were lower than those of ligands without central chirality. However, planar chiral [2.2] paracyclophane-based bisoxazoline ligands with an inserted benzene spacer that had a sterically demanding isopropyl substituent showed good enantioselectivities in inter- and intramolecular aromatic O-H insertion reactions, without the aid of central chirality.

Full text of DOI:10.1248/cpb.c18-00519

1006
Chem. Pharm. Bull. 66, 1006–1014 (2018)
Vol. 66, No. 10
Regular Article
Highlighted Paper selected by Editor-in-Chief
Planar Chiral [2.2]Paracyclophane-Based Bisoxazoline Ligands:
Design, Synthesis, and Use in Cu-Catalyzed Inter- and Intramolecular
Asymmetric O–H Insertion Reactions
,a
Shinji Kitagaki,* Shunsuke Murata,a Kisaki Asaoka,a Kenta Sugisaka,b Chisato Mukai,b
Naoko Takenaga,a and Keisuke Yoshidaa
a Faculty of Pharmacy, Meijo University; 150 Yagotoyama, Tempaku-ku, Nagoya 468–8503, Japan: and b Division of
Pharmaceutical Sciences, Graduate School of Medical Sciences, Kanazawa University; Kakuma-machi, Kanazawa,
Ishikawa 920–1192, Japan.
Received July 7, 2018; accepted July 24, 2018
Centrally chiral bisoxazolines connected directly to a planar chiral [2.2]paracyclophane backbone were
synthesized and evaluated as asymmetric ligands in Cu-catalyzed intermolecular ethanolic O–H insertion
reactions of α-diazo esters. The reactivities and enantioselectivities of Cu complexes of the synthesized
bisoxazoline ligands were lower than those of ligands without central chirality. However, planar chiral
[2.2]paracyclophane-based bisoxazoline ligands with an inserted benzene spacer that had a sterically de-
manding isopropyl substituent showed good enantioselectivities in inter- and intramolecular aromatic O–H
insertion reactions, without the aid of central chirality.
Key words cyclophane; planar chirality; bisoxazoline; O–H insertion; copper
Substituted [2.2]paracyclophanes (PCPs) have been used as its characteristic substituents (R3 and R4). We have already re-
planar chiral ligands or organocatalysts in a variety of asym- ported that the single spacer in PCP-based phosphine–phenol
metric reactions because of PCP features such as configura- catalysts (Sp)-A (Fig. 1, left) is crucial for achieving higher
tional stability and a diversity of possible chiral structures.1–5) reactivity and enantioselectivity in the aza-Morita–Baylis–
However, most PCP-based ligands or catalysts with a high Hillman reactions of N-tosylaldimines and vinyl ketones11,12)
asymmetric induction ability have central chirality along with (Chart 1a). The phosphine catalysts (Sp)-A can also be used in
their intrinsic planar chirality, except in the cases of phane- the highly enantioselective [3+2] annulations of allenoates and
phos (4,12-bis(diphenylphosphino)[2.2]paracyclophane),6) N- N-tosylaldimines13) (Chart 1b). We envisaged that the planar
heterocyclic carbene (NHC) carbenes bearing two PCP units,7) chirality of the C2-symmetric bisoxazoline (Box) ligand (Sp)-1,
and others.3,8–10) The potential ability of this planar chiral PCP which has a PCP backbone, could strictly control the enantio-
backbone for asymmetric induction has therefore not yet been selectivity of the metal-catalyzed asymmetric reaction without
sufficiently investigated.
We were interested in the use of pseudo-ortho-substituted
aryl-PCPs as catalyst scaffolds with no additional chiral
sources, which are expected to provide efficient asymmetric
environments different from those of known functionalized
PCPs. Our design concept is as follows. One or two spacer
aryl groups are connected to the pseudo-ortho position of
the PCP backbone and two functional groups (R1 and R2) are
located at the meta position of the spacer or directly on the
backbone (Fig. 1). The spacer has the conformational flexibil-
ity to achieve a distance between the two functional groups
that is suitable for a reaction to occur and also has a steric or
electronic effect, which depends on the aryl group itself and/or
Chart 1. PCP-Based Phosphine–Phenol-Catalyzed Asymmetric Reac-
Fig. 1. Design Concept of Planar Chiral PCP Catalysts
tions
*To whom correspondence should be addressed. e-mail: skitagak@meijo-u.ac.jp
© 2018 The Pharmaceutical Society of Japan

Vol. 66, No. 10 (2018)
Chem. Pharm. Bull.
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the aid of central chirality (Fig. 1, right, R1=R2=oxazolinyl). showed good enantioselectivities in intermolecular ethanolic
The known chiral Box ligands are easily prepared from chiral and phenolic O–H insertions.35) We now report the effects of
amino alcohols and usually have two centrally chiral oxazo- the introduction of central chirality of the oxazoline ring in
line rings.14) There are therefore few examples of the use of the PCP-based Box ligand on intermolecular ethanolic O–H
chiral Box ligands bearing achiral oxazoline units in catalytic insertion. Details of inter- and intramolecular aromatic O–H
asymmetric reactions.15–17)
The Cu-catalyzed O–H insertion of α-diazo esters is use- complexes are also given.
insertion reactions catalyzed by PCP-based Box (Sp)-1-Cu
ful for the construction of α-alkoxycarbonyl structures,
We previously investigated the use of five C2-symmetric
which are found in natural products and biologically active PCP-Box ligands (Sp)-1, which had spacers with differ-
compounds.18–20) A highly enantioselective version of this re- ent steric features, in the Cu-catalyzed insertion of methyl
action has recently been accomplished with bisazaferrocene α-diazophenylacetate (2) into the O–H bond of ethanol under
I,21) Spirobox II,17,22–26) or the imidazoindolephosphine ligand the conditions 5mol% of Cu(OTf)2, 6mol% of the Box ligand,
III27–34) (Fig. 2). However, there is still a need to develop other 6mol% of NaBArF (sodium tetrakis[3,5-bis(trifluoromethyl)-
ligands for this type of reaction because the more versatile phenyl]borate) as an additive, and 5-Å molecular sieves in
Box ligands IV and V are unsuitable.21–24,26) In this context, CH2Cl2 at 40°C35) (Chart 2). The phenyl-substituted ligand
we recently used our designed PCP-based Box ligands (Sp)-1, (Sp)-1b showed the highest enantioselectivity, namely 76%.
in which two achiral oxazoline units are located at the meta When the unsubstituted ligand (Sp)-1a was used, the enantio-
positions of the spacer, in Cu-catalyzed O–H insertion reac- selectivity for product 3 decreased significantly, and ligands
tions. We reported in a short communication that the ligands (Sp)-1c and (Sp)-1d, characterized by horizontal extension of
the substituent on the spacer, also showed lower selectivities.
However, the vertically extended ligand (Sp)-1e gave an enan-
tiomeric excess (ee) similar to that for (Sp)-1b. Ligand (Sp)-4a,
in which both oxazoline functionalities are directly connected
to the PCP backbone, gave a moderate level of asymmetric
induction (46% ee) in the opposite sense, despite having no
bulky substituent that could provide stereocontrol. We there-
fore investigated installation of central chirality in the (Sp)-4
ligand and use of the synthesized planar–central hybrid chiral
ligands to achieve improved enantioselectivity.
The effects of central chirality of the oxazoline ring in
PCP-based Box were investigated by using phenyl- and ben-
zyl-substituted Box ligands 4b and 4c, in which substituted
oxazolines were directly connected to the PCP backbone.
Fig. 2. Reported Bidentate Ligands Used in Cu-Catalyzed O–H Inser-
Phenyl-substituted Box ligands (Sp,S,S)- and (Rp,S,S)-4b were
tions
prepared from the corresponding (S)-amino alcohol and ra-
Chart 2. Box (Sp)-1-Cu-Catalyzed Intermolecular Ethanolic O–H Insertion of 2

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cemic pseudo-ortho-[2.2]cyclophanedicarboxylic acid (6), de- vision of an efficient asymmetric environment for O–H inser-
rived from bromocyclophanyl triflate (5),36) as shown in Chart tion. When the benzyl-substituted PCP-Box ligands (Sp,S,S)-
3. The benzyl-substituted Box ligands (Sp,S,S)- and (Rp,S,S)- and (Rp,S,S)-4c were used, the reactions were sluggish and
4c were prepared from (Sp)- and (Rp)-6, respectively.
incomplete, even at 60°C. The benzyl group on the oxazoline
With four Box ligands bearing both planar and central ring can protrude toward the reaction site and prevent forma-
chiralities in hand, we explored their use in O–H insertion tion of a Cu–carbene complex.
reactions. For all the ligands examined, the levels of asym-
The Box ligands 4 with both planar and central chirali-
metric induction were lower than that of Box (Sp)-4a bearing ties proved to be unsuitable for Cu-catalyzed intermolecular
achiral oxazoline rings, and the sense of the enantioselectivity ethanolic O–H insertion, therefore we next investigated the
depended on the planar chirality of the PCP backbone rather use of the Box ligands (Sp)-1, with only planar chirality, in
than the central chirality of the oxazoline ring (Table 1, en- inter- and intramolecular aromatic O–H insertion reactions.
tries 1–5). These results suggest that the alkyl groups on the For the intermolecular phenolic O–H insertion reaction of
oxazoline ring connected to the PCP do not contribute to pro- ethyl α-diazopropionate (7),22,27) the enantioselectivity of the
isopropyl-substituted ligand (Sp)-1d was better than that of
phenyl-substituted (Sp)-1b, and 80% ee was achieved, with the
S configuration being preferred35) (Table 2, entries 1, 3). The
insertion reaction proceeded smoothly at room temperature,
and the use of CuCl as a Cu source gave the highest enan-
tiocontrol, but the product yield was lower than that obtained
with Cu(OTf)2 (entry 6). Use of the sterically hindered tert-
butyl ester, lowering the reaction temperature to 0°C, or the
absence of 5-Å molecular sieves led to decreases in the prod-
uct ee values (entries 4, 7, and 8).
A catalyst prepared in situ from Cu(OTf)2 and (Sp)-1d was
used to investigate the effects of the substituent on the ben-
zene ring of the phenol. An electron-donating group at the
para position led to a slight increase in the product enantio-
selectivity, whereas an electron-withdrawing group led to a
decrease in both the reactivity and product ee (Table 3, entries
2 and 5). A substituent at the ortho position also led to a de-
crease in the reactivity (entries 3, 4, and 6). These trends are
similar to those reported for a chiral imidazoindolephosphine–
Cu complex.27)
Finally, intramolecular aromatic O–H insertion reactions17)
with Box (Sp)-1d-Cu as the catalyst were examined. In the
absence of a ligand, the CuOTf(C6H6)1/2-catalyzed reaction of
2-diazo-o-hydroxyphenylpropanoate 10a proceeded smoothly
at room temperature to afford the desired dihydrobenzofuran
product 11a in only 12% yield, along with the α,β-unsaturated
ester 12a (60%) (Table 4, entry 1). In contrast, the reaction
with the catalyst prepared in situ from CuOTf(C6H6)1/2 and
(Sp)-1d gave the cyclized product 11a as the major product in
58% yield with 72% ee, although a prolonged reaction time
(3h) was required (entry 2). Use of the ligand (Sp)-1d not only
Chart 3. Preparation of PCP-Based Box Ligands with Central Chirality
induced an asymmetric reaction but also prevented production
Table 1. Box 4-Cu-Catalyzed Intermolecular Ethanolic O–H Insertion of 2a)
Entry
Ligand
Solvent
Temp (°C)
Time (h)
Yield (%)b)
ee (%)c)
1
2
3
4
5
(Sp)-4a
CH2Cl2
CH2Cl2
40
40
40
60
60
9
4
77
83
68
13
19
46 (R)
15 (R)
11 (S)
25 (R)
1 (S)
(Sp,S,S)-4b
(Rp,S,S)-4b
(Sp,S,S)-4c
(Rp,S,S)-4c
CH2Cl2
9
(CH2Cl)2
(CH2Cl)2
20
38
a) All reactions were performed with ligand (0.012mmol), Cu(OTf)2 (0.010mmol), NaBArF (0.012mmol), 5-Å molecular sieves (300mg), ethanol (1.0mmol), and diazo
ester 2 (0.20mmol) in CH2Cl2 (total 2mL). b) Isolated yield. c) Determined by HPLC analysis (Daicel CHIRALCEL OD-H).

Vol. 66, No. 10 (2018)
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Table 2. Box (Sp)-1-Cu-Catalyzed Intermolecular Phenolic O–H Insertion
of 7a)
diazoarylpropanoates 10b and 10c were similar to those of
10a in the presence of the Box (Sp)-1d-CuOTf(C6H6)1/2 catalyst
(entries 7 and 9). However, chloro-substituted 10d gave a poor
insertion product yield, as expected (entry 11). The dihydro-
benzopyran product 11e was obtained from diazobutanoate
10e in moderate yield and with good enantioselectivity under
the same conditions (entry 13).
It is generally accepted that X–H insertion via metal-
catalyzed decomposition proceeds by a stepwise mechanism,
which involves generation of a metal-associated onium ylide
and proton transfer from the metal-associated ylide or from
the free ylide, if the X atom has lone-pair electrons.19,20,40)
Zhou and colleagues reported that proton transfer is involved
in the rate-limiting step in both Cu-catalyzed O–H insertion
with water23) and N–H insertion with aniline.41,42) On the basis
of the catalyst structure, Zhou proposed a chiral induction
model for the N–H insertion process, in which the conforma-
tion of the copper carbenoid and the direction of attack of
aniline followed by a configuration-retaining proton transfer
are controlled by the C2-symmetric chiral pocket41,42) (Fig. 3a).
Considering the similarity between O–H insertion and N–H
insertion, and the observation that the preferred absolute ste-
reochemistries of all products (ethanolic O–H insertion prod-
uct 3, intramolecular aliphatic O–H insertion product,35) and
inter- and intra-molecular aromatic O–H insertion products 9
and 11) obtained with the Box (Sp)-1-Cu catalysts are opposite
to those obtained with Zhou’s Spirobox (Sa,S,S)-5-Cu,22,24–26)
the C2-symmetric catalyst conformation during the Cu-cata-
lyzed O–H insertion reaction is proposed to be that shown in
Fig. 3b. However, there is no experimental support for this ac-
tive catalyst structure at this stage. In Box ligands 4, the sub-
stituents at position 4 on the oxazoline ring come into close
proximity to the Cu atom, because of their large bite angles.
When the R groups at position 4 are bulky, copper carbenoid
formation might be inhibited and/or catalyst dissociation be-
fore the stereochemistry-determining step might be acceler-
ated because the R group is too close to the Cu atom, leading
to lower reactivity and selectivity (Fig. 3c).
Entry
Ligand
Cu salt
Time (h) Yield (%)b) ee (%)c)
1
(Sp)-1b
(Sp)-1c
(Sp)-1d
(Sp)-1d
(Sp)-1d
(Sp)-1d
(Sp)-1d
(Sp)-1d
CuCl
CuCl
2
2
1
2
1
1
2
2
73
65
66
68
78
75
61
48
61
39
80
36
57
76
67
24
2
3
4d)
CuCl
CuCl
5
CuOTf(C6H6)1/2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
6
7e)
8f)
a) All reactions were performed with ligand (0.012mmol), [Cu] (0.010mmol),
NaBArF (0.012mmol), 5-Å molecular sieves (300mg), PhOH 8a (1.0mmol), and
diazo ester 7 (0.20mmol) in CH2Cl2 (total 2mL) unless otherwise stated. b) Isolated
yield. c) Determined by HPLC analysis (Daicel CHIRALCEL OD-H). d) tert-Butyl
α-diazopropionate was used instead of ethyl α-diazopropionate (7). e) Reaction was
performed at 0°C. f) Reaction was performed without 5-Å molecular sieves.
Table 3. Box (Sp)-1d-Cu-Catalyzed Intermolecular Aromatic O–H Inser-
tion of 7a)
In summary, we found that Cu complexes with C2-symmet-
ric planar chiral PCP-Box ligands with no central chirality
catalyzed inter- and intramolecular O–H insertion reactions.
A bulky substituent on the benzene spacer of the PCP-based
Box ligand was important for achieving a high level of planar-
chirality-controlled asymmetric induction (up to 80% ee). The
introduction of central chirality of the oxazoline ring connect-
ed directly to the PCP backbone led to unfavorable results.
Further investigations of the catalyst structure and use of the
planar chiral ligands in other catalytic asymmetric reactions
are currently underway.
Experimental
General Information Melting point (mp) was measured
by Yanaco melting point apparatus MP-500D and uncor-
rected. Optical rotations were measured on a JASCO P-2200.
IR spectra were measured with a SHIMADZU FTIR-8700
a) All reactions were performed with ligand (0.006mmol), Cu(OTf)2 (0.005mmol),
NaBArF (0.006mmol), 5-Å molecular sieves (150mg), phenol 8 (0.5mmol), and
diazo ester 7 (0.10mmol) in CH2Cl2 (total 1mL). b) Isolated yield. c) Determined by
HPLC analysis.
1
spectrometer for samples in CHCl3. H- and 13C-NMR spec-
tra were recorded by a JNM-ECA500 or a JNM-ECA600 or
of the undesired β-hydride elimination37,38) product 12.39) In a Bruker Avance III 600 spectrometer for samples in CDCl3
screening of the Cu source, copper(I) halides showed good with tetramethylsilane (δ=0.0ppm) as an internal standard.
enantioselectivities but low reactivities (entries 4 and 5). The The data are reported as follows: chemical shift in ppm (δ),
reactivities and enantioselectivities of methoxy-substituted multiplicity (s=singlet, d=doublet, t=triplet, q=quartet,

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Vol. 66, No. 10 (2018)
Table 4. Box (Sp)-1d-Cu-Catalyzed Intramolecular Aromatic O–H Insertion of 10a)
a) All reactions were performed with ligand (0.006mmol), CuOTf(C6H6)1/2 (0.005mmol),
NaBArF (0.006mmol), 5-Å molecular sieves (250mg), and diazo ester 10 (0.10mmol) in CH2Cl2
(total 1mL), unless otherwise stated. b) Isolated yield of 11. c) The values in parentheses are yields
of α,β-unsaturated ester 12. d) Determined by HPLC analysis. e) Reaction was performed without
ligand (Sp)-1d.
m=multiplet), integration, and coupling constant (Hz). High
Procedure for the Preparation of PCP-Based Box
resolution MS were measured with a JEOL JMS-T100TD. Ligands with the Central Chirality
Analytical TLC was performed on MERCK silica gel, grade
(Sp,S,S)-(–)-4,12-Bis(4-phenyl-2-oxazolinyl)[2.2]para-
60F254. The spots and bands were detected by UV light of cyclophane ((Sp,S,S)-4b) and (Rp,S,S)-(+)-4,12-Bis(4-phenyl-
irradiation (254nm) and/or by staining with 5% phosphomo- 2-oxazolinyl)[2.2]paracyclophane ((Rp,S,S)-4b)
lybdic acid followed by heating. Column chromatography for
To a mixture of rac-6 (56.0mg, 0.189mmol), (S)-phenyl-
isolation of the products was carried out on KANTO Sillica glycinol (57.0mg, 0.416mmol), 1-hydroxybenzotriazole (HOBt)
Gel 60 (230–400 mesh). HPLC analyses were performed using (56.2mg, 0.416mmol) and dicyclohexylcarbodiimide (DCC)
Interigent UV/VIS Detector JASCO UV-7500. The chiral col- (164mg, 0.795mmol) was added tetrahydrofuran (THF) (2mL)
umns included CHIRALCEL OD-H, OJ-H and CHIRALPAK at −5°C. After stirring for 1h at the same temperature, the
AS-H (Daicel Chemical Industries, Ltd., ϕ0.46×25cm). All reaction mixture was warmed to room temperature, and fur-
reactions were carried out under a nitrogen atmosphere unless ther stirred for 16h. The mixture was concentrated and chro-
otherwise stated. Organic extracts were dried over anhydrous matographed with AcOEt to afford a mixture of (Rp,S,S)- and
Na2SO4. Ligands (Sp)-1b, 1c, 1d and 4a,35) compounds rac-536) (Sp,S,S)-4,12-bis[N-(2-hydroxy-1-phenylethyl)carbamoyl][2.2]-
and 6,35) and diazo esters 2,21) 743) and 10a–e17) were prepared paracyclophane (78.6mg, 78%) as a white solid.
according to the literature procedure.
To a solution of the above mixture of amides (78.6mg,
0.148mmol) and PPh3 (253mg, 0.964mmol) in MeCN (2mL)
were added Et3N (0.13mL, 1.0mmol) and CCl4 (0.1mL,
1mmol) at room temperature. After stirring for 10h at the
same temperature, the reaction mixture was diluted with

Vol. 66, No. 10 (2018)
Chem. Pharm. Bull.
1011
Fig. 3. Proposed Catalyst Structure Deduced from Product Stereochemistry
AcOEt, washed with water and brine, dried and concentrated 0.110mmol), HOBt (14.9mg, 0.110mmol) and DCC (43.2mg,
to dryness. The residue was chromatographed with hex- 0.209mmol), and the title compound (8.9mg, 34%) was
ane–CH2Cl2–AcOEt (13:6:1) to afford (Sp,S,S)-4b (28.5mg, prepared using PPh3 (52.4mg, 0.200mmol), Et3N (30µL,
39%) as a white solid and (Rp,S,S)-4b (25mg, 34%) as a 0.22mmol) and CCl4 (19 µL, 0.20mmol), according to the
white solid; (Sp,S,S)-4b: mp 193–195°C; [α]D22 −178.6 (c=1.00, procedure for preparation of 4b. A white solid; mp 109–111°C;
1
CHCl3); IR (CHCl3) cm−1: 3009, 2932, 2899, 1632; H-NMR [α]D22 −68.5 (c=0.47, CHCl3); IR (CHCl3) cm−1: 3009, 2934,
1
(600MHz, CDCl3) δ: 7.50 (d, 4H, J=7.2Hz), 7.39 (t, 4H, 2856, 1636; H-NMR (600MHz, CDCl3) δ: 7.39–7.35 (m, 8H),
J=7.2Hz), 7.33–7.30 (m, 4H), 6.67 (dd, 2H, J=7.2, 1.8Hz), 7.29–7.25 (m, 2H), 7.17 (d, 2H, J=1.8Hz), 6.65 (dd, 2H, J=7.8,
6.59 (d, 2H, J=7.2Hz), 5.38 (t, 2H, J=9.6Hz), 4.69 (dd, 2H, 1.8Hz), 6.55 (d, 2H, J=7.8Hz), 4.64–4.59 (m, 2H), 4.32–4.28
J=9.6, 7.8Hz), 4.37–4.33 (m, 2H), 4.17 (t, 2H, J=9.0Hz), (m, 4H), 4.07 (t, 2H, J=7.2Hz), 3.37 (dd, 2H, J=13.8, 6.0Hz),
3.17–3.09 (m, 4H), 2.87–2.80 (m, 2H); 13C-NMR (150MHz, 3.25–3.18 (m, 2H), 3.13–3.09 (m, 2H), 2.88 (dd, 2H, J=13.8,
CDCl3) δ: 164.1 (2C), 142.8 (2C), 141.2 (2C), 140.2 (2C), 8.4Hz), 2.85–2.79 (m, 2H); 13C-NMR (100MHz, CDCl3) δ:
135.8 (2C), 135.1 (2C), 132.6 (2C), 128.7 (4C), 127.7 (2C), 163.4 (2C), 141.0 (2C), 140.1 (2C), 138.6 (2C), 135.8 (2C),
127.5 (2C), 127.0 (4C), 73.5 (2C), 71.1 (2C), 35.7 (2C), 33.9 134.9 (2C), 132.0 (2C), 129.2 (4C), 128.6 (4C), 127.9 (2C),
(2C); MS (DART) m/z 499 (100.0, M++1); high resolution 126.4 (2C), 70.6 (2C), 68.7 (2C), 42.2 (2C), 35.6 (2C), 33.6
(HR)-MS Calcd for C34H31N2O2: 499.2386. Found 499.2400; (2C); MS (DART) m/z 527 (100.0, M++1); HR-MS Calcd for
HPLC: OD-H column; λ=254nm; eluent: hexane–isopro- C36H35N2O2: 527.2699. Found 527.2697.
panol=85:15; flow rate: 1.0mL/min; tR=15.1min for major;
(Rp,S,S)-(+)-4,12-Bis(4-benzyl-2-oxazolinyl)[2.2]-
de%=>99.9%. (Rp,S,S)-4b: mp 43–45°C; [α]D23 +55.4 (c=1.30, paracyclophane ((Rp,S,S)-4c)
CHCl3); IR (CHCl3) cm−1: 3032, 3009, 2932, 2897, 2856, 1634;
The amide precursor of title compound was prepared from
1H-NMR (600MHz, CDCl3) δ: 7.37–7.35 (m, 8H), 7.30 (d, 2H, (Rp)-6 (9.6mg, 0.032mmol), (S)-phenylalaninol (10.8mg,
J=1.8Hz), 7.29–7.24 (m, 2H), 6.69 (dd, 2H, J=7.8, 1.8Hz), 0.0713mmol), HOBt (9.6mg, 0.071mmol) and DCC (28.1mg,
6.60 (d, 2H, J=7.8Hz), 5.46 (dd, 2H, J=10.2, 8.4Hz), 4.63 (dd, 0.136mmol), and the title compound (5.9mg, 35%) was
2H, J=10.2, 7.8Hz), 4.37–4.33 (m, 2H), 4.12 (t, 2H, J=8.4Hz), prepared using PPh3 (24.6mg, 0.0938mmol), Et3N (50µL,
3.20–3.14 (m, 4H), 2.84 (dt, 2H, J=13.2, 9.0Hz); 13C-NMR 0.36mmol) and CCl4 (50 µL, 0.52mmol), according to the
(150MHz, CDCl3) δ: 164.4 (2C), 142.9 (2C), 141.3 (2C), 140.1 procedure for preparation of 4b. A white solid; mp 37–39°C;
(2C), 135.8 (2C), 135.0 (2C), 132.7 (2C), 128.6 (4C), 127.6 [α]D24 +13.9 (c=0.72, CHCl3); IR (CHCl3) cm−1: 3030, 3011,
1
(2C), 127.4 (2C), 126.8 (4C), 73.8 (2C), 70.6 (2C), 36.3 (2C), 2932, 2858, 1636; H-NMR (600MHz, CDCl3) δ: 7.33–7.21
34.1 (2C); MS (DART) m/z 499 (100.0, M++1); HR-MS Calcd (m, 10H), 7.12 (d, 2H, J=1.8Hz), 6.65 (dd, 2H, J=7.8, 1.8Hz),
for C34H31N2O2: 499.2386. Found 499.2394; HPLC: OD-H col- 6.56 (d, 2H, J=7.8Hz), 4.66–4.61 (m, 2H), 4.31–4.19 (m, 4H),
umn; λ=254nm; eluent: hexane–isopropanol=85:15; flow rate: 4.03 (t, 2H, J=7.2Hz), 3.24 (dd, 2H, J=13.8, 4.8Hz), 3.15–3.09
1.0mL/min; tR=15.6min for minor, tR=11.3min for major; (m, 4H), 2.84–2.79 (m, 2H), 2.66 (dd, 2H, J=13.8, 9.6Hz);
de%=98.7%.
(Sp,S,S)-(−)-4,12-Bis(4-benzyl-2-oxazolinyl)[2.2]- (2C), 138.4 (2C), 135.7 (2C), 134.8 (2C), 132.5 (2C), 129.2
paracyclophane ((Sp,S,S)-4c) (4C), 128.5 (4C), 127.9 (2C), 126.4 (2C), 70.7 (2C), 68.4 (2C),
13C-NMR (150MHz, CDCl3) δ: 163.6 (2C), 141.0 (2C), 140.1
The amide precursor of title compound was prepared from 41.9 (2C), 36.0 (2C), 34.0 (2C); MS (DART) m/z 527 (100.0,
(Sp)-6 (14.2mg, 0.0479mmol), (S)-phenylalaninol (16.6mg, M++1); HR-MS Calcd for C36H35N2O2: 527.2699. Found

1012
Chem. Pharm. Bull.
Vol. 66, No. 10 (2018)
527.2687.
(−)-Ethyl 2-(2,4-Dimethylphenoxy)propionate (9d)22)
General Procedure for Asymmetric EtOH Insertion
A colorless oil; 61% yield; [α]D25 −21.0 (c=0.36, EtOH);
1H-NMR (500MHz, CDCl3) δ: 6.95 (s, 1H), 6.88 (dd, 1H,
J=8.5, 1.5Hz), 6.59 (d, 1H, J=8.5Hz), 4.67 (q, 1H, J=7.0Hz),
Reaction of 2
Methyl 2-Ethoxy-2-phenylacetate (3)21)
To a suspension of ligand (0.012mmol) and MS5A (300mg) 4.20 (q, 2H, J=7.0Hz), 2.24 (s, 3H), 2.24 (s, 3H), 1.60 (d,
in CH2Cl2 (1mL) were added NaBArF (10.6mg, 0.0120mmol) 3H, J=7.0Hz), 1.25 (t, 3H, J=7.0Hz); HPLC: OJ-H column;
and Cu(OTf)2 (3.6mg, 0.010mmol) at room temperature under λ=230nm; eluent: hexane–isopropanol=99:1; flow rate:
an argon atmosphere. After stirring for 4h at the same tem- 0.8mL/min; tR=28.96min for major isomer, tR=11.74min for
perature, EtOH (56µL, 1.0mmol) and then a solution of 2 minor isomer. Compound 9d was determined to be 71% ee.
(35mg, 0.20mmol) in CH2Cl2 (1mL) were added to the reac-
tion mixture at 40°C. The mixture was stirred at the same
(S)-(−)-Ethyl 2-(4-Chlorophenoxy)propionate (9e)22)
A colorless oil; 58% yield; [α]D26 −23.0 (c=0.41, CH2Cl2)
temperature for 4-38h and filtered, and filtrate was concen- [lit.: [α]D18 +47.6 (c=1.0, CH2Cl2) for (R)]22); 1H-NMR
trated to dryness. The residue was chromatographed with (500MHz, CDCl3) δ: 7.22 (d, 2H, J=9.3Hz), 6.81 (d, 2H,
1
hexane–Et2O (20:1) to afford 3 as a colorless oil; H-NMR J=9.3Hz), 4.69 (q, 1H, J=7.0Hz), 4.21 (q, 2H, J=7.0Hz), 1.61
(600MHz, CDCl3) δ: 7.45 (d, 2H, J=7.6Hz), 7.37–7.31 (m, (d, 3H, J=7.0Hz), 1.25 (t, 3H, J=7.0Hz); HPLC: OJ-H col-
3H), 4.89 (s, 1H), 3.71 (s, 3H), 3.60 (m, 1H), 3.50 (m, 1H), 1.27 umn; λ=254nm; eluent: hexane–isopropanol=9:1; flow rate:
(t, 3H, J=7.0Hz); 13C-NMR (150MHz, CDCl3) δ: 171.4, 136.6, 1.0mL/min; tR=8.92min for major isomer, tR=6.88min for
128.6, 128.6, 127.1, 80.8, 65.3, 52.2, 15.1; HPLC: OD-H col- minor isomer. Compound 9e was determined to be 48% ee.
umn; λ=254nm; eluent: hexane–isopropanol=99.5:0.5; flow
rate: 1.0mL/min; tR=11.6min for (R)-enantiomer, tR=9.9min
for (S)-enantiomer.
(S)-(+)-Ethyl 2-(Naphtalen-1-yloxy)propionate (9f)22)
A colorless oil; 44% yield; [α]D26 +24.7 (c=0.34, CHCl3) [lit.:
1
[α]D18 −35.0 (c=0.4, CHCl3) for (R)]22); H-NMR (500MHz,
Typical Procedure for Asymmetric Intermolecular CDCl3) δ: 8.36 (m, 1H), 7.79 (m, 1H), 7.50–7.45 (m, 3H),
Aromatic O–H Insertion Reaction of 7 (Table 2, Entry 3)
7.32 (t, 1H, J=8.0Hz), 6.70 (d, 1H, J=7.5Hz), 4.93 (q, 1H,
J=6.5Hz), 4.22 (q, 2H, J=7.0Hz), 1.75 (d, 3H, J=6.5Hz), 1.23
(S)-(−)-Ethyl 2-Phenoxypropionate (9a)22)
To a suspension of (Sp)-1d (7.3mg, 0.013mmol) and MS5A (t, 3H, J=7.0Hz); HPLC: OD-H column; λ=210nm; eluent:
(315mg) in CH2Cl2 (1mL) were added NaBArF (11.2mg, hexane–isopropanol=93:7, flow rate: 1.0mL/min; tR=9.81min
0.0126mmol) and CuCl (1.0mg, 0.010mmol) at room tem- for minor isomer, tR=6.27min for major isomer. Compound 9f
perature under an argon atmosphere. After stirring for 4h was determined to be 75% ee.
at the same temperature, a solution of PhOH (8a) (98.8mg,
1.05mmol) in CH2Cl2 (0.5mL) was added to the mixture. Aromatic O–H Insertion Reaction of 10 (Table 4, Entry 4)
After 20min, a solution of 7 (28.0mg, 0.219mmol) in CH2Cl2
(−)-Methyl 2,3-Dihydrobenzofuran-2-carboxylate (11a)17)
(0.6mL) was added to the mixture, which was stirred at room and (E)-Methyl 3-(2′-Hydroxyphenyl)-2-propenoate (12a)44)
temperature for 1h. The mixture was filtered and filtrate was To a suspension of (Sp)-1d (7.0mg, 0.012mmol, 6mol%) and
Typical Procedure for Asymmetric Intramolecular
concentrated to dryness. The residue was chromatographed MS5A (250mg) in CH2Cl2 (1mL) were added CuCl (1.1mg,
with hexane–AcOEt (30:1) to afford 9a (28.0mg, 66%) as a 0.011mmol, 5mol%) and NaBArF (10.6mg, 0.012mmol,
colorless oil; [α]D23 −37.5 (c=0.43, MeOH) [lit.: [α]D18 +47.2 6mol%) at room temperature under an argon atmosphere.
(c=0.5, MeOH) for (R)]22); 1H-NMR (600MHz, CDCl3) After stirring for 4h at the same temperature, a solution of
δ: 7.29–7.25 (m, 2H), 6.97 (t, 1H, J=7.8Hz), 6.88 (d, 2H, 10a (42.4mg, 0.206mmol) in CH2Cl2 (0.5mL) was added to
J=7.8Hz), 4.74 (q, 1H, J=6.6Hz), 4.22 (q, 2H, J=6.6Hz), 1.61 the mixture, which was stirred for 19h. The mixture was
(d, 3H, J=6.6Hz), 1.25 (t, 3H, J=6.6Hz); 13C-NMR (150MHz, filtered and filtrate was concentrated to dryness. The residue
CDCl3) δ: 172.3, 157.6, 129.5, 121.5, 115.1, 72.6, 61.2, 18.6, was chromatographed with hexane–AcOEt (8:1) to affored
14.1; HPLC: OD-H column; λ=254nm; eluent: hexane–iso- 11a (20.1mg, 54%) as a colorless oil and 12a (8.1mg, 22%)
1
propanol=9:1; flow rate: 1.0mL/min; tR=8.8min for (R)- as a white solid. 11a: [α]D26 −11.7 (c 0.9, CHCl3); H-NMR
enantiomer, tR=4.9min for (S)-enantiomer. Compound 9a was (600MHz, CDCl3) δ: 7.16–7.13 (m, 2H), 6.90–6.87 (m, 2H),
determined to be 80% ee.
5.20 (dd, 1H, J=10.8, 6.6Hz), 3.80 (s, 3H), 3.55 (dd, 1H,
J=15.6, 10.8Hz), 3.38 (dd, 1H, J=15.6, 6.6Hz); HPLC: OD-H
(−)-Ethyl 2-(4-Methoxyphenoxy)propionate (9b)22)
A colorless oil; 73% yield; [α]D26 −43.8 (c=0.51, EtOH); column; λ=280nm; eluent: hexane–isopropanol=75:25; flow
1H-NMR (500MHz, CDCl3) δ: 6.85–6.80 (m, 4H), 4.65 (q, rate: 1.0mL/min; tR=7.18min for minor isomer, tR=6.33min
1H, J=7.0Hz), 4.21 (q, 2H, J=7.0Hz), 3.76 (s, 3H), 1.59 for major isomer. Compound 11a was determined to be
(d, 3H, J=7.0Hz), 1.25 (t, 3H, J=7.0Hz); HPLC: OJ-H col- 74% ee. 12a: 1H-NMR (600MHz, CDCl3) δ: 8.02 (d, 1H,
umn; λ=254nm; eluent: hexane–isopropanol=9:1; flow rate: J=16.2Hz), 7.46 (dd, 1H, J=7.8, 1.8Hz), 7.23 (td, 1H, J=7.8,
1.5mL/min; tR=13.17min for major isomer, tR=11.02min for 1.8Hz), 6.93 (td, 1H, J=7.8, 0.6Hz), 6.82 (dd, 1H, J=7.8,
minor isomer. Compound 9b was determined to be 79% ee.
0.6Hz), 6.60 (d, 1H, J=16.2Hz), 6.04 (s, 1H), 3.81 (s, 3H).
(−)-Methyl 5-Methoxy-2,3-dihydrobenzofuran-2-carboxylate
(−)-Ethyl 2-(2-Methoxyphenoxy)propionate (9c)22)
A yellow oil; 61% yield; [α]D25 −43.3 (c=0.43, EtOH); (11b)17)
1H-NMR (500MHz, CDCl3) δ: 6.98–6.85 (m, 4H), 4.75 (q,
A colorless oil (entry 7); 47% yield; [α]D26 −3.7 (c=0.43,
1
1H, J=7.0Hz), 4.23–4.19 (m, 2H), 3.86 (s, 3H), 1.64 (d, 3H, CHCl3); H-NMR (600MHz, CDCl3) δ: 6.79 (d, 1H, J=9.0Hz),
J=7.0Hz), 1.25 (t, 3H, J=7.0Hz); HPLC: OD-H column; 6.75 (d, 1H, J=2.4Hz), 6.68 (dd, 1H, J=9.0, 2.4Hz), 5.19 (dd,
λ=254nm; eluent: hexane–isopropanol=9:1; flow rate: 1H, J=10.2, 6.6Hz), 3.80 (s, 3H), 3.75 (s, 3H), 3.53 (dd, 1H,
1.5mL/min; tR=10.80min for minor isomer, tR=5.45min for J=15.9, 10.2Hz), 3.36 (dd, 1H, J=15.9, 6.6Hz). HPLC: OD-H
major isomer. Compound 9c was determined to be 71% ee.
column; λ=230nm; eluent: hexane–isopropanol=70:30; flow

Vol. 66, No. 10 (2018)
Chem. Pharm. Bull.
1013
5) Wang X., Chen Z., Duan W., Song C., Ma Y., Tetrahedron Asym-
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for major isomer. Compound 11b was determined to be 74%
ee.
(−)-Methyl 7-Methoxy-2,3-dihydrobenzofuran-2-carboxylate
(11c)17)
A colorless oil (entry 9); 65% yield; [α]D26 −29.3 (c=0.54,
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CHCl3); H-NMR (600MHz, CDCl3) δ: 6.84 (t, 1H, J=7.8Hz),
6.78 (dd, 1H, J=7.8, 1.2Hz), 6.76 (d, 1H, J=7.8Hz), 5.24 (dd,
1H, J=10.2, 6.6Hz), 3.88 (s, 3H), 3.79 (s, 3H), 3.57 (dd, 1H,
J=15.6, 10.2Hz), 3.40 (dd, 1H, J=15.6, 6.6Hz); HPLC: OD-H
column; λ=220nm; eluent: hexane–isopropanol=70:30; flow
rate: 1.0mL/min; tR=31.96min for minor isomer, tR=11.01min
for major isomer. Compound 11c was determined to be 69%
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(S)-(+)-Methyl 5-Chloro-2,3-dihydrobenzofuran-2-carboxylate
(11d)17)
A white solid; 21% yield (entry 11); [α]D26 +5.2 (c=0.11,
1
CHCl3) [lit.: [α]D17 −12.8 (c=2.1, CHCl3) for (R)]17); H-NMR
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1H, J=16.2, 10.8Hz), 3.35 (dd, 1H, J=16.2, 6.6Hz); HPLC:
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flow rate: 1.0mL/min; tR=10.23min for minor isomer,
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(+)-Methyl Chroman-2-carboxylate (11e)17)
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CHCl3); H-NMR (600MHz, CDCl3) δ: 7.12 (td, 1H, J=7.8,
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1.2Hz), 6.87 (td, 1H, J=7.8, 1.2Hz), 4.73 (dd, 1H, J=7.8,
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HPLC: AS-H column; λ=220nm; eluent: hexane–isopro-
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5.82 (dt, 1H, J=15.6, 1.8Hz), 4.90 (s, 1H), 3.69 (s, 3H), 3.53
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Acknowledgments We thank Mr. Yudo Murai for
checking spectral data. This work was supported by JSPS
KAKENHI Grant Number 24590006 and the Hoansha Foun-
dation. We thank Helen McPherson, Ph.D. and Leo Holroyd,
Ph.D., for editing a draft of this manuscript.
Conflict of Interest The authors declare no conflict of
interest.
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