Asymmetric dearomatizing spirolactonization of naphthols catalyzed by spirobiindane-based chiral hypervalent iodine species

Article abstract of DOI:10.1021/ja401074u

This report details the development of a spirobiindane-based chiral hypervalent iodine reagent, especially focusing on its structural elucidation for effective asymmetric induction of the chiral spiro center during the oxidative dearomatizing spirolactonization of naphthols. In this study we synthesized a new series of ortho-functionalized spirobiindane catalysts and demonstrated that the enantioselectivity can be dramatically improved by the presence of the substituents ortho to the iodine atom. The structural elucidation of a spirobiindane-based hypervalent iodine catalyst has led to further improvement in the stereoselective construction of the spiro center during the oxidative dearomatizing spirolactonization of naphthols. Thus, catalytic oxidation with the highest reported level of enantioselectivity in hypervalent iodine chemistry has been achieved with also an excellent level of asymmetric induction (92% ee for substrate 3a). As a result, this study, dealing with a series of modified iodine catalysts, can provide important clues about the transition state and reaction intermediate to help scientists understand the origin of the stereoselectivity. A plausible transition-state model and intermediate in the reaction for the stereoselective formation of spirolactone products are postulated by considering the ortho-substituent effect and the results of X-ray analysis. In this reaction model, the high enantiomeric excess obtained by using the spirobiindane catalysts could be well explained by the occupation of the equatorial site and extension of the surroundings around the hypervalent iodine bonds by the introduced ortho-substituent. Thus, this study would contribute to estimation of the chiral hypervalent iodine compounds in asymmetric reactions.

Full text of DOI:10.1021/ja401074u

Article
pubs.acs.org/JACS
Asymmetric Dearomatizing Spirolactonization of Naphthols
Catalyzed by Spirobiindane-Based Chiral Hypervalent Iodine Species
†
‡,⊥
†
†
§
Toshifumi Dohi, Naoko Takenaga, Tomofumi Nakae, Yosuke Toyoda, Mikio Yamasaki,
§
‡
‡
,†,∥
Motoo Shiro, Hiromichi Fujioka, Akinobu Maruyama, and Yasuyuki Kita*
†College of Pharmaceutical Sciences, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan
‡Graduate School of Pharmaceutical Sciences, Osaka University, 1-6 Yamada-oka, Suita, Osaka, Japan
§Rigaku Corporation, 3-9-12 Matsubara, Akishima, Tokyo, Japan
*
S Supporting Information
ABSTRACT: This report details the development of a spirobiindane-based
chiral hypervalent iodine reagent, especially focusing on its structural
elucidation for effective asymmetric induction of the chiral spiro center
during the oxidative dearomatizing spirolactonization of naphthols. In this
study we synthesized a new series of ortho-functionalized spirobiindane
catalysts and demonstrated that the enantioselectivity can be dramatically
improved by the presence of the substituents ortho to the iodine atom. The
structural elucidation of a spirobiindane-based hypervalent iodine catalyst has
led to further improvement in the stereoselective construction of the spiro
center during the oxidative dearomatizing spirolactonization of naphthols. Thus, catalytic oxidation with the highest reported
level of enantioselectivity in hypervalent iodine chemistry has been achieved with also an excellent level of asymmetric induction
(
92% ee for substrate 3a). As a result, this study, dealing with a series of modified iodine catalysts, can provide important clues
about the transition state and reaction intermediate to help scientists understand the origin of the stereoselectivity. A plausible
transition-state model and intermediate in the reaction for the stereoselective formation of spirolactone products are postulated
by considering the ortho-substituent effect and the results of X-ray analysis. In this reaction model, the high enantiomeric excess
obtained by using the spirobiindane catalysts could be well explained by the occupation of the equatorial site and extension of the
surroundings around the hypervalent iodine bonds by the introduced ortho-substituent. Thus, this study would contribute to
estimation of the chiral hypervalent iodine compounds in asymmetric reactions.
INTRODUCTION
■
Hypervalent iodine reagent has gained importance as a useful
alternative to toxic heavy-metal oxidants, such as lead, thallium,
mercury, etc., due to its low toxicity, high stability, and easy
1
handling, in combination with a wide array of reactivities. It is
widely recognized as a valuable oxidant for performing
environmentally benign reactions. Oxidations by some
representative hypervalent iodine reagentsphenyliodine(III)
diacetate (PIDA) and phenyliodine(III) bis(trifluoroacetate)
(
PIFA)have already been applied in many total syntheses of
2
natural products and their pivotal intermediates, in which the
reagents are conventionally used as stoichiometric oxidants.
From economic and environmental viewpoints, the recycling
and catalytic utilization of hypervalent iodine reagent3s
developed in recent years enhances their practical applications.
In spite of the significant advances and usefulness of
hypervalent iodine reagents as mild organo-oxidants in many
synthetic reactions, asymmetric variants with control of the
chiral hypervalent iodine compound have been rarely
accomplished, and realization of a high enantioselectivity is
Figure 1. Chiral hypervalent iodine(III) reagents reported in early
studies.
molecules: those bearing a chiral leaving group attached to the
iodine atom, and the compounds in which the chiral moiety is
based on iodoarene backbones. The former type of compounds,
having chirality at the leaving ligands, such as optically active
5a,b
5c
5d,e
carboxylic acids,
sulfonic acids, and alcohols,
were
intensively investigated in early studies. Our research group also
4
−9
still a challenge. Early reports of the use of chiral hypervalent
iodine compounds for asymmetric reactions are shown in
Figure 1. These are classified by the two types of chiral
Received: January 31, 2013
©
XXXX American Chemical Society
A
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applied the strategy to the asymmetric oxidation of sulfides into
formation of almost racemic spirolactones 4. We also reported
the catalytic use of the chiral hypervalent iodine compound
combining the corresponding chiral iodoarene 2a and
stoichiometric m-chloroperbenzoic acid (mCPBA) as a co-
oxidant (Scheme 1, eq 2). This is a landmark that proved the
chiral iodine compound as a new entrance to asymmetric
sulfoxides and obtained moderate enantiomeric excess (ee)
6
values. However, oxidations mediated by hypervalent iodine
reagents liberate the chiral ligands from the iodine during the
early stages, and thus application of the system to other
asymmetric oxidations was quite difficult.
It appears that a hypervalent iodine reagent based on a chiral
ligand has not been effective for asymmetric induction in an
extensive array of reactions. Therefore, the latter types of chiral
compounds were recently explored with the aim of achieving
extensive asymmetric oxidations. Wirth’s research group has
designed new molecules having a chiral moiety ortho to the
15
organocatalysis.
Since our first report of the asymmetric dearomatization of
1
6,17
naphthols, several asymmetric phenolic oxidations
as other transformations
as well
18,19
have appeared in this area,
employing new chiral hypervalent iodine compounds. With
respect to our asymmetric spirolactonization and effective
catalytic systems as model cases, Ishihara’s group discovered in
iodine atom on the aromatic ring for the enantioselective7
2010 a new C -symmetric hypervalent iodine catalyst bearing
oxygenation of ketones and alkenes (up to 65% ee value).
2
20
two chiral lactate groups at the ortho positions of the
Ochiai and co-workers prepared chiral binaphthyl molecules
having an iodine group and applied the salts to the asymmetric
iodophenyl ring in their evaluation of catalysts for the
1
7
8
reactions. The new catalyst successfully provided the
α-arylation of ketones (up to 53% ee). These compounds,
spirolactone product 4a with up to 92% ee from the substrate
based on chiral iodoarene backbones, could expand the
possibilities for hypervalent iodine reagents in extensive
asymmetric oxidations, although there is still a requirement
for improved enantioselectivities.
3
a by 10−15 mol% loadings with stoichiometric mCPBA.
In this paper, we report new results on the highly
enantioselective organocatalytic spirolactonization of naphthols
based on the use of a chiral spirobiindane hypervalent iodine
3
Over the past few decades, we have been engaged in the
compound. Further elucidation of the chiral reagent’s structure
reveals that catalysts having a substituent at the ortho positions
of the iodoarene rings are more effective and promise better
stereocontrol for the enantioselective transformations. The
excellent and remarkable utility of the ortho-substituted
spirobiindane compound 2b (vide infra) as an asymmetric
catalyst in the presence of mCPBA as a co-oxidant is well
demonstrated.
development of new reactions using hypervalent iodine
10
reagents. In our continuous study of phenolic oxidations,
we have recently become interested in efficient construction of
the ortho-spirolactone architecture as a potentially useful
11
subunit for synthesizing some natural products. It has been
shown that the reactions that provided ortho-spirolactones well
proceeded using reactive oxygen-bridged hypervalent iodine
1
2
compounds. To control the chiral spiro center in the
products, we established in 2008 the first protocol for
enantioselective ortho-spirocyclization of naphthols 3 bearing
a carboxylic acid moiety as an intramolecular nucleophilic side
chain by designing a novel chiral hypervalent iodine(III)
compound 1a based on the spirobiindane backbone (Scheme 1,
RESULTS AND DISCUSSION
■
Initial Investigation of the Catalytic Reaction Using
Spirobiindane Iodoarene 2a. Recent progress in the field of
hypervalent iodine-catalyzed oxidation has given it a significant
3
1
3,14
position in synthetic chemistry. In one of the pioneering
eq 1).
unprecedented excellent level for asymmetric induction (up to
6% ee value) in the field of hypervalent iodine-mediated
Surprisingly, the reaction was found to proceed at an
studies, we reported an efficient in situ reoxidation hypervalent
21
iodine(III) species from organoiodine(I) compounds, and in
8
2
005 we successfully established a general catalytic cycle for the
transformations. The results are noteworthy because the use of
other chiral iodoarenes (as in Figure 1) resulted in the
oxidation of phenols and related compounds using mCPBA as a
co-oxidant. Based on previous results, we began the
22
optimization of the catalytic conditions of the asymmetric
spirolactonization using the original spirobindane catalyst (R)-
Scheme 1. Dearomatizing ortho-Spirocyclization of
Naphthols 3 Using Chiral Hypervalent Iodine Reagent 1a
and Its Precursor 2a
2a. The optimizations were carried out with substrate 3a and
stoichiometric mCPBA in the presence of 15 mol% of the chiral
iodoarene (R)-2a in several solvents (0.05 M concentration of
substrate 3a) at 0 °C.
As we expected, a remarkable solvent effect was observed
even under the catalytic conditions, and the enantioselectivity
was significantly affected by the solvent polarities, similar to the
13
stoichiometric reactions. Thus, less-polar solvents, chloro-
form, dichloromethane, and 1,2-dichloroethane, were used to
afford the spirolactone 4a with 47%, 66%, and 67% ee,
respectively (for chemical yields 66%, 61%, and 57%,
respectively, after 3 h), while the use of highly polar solvents,
acetonitrile and 1,1,1,3,3,3-hexafluoro-2-propanol, gave the
product 4a with inferior and disappointing ee values (17%
and 3% ee, respectively). With the aim of promoting the
reaction rate, a higher concentration of the substrate 3a was
examined, but it did not improve the product yield.
Subsequently, additives for the effective generation of the in
situ reactive hypervalent iodine(III) species were also explored.
Addition of 1 equiv of acetic acid to the reaction mixture of the
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substrate 3a, stoichiometric mCPBA, and 15 mol% of (R)-2a in
dichloromethane did not affect the ee value and gave only little
improvement in the chemical yield (70% yield after 3 h, 65%
ee; cf. the result in the absence of acetic acid mentioned above,
of the spirobiindane catalyst in the hypervalent iodine state. In
the catalyst (R)-1′, one side of the iodine atom of the reactive
species is effectively shielded by the indane plane (Figure 3,
left). Due to the rigid spirobiindane backbone of the catalyst,
the conformation is almost maintained throughout the course
of the reactions.
61% yield after 3 h, 66% ee). Finally, we checked the influence
of the loading amount of (R)-2a on the yield and ee value. The
use of 5 mol% of (R)-2a significantly decreased the chemical
yield (16% yield after 12 h, 63% ee) at 0 °C, but when it was
used at room temperature, a result comparable to that obtained
with the higher catalyst loading (62% yield after 12 h, 61% ee
for 5 mol% (R)-2a) was obtained after a prolonged reaction
time. Therefore, the use of a lower amount of the catalyst still
maintained the enantioselectivity but affected the chemical yield
of the product 3a to some extent.
We then confirmed the catalytic reaction for several 1-
naphthol derivatives 3b−d (Figure 2) under the standard
Figure 3. Extension of the equatorial surroundings around the iodine
atom by ortho-substituent (R) to affect the chiral environment.
Figure 2. 4-Substituted derivatives of naphtholcarboxylic acids 3b−d
and their corresponding spirolactone products 4b−d.
Previously, Wirth and co-workers reported that the
introduction of an achiral substituent at the ortho-aryl ring
position relative to the iodine atom improved the enantiose-
lectivity in the α-oxygenation of ketones and dioxygenation of
conditions, treating 15 mol% of (R)-2a and stoichiometric
mCPBA in the presence of acetic acid in dichloromethane
(Scheme 2). Reaction of the 4-substituted derivatives gave the
7b,c
alkenes. They concluded that the enhanced enantioselectiv-
ity is due to a well-organized transition state in which the
reactant is distributed more closely to the chiral environment,
positioned in the axial space by an additional steric effect on the
reaction sequence. In this scenario, introduction of an ortho-
substituent at the equatorial position of the hypervalent iodine
bonds also seems to produce some steric influence on the
apically positioned ligand (L) and substrate 3 in the case of our
chiral iodoarene catalyst (R)-2a (Figure 3, right), thus affecting
the enantioselectivity. More extensively surrounding the
iodine(III) atom by both the spirobiindane ring and a newly
introduced ortho-substituent might positively affect the
enantioselective performance during the reactions.
Scheme 2. Catalytic Asymmetric Spirocyclization of
Naphthols 3a−d Using 15 mol% (R)-2a
a
a
The ee value in parentheses is that obtained after a single
recrystallization.
corresponding spirolactones 4 in moderate to good yields and
enantioselectivities, except for the substrate 3d. In accord with
the theoretical studies and experimental evidence for generation
The ortho-functionalized chiral spirobiindane-based iodoar-
1
4
ene catalysts have been developed from the diamine (R)-5,
a
13
precursor of (R)-2a described in our previous report, as a
common intermediate in the synthetic sequences shown in
Scheme 3. To prepare the spirobiindanes having ortho-alkyl
groups, iodination of (R)-6, after introduction of an acetyl
group into the ortho positions of the amines by the Houben−
Hoesch-type reaction, by the conventional protocol afforded
(R)-7, which is a key compound in other ortho-functionalized
derivatives. (R)-7 was then transformed into the iodoarene (R)-
2b having a linear alkyl substituent, while the ketone group of
(R)-7 was subjected to reduction by sodium tetrahydroborate
23
of phenoxenium ion during phenolic oxidations, apparently
the presence of the electron-donating group in substrate 3d
decreased the ee of the reaction. For a well-crystallized
24
spirolactone such as 4b, we could obtain for the first time
the enantiomerically pure spirolactone 4b from the enantio-
enriched sample after recrystallization.
25
It was revealed by this study that the chiral iodine compound
a offers a very moderate stereocontrol for the representative
2
naphtholcarboxylic acids 3a−d even under catalytic conditions,
although there is a need for improved enantioselectivities. Thus,
we next considered optimizing the structure of the
spirobiindane catalyst 2a to realize a highly selective
asymmetric cyclization.
(NaBH ) to give the secondary alcohol (R)-8 as the precursor
4
of (R)-2c−f. The iodoarenes (R)-2g−i were readily prepared
by controlled halogenation of the diamine (R)-5, followed by
the same iodination procedure.
Design and Use of ortho-Functionalized Chiral
Spirobiindane Iodoarene Catalysts. With the aim of
designing an iodine catalyst that shows a higher enantiose-
lectivity, we resumed our investigation of the possible structure
The enantioselectivities of the ortho-functionalized iodoar-
enes (R)-2b−i in the dearomatizing spirocyclization were
evaluated using naphtholcarboxylic acid 3a as a reference
substrate (Table 1). To our delight, newly prepared iodoarene
C
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Scheme 3. Synthesis of ortho-Substituted Chiral Iodoarenes
(R)-2b (10 mol%), having a linear alkyl substituent at the ortho
position, could produce the spirolactone 4a with a higher
enantioselectivity (entry 1, 82% ee) in comparison to the
original spirobiindane (R)-2a (65% ee). Similarly, a series of
branched secondary groups in the iodoarenes (R)-2c−f
a
2
b−i
26
improved the enantioselectivities, among which the smallest
substituent (R = Me; (R)-2c) showed the best result of 80% ee
(
entry 2). As the size of the substituent increased (R = Bn, Ac,
and Piv; (R)-2d−f), spirolactone 4a was obtained with slightly
lower ee’s between 72 and 78% (entries 3−5). As we expected,
halogenated iodoarenes (R)-2g−i also gave better enantiose-
27a
lectivities, but the reactions were stopped within a few hours
and the chemical yields dropped significantly (entries 6−8).
This seemed to be the result of inefficient generation of the
reactive iodine(III) species from the electron-deficient iodine
atom of (R)-2g−i caused by the electron-withdrawing nature of
3
c,22
the halogen groups.
All the iodoarene catalysts (R)-2b−i
are likely to produce the same enantiomer (R)-4a as the non-
ortho-substituted one, (R)-2a, during the reactions.
Thus, (R)-2b was the best iodoarene catalyst in terms of both
reactivity and enantioselectivity among those examined. To
confirm the catalyst changes, the versatility and scope of the o-
ethyl-substituted iodoarene (R)-2b in our dearomatizing
spirolactonization were tested for various naphthol derivatives
3
under the catalytic conditions. Representative results are
given in Table 2. Using chloroform as the solvent instead of
dichloromethane, spirolactone 4a was obtained with a slightly
higher ee value, 87% (entry 1). A lower catalyst loading still
permitted good enantioselectivities, but the reactions required
higher concentrations and the addition of acetic acid (see eq 2).
As the polarity of the solvent decreased upon addition of
hexane, the selectivity further improved to 92% ee (entry 2).
Catalytic oxidation of the 4-substituted naphthol derivatives 3b
and 3c gave the corresponding spirolactones 4b and 4c with
improved enantioselectivities (entries 3 and 4). In accord with
our previous experience using the original chiral spirobiindane
catalyst 2a (vide supra), racemic spirolactone 4d was obtained
from the substrate 3d with a methoxy substituent (entry 5).
The present system was widely applicable for 3- (entry 7), 5-
a
Reagents and conditions: (a) BCl , dichloromethane/acetonitrile,
3
ZnCl , reflux, then HCl (aq) reflux. (b) NaNO , TFA, KI, H O. (c)
CF SO H, Et SiH, dichloromethane. (d) NaBH , tetrahydrofuran
2
2
2
3
3
3
4
(
THF)/methanol. (e) NaH, MeI, THF. (f) NaH, BnBr, THF. (g)
Ac O, NEt , cat. 4-DMAP, dichloromethane. (h) PivCl (Piv =
pivaloyl), NEt , cat. 4-DMAP, dichloromethane. (i) NCS or NBS,
dichloromethane.
2
3
3
(
entries 8−11), and 6-substituted (entries 12 and 13)
naphtholcarboxylic acids 3f−l, maintaining good ee values.
However, the efficiency of the reaction as well as the
stereoselectivity with the spirobiindane catalyst 2b became
Table 1. Screening of the ortho-Functionalized
Spirobiindane Catalysts 2b−i
significantly lower for substrate 3m, having a peri-methyl group
5
(
entry 14, R = Me). Hence, the substituent at the 8-position of
the 1-naphthol 3m might cause steric congestion between the
spirobiindane ring of the catalyst 2 and the substrate 3m,
hindering organization of a plausible catalyst−substrate pair
a
28
entry
ortho-substituted iodoarene (2)
time
yield, ee
during the reaction. After the reactions, most of the iodoarene
b
,
c
1
2
3
4
5
6
7
8
(R)-2b
(R)-2c
(R)-2d
(R)-2e
(R)-2f
(R)-2g
(R)-2h
(R)-2i
15 h
46 h
18 h
40 h
10 h
12 h
6 h
62%, 82% ee
51%, 80% ee
42%, 72% ee
45%, 77% ee
52%, 75% ee
39%, 81% ee
15%, 83% ee
15%, 77% ee
2b (at least 90% used) could be recovered in a pure form by
column chromatography, and when we reused the recovered
catalyst 2b, the reaction proceeded with the same level of yields
and ee values.
c
Enantiomerically pure spirolactones (>99% ee) could be
obtained in gram quantities after one-time recrystallization of
the products 4 formed with the catalyst (R)-2b. Using the well-
crystallized spirolactone 4b, the absolute stereochemistry at the
spiro carbon for the product (+)-4b was determined to be R by
c
6 h
a
Determined by HPLC analysis using chiral separation column.
Reaction was performed at −10 °C. Performed with 10 mol% of the
catalysts. Compare these results with 70% yield of the product
b
c
29
X-ray analysis (Figure 4).
Based on these results regarding the extensive use of catalysts
2 and substrates 3, we postulated a reasonable transition-state
model for the formation of spirolactone products with the R-
configuration caused by the catalyst (R)-2b as shown in
(
+)-(R)-4a with 65% ee obtained with the original spirobiindane
catalyst (R)-2a under the same conditions.
D
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a
Table 2. Examples of the Asymmetric Spirocyclization Substrates 3a−m for ortho-Substituted Spirobiindane Catalyst (R)-2b
b
entry
1
substrate
product
R1
H
R2
R3
R4
H
R5
H
time
8 h
yield, ee
3a
(+)-(R)-4a
H
H
83%, 87% ee
60%, 87% ee
85%, 83% ee
56%, 92% ee
96%, 82% ee
80%, 80% ee
c
c
1
2 h
2 h
,
d
1
e
2
3a
3b
3c
3d
3e
3f
(+)-(R)-4a
(+)-(R)-4b
(+)-4c
H
H
H
H
H
Ph
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
10 h
7 h
8 h
8 h
8 h
8 h
d
3
Br
e
4
Bn
OMe
Ph
H
5
(±)-4d
(+)-4e
16%, <1% ee
64%, 83% ee
66%, 90% ee
e
6
d
7
4f
f
1
0 h
88%, 86% ee
94%, 85% ee
70%, 80% ee
68%, 86% ee
52%, 84% ee
50%, 88% ee
67%, 87% ee
20%, 40% ee
d
8
3g
3h
3i
4g
4h
4i
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Ph
Br
H
H
7 h
7 h
8 h
8 h
7 h
8 h
7 h
d
9
H
H
10
11
12
13
14
N3
OBn
H
H
H
3j
4j
H
H
3k
3l
4k
4l
Ph
N3
H
H
H
H
3m
4m
H
Me
a
b
Reactions were performed in chloroform with 10 mol% of (R)-2b at 0.02 M concentration of substrate 3. Determined by HPLC analysis using
c
d
chiral separation column. 5 mol% catalyst (R)-2b was used. 1 equiv of acetic acid was added at higher concentration (0.05 M concentration of
e
f
substrate 3). Reaction performed in a mixed solvent of chloroform/hexanes 1/1 with 15 mol% of the catalyst (R)-2b. Chloroform/hexanes 1/1,
10 °C.
−
iodine(III) atom would occur from the less-hindered face of the
spirobiindane ring of (R)-2b′. Thus, the catalytic species (R)-
3
0
2
b′ could undergo ligand exchange with the naphthol oxygen
31
to give intermediate (A). A similar intermediate having a
hypervalent bond between the iodine(III) atom and naphtholic
32
oxygen was previously isolated. The presence of two iodine
groups on the catalyst is important in order to restrict the
33
substrate access and repositioning. As such, the iodine groups
can block access to substrate 3 from the major groove of the
two spirobiindane planes and opposite apical position of the
iodine(III) by forming an oxygen bridge. In intermediate (A),
the pendant carboxylic acid would then preferentially attack the
ipso position of the naphthol ring from the unshielded Re-face
of substrate 3, resulting in formation of the R enantiomer of the
product 4 with good enantioselectivity, with the carboxylic acid
in the naphthol plane of substrate 3 located away from the
indane plane to avoid steric congestion of the just-formed
lactone ring of the product 4. One can expect a π−π interaction
Figure 4. Crystal structure of the spirolactone product (+)-(R)-4b by
X-ray analysis.
Scheme 4. Initially, catalyst (R)-2b was oxidized by mCPBA to
13
give the reactive iodine(III) species (R)-2b′. The apical
approach and repositioning of the naphthol substrates 3 on the
Scheme 4. Plausible Reaction Course and Transition-State Model for the Formation of the Product of R Configuration
E
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between the naphthol and equatorial indane rings of substrate 3
and catalyst (R)-2b′, but it seems unlikely in this case when
considering the shorter bond length of the hypervalent iodine−
chloroform−hexane mixed solvent. Regarding the iodine
content (20−30 mol% iodine) in the catalyst loading of 2a
(10−15 mol%), they also stated that their catalyst 10 well
works even with a lower amount of catalyst loading (10−15
mol%), which is claimed as one of the significant merits of the
new catalyst 10 over our catalyst. In our comparison of the
required catalyst molecular weight, the used masses are not very
different from each other ((R)-2b, MW = 528.2083;
monoiodoarene catalyst 10, MW = 614.5144). Besides, the
favorable characteristics of our spirobiindane catalyst 2b are as
follows: (1) It is easy to handle and recover in a pure form by
column chromatography (hexanes extraction) after the
reactions due to the low polarity of the catalyst 2b consisting
of only carbon, hydrogen, and iodine elements (while the
catalyst 10 is a highly polar compound). (2) The polarity
difference between the nonpolar catalyst 2b and polar product
4 also allows easy separation by column chromatography, and
both enantiomers (R)- and (S)-2b are readily obtained, which
could lead to utility of 2b for flexible applications involving two
enantiomers.
34
oxygen atoms (2.0−2.2 Å) compared to the typicallly
required stacking distance for forceful interaction of two
aromatic ring orbitals (around 3.5 Å). In fact, the opposite facial
approach of the naphthalene ring of substrate 3 toward the
catalyst (R)-2b′ should involve attack of the very crowded Re-
face of the naphthalene by the carboxylic acid to form the
observed product 4 with the R configuration, thus ruling out a
3
5
π−π interaction. Therefore, introducing an ethyl group into
the ortho position of the iodine atom of the spirobiindane
catalyst 2b can improve the selectivity without altering the
stereochemistry of the products 4 by increasing the steric
demand between the carboxylic side chains of the substrates 3
and the ortho-ethyl substituent of the catalyst as the result of
extending the Si-face shielding of the naphthol ring of the
substrates 3 in the intermediate (A). This transition-state
model and formation of the intermediate are in accord with the
unfavorable effect of the introduction of the 8-substituent of
naphthol substrate 3m, which shows a low enantioselectivity.
The experimental results all match with the hypothesis
regarding the reaction mechanism and intermediate.
CONCLUSIONS
■
We have demonstrated the theoretical elucidation of our
spirobiindane-based iodoarene catalyst 2 and synthesized a
series of ortho-functionalized catalysts. Evaluation of these
catalysts for the asymmetric dearomatizing spirolactonization of
naphtholcarboxylic acid 3a indicates that the enantioselectivity
is improved by the presence of the ortho substituents.
Additionally, based on the results of an X-ray analysis, we
have proposed a reasonable transition-state model for the
formation of the (R)-spirolactone product 4 from the (R)-2a
spirobiindane catalyst, which is in good agreement with the
results of the reactions performed for various naphthol
derivatives 3. Our chiral iodoarene compounds based on the
spirobiindane structure can realize the highest level of
asymmetric induction for the catalytic reaction in hypervalent
iodine chemistry. In fact, an enantioselectivity of up to 92% ee
for simple substrate 3a has been achieved using the elucidated
spirobiindane catalyst 2b. Recently, our dearomatizing reaction
After we accomplished the spirobiindane-catalyzed asym-
13
metric oxidative dearomatization shown in eq 2, Ishihara and
co-workers reported in 2010 a new type of C -symmetric chiral
2
hypevalent iodine catalyst 10 for the same reaction and catalytic
17
conditions using mCPBA. They concluded that the new
catalyst 10 is a superior alternative to our original reagent 2a in
their own evaluation and comparison of the reaction
selectivities. Indeed, it was reported that the product (R)-4a
was obtained in 60% yield with 92% ee in chloroform and in
6
5% yield with 90% ee in a chloroform−nitromethane mixture
using 15 mol% of the catalyst 10 (Scheme 5). In comparison,
our modified reagent (R)-2b can afford the same product (R)-
4
a in a comparable yield and ee value, that is, 83% yield with
27b
87% ee in chloroform,
and 56% yield with 92% ee in a
Scheme 5. Yield and Enantioselectivity by Reported
Hypervalent Iodine Catalysts in Our Dearomatizing
conditions and the catalytic systems have been applied to
a
Spirocyclization of Simple Naphthol 3a (Eq 3)
16,17
further catalyst screening by others.
Consequently, the
findings of this research study will contribute to the design and
discovery of effective chiral hypervalent iodine catalysts for
organic synthesis.
EXPERIMENTAL SECTION
General. Melting point (mp) was measured by using a Stuart
SMP3 melting point apparatus with AC input 100 V. H and
■
1
13
C
NMR spectra were recorded on a JEOL JMN-400 or -300
spectrometer operating at 400 or 300 MHz (100 or 75 MHz for
13
C NMR) at 25 °C with tetramethylsilane (δ = 0.0 ppm) as an
internal standard. The NMR data are reported as follows: chemical
shift in ppm (δ), integration, multiplicity (s = singlet, d = doublet, t =
triplet, m = multiplet), and coupling constant (Hz). Infrared (IR)
spectra were obtained using a Hitachi 270-50 spectrometer;
−1
absorptions are reported in reciprocal centimeters (cm ). High-
resolution mass spectrometry (HRMS) was performed by the
Elemental Analysis Section of Osaka University and Osaka University
Pharmaceutical Sciences. Analytical thin-layer chromatography (TLC)
was performed on MERCK silica gel, grade 60 F254. The spots and
bands were detected by irradiation with UV light (254, 365 nm) and/
or by staining with 5% phosphomolybdic acid followed by heating.
Column chromatography for isolation of the products 4 was carried
out on Merck silica gel 60 (230−400 mesh). Naphtholcarboxylic acid
a
Mes = mesityl.
F
dx.doi.org/10.1021/ja401074u | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
3
a was prepared by hydrolysis of 3,4-dihydro-2H-naphtho[1,2-
chromatography on silica gel (eluent n-hexane/ethyl acetate 8/1) to
give (+)-(R)-spiro[tetrahydrofuran-2,2′-(1′H-naphthalene)]-1′,5-
dione 3a (89 mg, 0.42 mmol) in 83% yield. The ee of the product
was determined by HPLC analysis using a chiral separation column.
Use of the (S) enantiomer of the catalyst 2b could preferentially
afford as product the opposite enantiomer, (−)-(S)-4a, in a
comparable yield and ee.
Determination of the Enantioselectivity of the Reactions.
The ee values of all the spirolactone products 4 obtained using the
spirobiindane catalyst (R)-2b were measured by HPLC analyses using
a JASCO DM-2010 multiwavelength detector. The chiral columns
included Chiralcel OD and OD-H, and Chiral-pac IA (Daicel
Chemical Industries, Ltd., 0.46 × 25 cm). The separation conditions
are summarized as follows for each compound.
Spiro[tetrahydrofuran-2,2′-(1′H-naphthalene)]-1′,5-dione (4a).
The ee value was determined by HPLC, OD-H chiral column, eluent
n-hexane/isopropanol 85/15, flow rate 1.0 mL/min, 25 °C, λ = 230
nm, t(major, R isomer) = 16.07 min, t(minor, S isomer) = 21.37 min.)
4′-Bromospiro[tetrahydrofuran-2,2′-(1′H-naphthalene)]-1′,5-
dione (4b). By HPLC, OD-H chiral column, eluent n-hexane/
isopropanol 85/15, flow rate 0.7 mL/min, 25 °C, λ = 235 nm, t(major)
3
6a
b]pyran-2-one.
Other substituted derivatives (3b−m) were
prepared from the corresponding naphthols according to literature
procedures.
reactions and chromatography in this study were obtained from
commercial suppliers and used as received without further purification.
Preparation of ortho-Substituted Iodoarene Compound 2b.
Enantiopure 1,1′-spirobiindane-7,7′-diamine 5 was obtained according
to the literature. To a solution of (R)-5 (1.0 g, 4.0 mmol) in 1,2-
dichloroethane (80 mL), boron trichloride in dichloromethane (1.0 M
solution, 40 mL, 40 mmol) was added under nitrogen at 0 °C, and the
mixture was stirred for 40 min. At the same temperature, zinc(II)
chloride (2.7 g, 20 mmol) and acetonitrile (3 mL) were added. The
mixture was stirred for 15 min at room temperature and then refluxed
at 80 °C for 20 h. After the mixture cooled to room temperature, 10%
aqueous hydrogen chloride was added, and then the mixture was
refluxed at 80 °C for 30 min. After cooling to ambient temperature,
the reaction mixture was extracted with dichloromethane. The organic
layer was dried over anhydrous sodium sulfate and concentrated in
vacuo to give the crude (R)-6,6′-diacetyl-7,7′-diamino-1,1′-spirobiin-
dane (R)-6 (1.22 g, 3.64 mmol) in 91% yield, which was used for a
further transformation.
36b
Unless otherwise noted, all other chemicals for the
14c
=
26.79 min, t(minor) = 31.76 min.
4
′-Benzylspiro[tetrahydrofuran-2,2′-(1′H-naphthalene)]-1′,5-
The obtained (R)-6 (251 mg, 0.75 mmol) was dissolved in 10 mL
of trifluoroacetic acid under nitrogen. Sodium nitrite (207 mg, 3
mmol) was added to the solution, which was stirred for 30 min at 0
dione (4c). OD chiral column, eluent n-hexane/isopropanol 85/15,
flow rate 1.0 mL/min, 25 °C, λ = 235 nm, t(major) = 22.56 min,
t(minor) = 52.23 min.
°
C. The reaction mixture was added to a solution of potassium iodide
4
′-Methoxyspiro[tetrahydrofuran-2,2′-(1′H-naphthalene)]-1′,5-
(
1.0 g, 6 mmol) in water (15 mL) at room temperature, and it was
dione (4d). OD-H chiral column, eluent n-hexane/isopropanol 85/15,
flow rate 0.7 mL/min, 25 °C, λ = 230 nm, t = 26.47, 33.87 min.
stirred for an additional 15 min, and then for 18 h at 50 °C. Ice−water
was added, and the reaction mixture was extracted with dichloro-
methane. The organic phase was washed with dilute aqueous sodium
thiosulfate and saturated aqueous sodium chloride and then dried over
anhydrous sodium sulfate. After removal of the solvent, the residue
was subjected to column chromatography (eluent n-hexane/ethyl
acetate 10/1) on silica gel to give (R)-6,6′-diacetyl-7,7′-diiodo-1,1′-
spirobiindane (R)-7 (242 mg, 0.435 mmol, 58% yield) as a white
powder.
4
′-Phenylspiro[tetrahydrofuran-2,2′-(1′H-naphthalene)]-1′,5-
dione (4e). OD-H chiral column, eluent: n-hexane/isopropanol: 90/
0, flow rate: 0.8 mL/min, 25 °C, λ = 230 nm, t(minor) = 33.97 min,
t(major) = 43.13 min.
1
3
′-Phenylspiro[tetrahydrofuran-2,2′-(1′H-naphthalene)]-1′,5-
dione (4f). OD-H chiral column, eluent n-hexane/isopropanol 85/15,
flow rate 1.0 mL/min, 25 °C, λ = 230 nm, t(minor) = 13.80 min,
t(major) = 18.98 min.
The diiodide (R)-7 (834 mg, 1.50 mmol) was dissolved in 15 mL of
5
′-Phenylspiro[tetrahydrofuran-2,2′-(1′H-naphthalene)]-1′,5-
dichloromethane at 0 °C, and trifluoromethanesulfonic acid (0.13 mL,
dione (4g). IA chiral column, eluent n-hexane/isopropanol 85/15,
flow rate 0.9 mL/min, 25 °C, λ = 230 nm, t(minor) = 12.35 min,
t(major) = 13.93 min.
1
.5 mmol) in dichloromethane (15 mL) was slowly added. A solution
of triethylsilane (1.8 mL, 11.3 mmol) in dichloromethane (15 mL) was
then added. After being stirred for 15 h, the reaction mixture was
poured into saturated aqueous sodium hydrogen carbonate, extracted
with dichloromethane, and dried over anhydrous sodium sulfate. After
removal of the solvent, the residue was subjected to column
chromatography on silica gel (eluent n-hexane) to give (R)-6,6′-
diethyl-7,7′-diiodo-1,1′-spirobiindane (R)-2b (634 mg, 1.20 mmol,
5′-Bromospiro[tetrahydrofuran-2,2′-(1′H-naphthalene)]-1′,5-
dione (4h). OD-H chiral column, eluent n-hexane/isopropanol 85/15,
flow rate 0.8 mL/min, 25 °C, λ = 230 nm, t(major) = 24.48 min,
t(minor) = 29.51 min.
5′-Azidospiro[tetrahydrofuran-2,2′-(1′H-naphthalene)]-1′,5-
dione (4i). OD-H chiral column, eluent n-hexane/isopropanol 85/15,
flow rate 0.8 mL/min, 25 °C, λ = 230 nm, t(major) = 28.73 min,
t(minor) = 33.11 min.
8
0% yield).
+)-(R)-6,6′-Diethyl-7,7′-diiodo-1,1′-spirobiindane (2b, (R)-
(
25
Enantiomer). Colorless solid, mp 105−106 °C, [α]
1
+21.0 (c
5′-Benzyloxyspiro[tetrahydrofuran-2,2′-(1′H-naphthalene)]-1′,5-
dione (4j). IA chiral column, eluent n-hexane/isopropanol 85/15, flow
rate 0.9 mL/min, 25 °C, λ = 230 nm, t(minor) = 22.34 min, t(major)
D
1
.09, chloroform); H NMR (300 MHz, CDCl ) δ 1.17 (6H, t, J = 7.6
3
Hz), 2.18−2.24 (2H, m), 2.41−2.69 (2H, m), 2.70−2.80 (4H, m),
=
27.16 min.
2
.98−3.10 (4H, m), 7.07 (2H, d, J = 7.6 Hz), 7.16 (2H, d, J = 7.6 Hz)
13
6
′-Phenylspiro[tetrahydrofuran-2,2′-(1′H-naphthalene)]-1′,5-
ppm; C NMR (75 MHz, CDCl ) δ 15.0, 30.1, 34.4, 36.9, 66.9, 123.5,
3
dione (4k). OD-H chiral column, eluent n-hexane/isopropanol 85/15,
flow rate 0.9 mL/min, 25 °C, λ = 230 nm, t(major) = 30.83 min,
t(minor) = 36.41 min.
1
1
24.4, 126.7, 126.8, 142.8, 143.8 ppm; IR (KBr) 2967, 2902, 2253,
815, 1792, 1454, 1454, 1385, 1384, 1095, 912, 745 cm ; HRMS
−1
+
(
FAB) calcd for C H I [M] , 527.9811, found 527. 9813.
21 22 2
6
′-Azidospiro[tetrahydrofuran-2,2′-(1′H-naphthalene)]-1′,5-
Typical Procedure for Dearomatizing Spirolactonization by
ortho-Substituted Iodoarene Catalyst 2b. To a stirred solution of
R)-6,6′-diethyl-7,7′-diiodo-1,1′-spirobiindane (R)-2b (26.4 mg, 0.050
mmol) in chloroform (25 mL), the wet m-chloroperbenzoic acid (ca.
9% purity including ∼25% water, 163 mg, 0.65 mmol) was added at
room temperature. The mixture was cooled to 0 °C for 10 min, and 3-
1-hydroxy-2-naphthyl)propionic acid 3a (108 mg, 0.50 mmol) was
dione (4l). OD-H chiral column, eluent n-hexane/isopropanol 85/15,
flow rate 1.0 mL/min, 25 °C, λ = 235 nm, t(major) = 18.50 min,
t(minor) = 22.67 min.
(
8
′-Methylspiro[tetrahydrofuran-2,2′-(1′H-naphthalene)]-1′,5-
6
dione (4m). OD-H chiral column, eluent n-hexane/isopropanol 85/15,
flow rate 0.9 mL/min, 25 °C, λ = 230 nm, t(major) = 20.08 min,
t(minor) = 23.80 min.
(
then added. The mixture was stirred for 8 h at the same temperature,
while the reaction progress was monitored by TLC. After the reaction
was completed, saturated aqueous sodium hydrogen carbonate was
added to the mixture. The organic layer was separated, and the
aqueous phase was extracted with dichloromethane several times. The
combined organic extract was dried over anhydrous sodium sulfate and
evaporated to dryness. The residue was purified by column
ASSOCIATED CONTENT
■
*
S
Supporting Information
Full experimental details and detailed spectroscopic data for
1
13
new materials and products, including H and C NMR
G
dx.doi.org/10.1021/ja401074u | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
spectra, and CIF file for (+)-4b. This material is available free of
charge via the Internet at http://pubs.acs.org.
S. M.; French, A. N.; Richter, C.; Bahar, A. M.; Witulski, B.; Wirth, T.
Eur. J. Org. Chem. 2008, 5315.
(8) (a) Ochiai, M.; Kitagawa, Y.; Takayama, N.; Takaoka, Y.; Shiro,
M. J. Am. Chem. Soc. 1999, 121, 9233. (b) Ochiai, M.; Takaoka, Y.;
Masaki, Y.; Nagao, Y.; Shiro, M. J. Am. Chem. Soc. 1990, 112, 5677.
AUTHOR INFORMATION
Corresponding Author
kita@ph.ritsumei.ac.jp
Present Address
N.T.: Department of Chemical Science and Technology,
Faculty of Bioscience and Applied Chemistry, Hosei University,
-7-2 Kajino-cho, Koganei, Tokyo
■
(9) For an early study of the latter type of chiral pentavalent iodine
compounds by Zhdankin’s research group, see: (a) Ladziata, U.;
Carlson, J.; Zhdankin, V. V. Tetrahedron Lett. 2006, 47, 6301. See also:
(b) Altermann, S. M.; Schafer, S.; Wirth, T. Tetrahedron 2010, 66,
⊥
5
(
902.
10) We previously demonstrated that p-cyclohexadienone structures
3
can be effectively constructed by intramolecular introduction of
carboxylic acids, alcohols, amides, and electron-rich aromatic rings in
the hypervalent iodine-mediated phenolic oxidations: (a) Tamura, Y.;
Yakura, T.; Haruta, J.; Kita, Y. J. Org. Chem. 1987, 52, 3927. (b) Kita,
Y.; Tohma, H.; Kikuchi, K.; Inagaki, M.; Yakura, T. J. Org. Chem. 1991,
56, 435. (c) Kita, Y.; Tohma, H.; Inagaki, M.; Hatanaka, K.; Yakura, T.
J. Am. Chem. Soc. 1992, 114, 2175. (d) Kita, Y.; Arisawa, M.; Gyoten,
M.; Nakajima, M.; Hamada, R.; Tohma, H.; Takada, T. J. Org. Chem.
Notes
The authors declare no competing financial interest.
∥
Y.K. is Emeritus Professor of Osaka University
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Scientific
Research (A) and Encouragement of Young Scientists (A) from
JSPS, a Grant-in-Aid for Scientific Research on Innovative
Areas “Advanced Molecular Transformations by Organo-
catalysts” from MEXT, and Ritsumeikan Global Innovation
Research Organization (R-GIRO) project. T.D. is thankful for
financial support from The Sumitomo Foundation and the
Industrial Technology Research Grant Program from NEDO of
Japan.
1
998, 63, 6625. (e) Tohma, H.; Harayama, Y.; Hashizume, M.; Iwata,
M.; Egi, M.; Kita, Y. Angew. Chem., Int. Ed. 2002, 41, 348. (f) Tohma,
H.; Harayama, Y.; Hashizume, M.; Iwata, M.; Kiyono, Y.; Egi, M.; Kita,
Y. J. Am. Chem. Soc. 2003, 125, 11235.
(11) For recent reports of natural product syntheses containing
related o-spirolactone structures, see: (a) Cox, C.; Danishefsky, S. J.
Org. Lett. 2000, 2, 3493. (b) Siu, T.; Cox, C. D.; Danishefsky, S. J.
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(
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(
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(
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(
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(
(
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(
c) Hirt, U. H.; Schuster, M. F. H.; French, A. N.; Wiest, O. G.; Wirth,
̈
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2
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face attack and enantioselectivity; thus it is ruled out.
3
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1
(
382.
26) Compound (R)-8 was obtained as a racemic form at the
stereogenic centers of the secondary alcohol groups by the reduction.
We separated some of these stereoisomers by HPLC and also
examined the reaction of 3a using them, which seemed not to show
significant alternation in the enantioselectivity compared to the
racemic samples.
(
27) (a) Dohi, T.; Takenega, N.; Kato, D.; Uchiyama, T.; Maruyama,
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reported similar results in early 2010 at an academic conference:
(
b) Dohi, T.; Takenaga, N.; Uchiyama, T.; Kato, D.; Maruyama, A.;
Fujioka, H.; Kita, Y. 8th Symposium on Organic ChemistryThe Next
Generation, Tokyo, May 2010, 1−14.
(
28) Other substituents, such as phenyl and halogen groups, at the 8-
position of 1-naphtholcarboxylic acids also negatively affected the
enantioselectivity.
(
29) CCDC 821310 contains the supplementary crystallographic
data for compound (+)-4b. The data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif. For more detailed crystallographic data, see
the CIF file.
(
30) (a) Bell, R.; Morgan, K. J. J. Chem. Soc. 1960, 1209. (b) Moriarty,
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31) For the mechanism of phenolic oxidations using hypervalent
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u, L.; Deffieux, D. Synlett 2008, 467.
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2
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Lamidey, A.-M.; Miqueu, K.; Sotiropoulos, J.-M.; Quideau, S. Angew.
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(
33) The presence of the two iodine groups in the spirobiindane
backbone is essential for high enantioselective control of the catalysts
. Indeed, a spirobiindane compound having one iodine group at the
2
same position gave the product 4a with a poorer ee value (19% ee)
using the same reaction system and conditions. See the results in SI.
For spirobiindane compounds having one iodine group, see also the
I
dx.doi.org/10.1021/ja401074u | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX