Chiral ammonium hypoiodite-catalyzed enantioselective oxidative dearomatization of 1-naphthols using hydrogen peroxide

Article abstract of DOI:10.1246/cl.141012

A highly enantioselective oxidative dearomatization of 1-naphthol derivatives (Kita spirolactonization) catalyzed by chiral hypoiodite species prepared in situ from chiral quaternary ammonium iodide in the presence of hydrogen peroxide is reported.

Full text of DOI:10.1246/cl.141012

Received: November 7, 2014 | Accepted: November 17, 2014 | Web Released: February 5, 2015
CL-141012
Chiral Ammonium Hypoiodite-catalyzed Enantioselective
Oxidative Dearomatization of 1-Naphthols
Using Hydrogen Peroxide
Muhammet Uyanik,1 Niiha Sasakura,1 Erina Kaneko,1 Kento Ohori,1 and Kazuaki Ishihara*1,2
1Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya, Aichi 464-8603
2Japan Science and Technology Agency (JST), CREST, Nagoya University, Chikusa-ku, Nagoya, Aichi 464-8603
(E-mail: ishihara@cc.nagoya-u.ac.jp)
a) Hypervalent organoiodine(III)-catalyzed oxidative dearomatization3,4
A highly enantioselective oxidative dearomatization of 1-
naphthol derivatives (Kita spirolactonization) catalyzed by chiral
hypoiodite species prepared in situ from chiral quaternary am-
monium iodide in the presence of hydrogen peroxide is reported.
O
Chiral ArI precat.
OH
O
O
O
m-CPBA
OH
Et
Halogenated Solvent
–20 ~ 0 °C
R1
R1
Mes
H
Mes
Et
The oxidative dearomatization of phenol derivatives is an
important pathway in the biosynthesis of many biologically
active compounds.1 As a consequence, the asymmetric oxidative
dearomatization of phenols has emerged as a promising tool
for the synthesis of various natural products.2 Conventionally,
enantioselective transition-metal catalysis has been used for
these transformations.2b,2d,2f Recently, transition-metal-free ox-
idative dearomatization reactions3-5 have been reported using
chiral hypervalent organoiodines.6 In particular, Kita’s group3
and our group4 have developed highly enantioselective catalytic
oxidative dearomatizations of phenol derivatives (Kita spirolac-
tonization) using chiral iodoarenes (Scheme 1a). However, these
chiral organoiodine(III) catalysts were prepared in situ from the
corresponding iodoarenes and meta-chloroperbenzoic acid (m-
CPBA), and meta-chlorobenzoic acid (m-CBA) was generated
as a waste (Scheme 1c, left). Here, we report the enantio-
selective oxidative dearomatization of 1-naphthol derivatives 1
to spirolactones 2 using chiral ammonium hypoiodite catalysis7,8
(Scheme 1b). The chiral hypoiodite active species is generated
in situ from the corresponding chiral quaternary ammonium9
I
H
I
O
N
I
N
O
O
O
Kita et al. (2008 and 2013)3
Ishihara et al. (2010 and 2013)4
b) This work: Hypoiodite-catalyzed oxidative dearomatization
O
[R4N]+[I]– precat. ([3]+[I]–)
OH
O
O
O
30% H2O2
OH
Toluene/H2O
Room Temperature
R1
Ar
R1
1
2
CF3
CF3
Ar =
F3C
I–
N+
CF3
Ar
[3]+[I]–
c) ArI/m-CPBA vs. [R4N]+[I]–/H2O2 oxidation systems:
[R4N]+[IO]–
ArIL1L2
1
H2O
m-CBA
¹
iodide [3]+[I] and aqueous hydrogen peroxide as a mild and
inexpensive oxidant, and water is the only by-product generated
from the oxidant (Scheme 1c, right). Moreover, the use of non-
halogenated solvents such as toluene at ambient temperature is
a major advantage over organoiodine(III)-catalyzed oxidation
reactions, which often require halogenated solvents under low-
temperature conditions.
2
[R4N]+[I]–
H2O2
m-CPBA
ArI
H2O
H2O
Scheme 1. Oxidative dearomatization of phenols.
Recently, we developed an in situ-generated chiral quater-
nary ammonium hypoiodite catalysis for the enantioselective
oxidative cyclization of ketophenols to 2-acyl-2,3-dihydroben-
zofurans or 2-acylchromans with the use of hydrogen peroxide
or tert-butyl hydroperoxide (TBHP) as oxidants.7 In these
reactions, the chemoselective α-oxidation of carbonyl moieties
preferentially proceeded, and phenol moieties served as intra-
molecular nucleophiles. However, during our studies on six-
membered oxidative cyclization to chromans (Scheme 2),7b
we found that tuning of the acidity of the phenol moiety of
substrates with electron-withdrawing protective groups is crucial
for chemoselective oxidative carbon-oxygen coupling (i.e., 4 to
5), since electron-rich phenol moieties are easily dearomatized
(i.e., 4 to 6). Based on these preliminary findings, we envisioned
that the hypoiodite oxidation system could be applied to the
TsO
N
PhN
R = Ts
O
N
O
N
Ph
O
[3]+[I]–
(10 mol%)
5: 98% yield, 60% ee
H
RO
t-BuOO
TBSO
R = TBS
TBHP
(2 equiv)
MTBE, RT
O
O
H
+
O
O
4
6a: 50% yield
6b: 10% yield
Scheme 2. Preliminary findings for the oxidative dearomatiza-
tion of phenols using hypoiodite catalysis.7b
Chem. Lett. 2015, 44, 179–181 | doi:10.1246/cl.141012
© 2015 The Chemical Society of Japan | 179

Table 1. Hypoiodite-catalyzed oxidative dearomatization of 1aa
Table 2. Substrate scopea
[3]+[I]– (10 mol%)
O
30% H2O2 (2 equiv)
Precat. (10 mol%)
30% H2O2
O
OH
O
O
1
2
O
(2 equiv)
Toluene/H2O or Toluene, 20 °C
+
CO2H
Solvent
20 °C, 72 h
O
O
O
O
O
O
O
O
O
Br
Br
Br
1a
2a
7a
2a (7a)
Yield/%b
2a
ee/%c
Entry Precat.
Solvent
Cl
2b
Ph
2c
Ar
¹
1
[Bu4N]+[I]
Toluene
Toluene
Toluene/H2Oe
Toluene/H2Oe
Toluene/H2Oe
Toluene/H2Oe
69 (13)
52 (14)
<1 (28)
38 (21)
72 (<5)
<1 (10)
®
®
®
®
88
®
2d [Ar = 3,5-(CF3)2C6H3]
72 h: 75%, 92% ee
d
72 h: 84%, 88% ee
72 h: 90%, 85% ee
2
3
4
5
6
®
®
d
O
O
O
O
O
O
O
O
O
¹
[Bu4N]+[I]
¹
[3]+[I]
[3]+[Cl]
¹
OMe
Me
2e
OMe
2g
18 h: 41%, 0% ee
aUnless otherwise noted, a solution of 1a (0.1 mmol) and
30 wt % H2O2 (0.2 mmol) in solvent was stirred in the presence
of catalyst at 20 °C. Isolated yield of 2a is shown. The yield
2f
24 h: 80%, 89% ee
48 h: 71%, 74% ee
b
aA solution of 1 (0.1 mmol) and 30 wt % H2O2 (0.2 mmol) in
toluene-H2O (for 2b-2d) or toluene (for 2e-2g) was stirred in
the presence of [3]+[I] at 20 °C. The reaction time, isolated
yield, and enantiomeric excess of 2 are shown. Intramolecular
condensation by-products 7 were obtained in less than 5%
yield in each case. For details, see Supporting Information.
of 7a shown in parentheses was determined by NMR analysis.
cEnantiomeric excess and absolute configuration of 2a were
determined by HPLC analysis based on the literature.4 dIn the
¹
e
absence of catalyst. Toluene/H2O (2:1 v/v).
oxidative spirolactonization of 1-naphthols tethered to a carbox-
ylic acid moiety at the 2-position (Scheme 1b).
are: (1) milder reaction conditions (non-halogenated solvent), (2)
operational simplicity (ambient temperature), and (3) water is
the only by-product derived from the oxidant used. These results
highlight the substantial scope of chiral hypoiodite catalysis in
place of organoiodine catalysis.
A mixture of 4-bromo-1-naphthol derivative 1a and 30 wt %
aqueous hydrogen peroxide (2 equivalents) in toluene was stirred
¹
in the presence of 10 mol % of [Bu4N]+[I] at 20 °C to give the
desired spirolactone 2a and intramolecular condensation by-
product 7a in respective yields of 69% and 13% (Table 1,
Entry 1). Surprisingly, 2a was also obtained in the absence of
catalyst under the same conditions (Entry 2).10 To prevent an
uncatalyzed background reaction, the reaction conditions were
investigated and almost no oxidation reaction occurred in a
toluene-water biphasic system in the absence of catalyst (Entries
3 and 4). Next, the enantioselective oxidation reaction was
Financial support for this project was partially provided by
JSPS. KAKENHI (Nos. 24245020 and 25105722) and Program
for Leading Graduate Schools “Integrative Graduate Education
and Research in Green Natural Sciences,” MEXT, Japan, and
JSPS Research Fellowships for Young Scientists (N.S.). We
thank Takeshi Yasui for assistance during the initial experiments.
¹
performed using chiral ammonium iodide [3]+[I] , and 2a was
obtained in 72% yield with 88% ee (Entry 5). Importantly, high
enantioselectivity could be achieved in the present oxidative
dearomatization even in the absence of an imidazolyl auxiliary,
which was required for our previous oxidation reactions.7
Additionally, we confirmed that iodide is essential for the present
reaction, since no oxidation reaction occurred in the presence of
Supporting Information is available electronically on J-STAGE.
References and Notes
1
2
S. K. Jackson, K.-L. Wu, T. R. R. Pettus, in Biomimetic
Organic Synthesis, ed. by E. Poupon, B. Nay, Wiley-VCH,
Weinheim, 2011, Chap. 20. doi:10.1002/9783527634606.
ch20.
ammonium chloride [3]+[Cl] instead of [3]+[I] (Entry 6).
To explore the scope of the present dearomatization
reaction, several 1-naphthol derivatives 1 were examined as
substrates under optimized conditions (Table 2).11 The oxidation
of 4-substituted 1b-1e and 3-OMe-substituted 1f gave the
corresponding spirolactones 2b-2f in good to high yields with
good to high enantioselectivities. However, the oxidation of
4-methoxynaphthol derivative 1g gave racemic 2g, as did a
previous organoiodine(III) system.3,4 Unfortunately, no satisfac-
tory results were obtained for the oxidation of simple phenols
under these conditions.
¹
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enantioselective oxidative dearomatization of 1-naphthol deriv-
atives using hydrogen peroxide as a mild and inexpensive
oxidant. The most important advantages of the present method
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10 Among the substrates examined, uncatalyzed background
reaction was observed only for 1a.11 The reason for this
result is not yet clear.
11 For details, see the Supporting Information.
Chem. Lett. 2015, 44, 179–181 | doi:10.1246/cl.141012
© 2015 The Chemical Society of Japan | 181