Enantiodifferentiating endo-selective oxylactonization of ortho-alk-l-enylbenzoate with a lactate-derived aryl-λ3-iodane

Article abstract of DOI:10.1002/anie.201003503

It's the hype: The asymmetric synthesis of 3-alkyl-4-oxyisochroman-1-one is achieved by oxylactonization of ortho-alk-1-enylbenzoate with a series of optically active hypervalent iodine(III) reagents prepared from lactate or valine as a chiral source (see scheme). The oxylactonization is highly regio-, diastereo-, and enantioselective. 2010 Wiley-VCH Verlag GmbH & Co. KGaA.

Full text of DOI:10.1002/anie.201003503

Communications
DOI: 10.1002/anie.201003503
Oxidation
Enantiodifferentiating endo-Selective Oxylactonization of
ortho-Alk-1-enylbenzoate with a Lactate-Derived Aryl-l3-Iodane**
Morifumi Fujita,* Yasushi Yoshida, Kazuyuki Miyata, Akihiro Wakisaka, and Takashi Sugimura
Ongoing efforts have been dedicated to the development of
reaction processes controlled by chiral hypervalent iodine
reagents with high enantioselectivity.[1–12] The oxidation of
sulfides into sulfoxides,[2] the a-oxygenation of ketones,[3,4] the
dioxygenation of alkenes,[4,5,12] and the dearomatization of
phenols[6–8,9b] have been reported, and most of these reactions
resulted in an encouraging level of enantioselectivity. Kita
and co-workers reported dearomatizing spirolactonization of
naphthols (78–86% ee) using
a spirocyclic iodine(III)
reagent.[6] Ishihara and co-workers recently reported that
higher enantioselectivities were obtained for the spirolacto-
nization by using a chiral iodine compound derived from
lactic acid.[8]
Scheme 1. Optically active hypervalent iodine(III) reagents.
2-Ethenylbenzoic acid (7a) was subjected to the reaction
conditions with the optically active hypervalent iodine(III)
reagent. The reaction was carried out in the presence of para-
toluenesulfonic acid (TsOH) to activate the iodine reagent,
and the tosylate also worked as a nucleophile to give lactones
8a and 9a (Table 1). The reaction proceeded regioselectively
to give the d-lactone product 8a as the major product. The
reaction with 6 gave a higher ee value of 8a with high
regioselectitvity (Table 1, entry 5).
Our studies with optically active hypervalent iodine
compounds have been focused on mechanisms of the reaction
concerned[11] as well as synthetic applications.[12] Asymmetric
oxidation of 4-acyloxybut-1-ene into 3-acyloxytetrahydro-
furan (up to 64% ee) was achieved by using chiral hypervalent
iodine(III) reagents, 1 and 2, which have a lactate moiety as a
chiral source.[12] During the course of these studies for the
asymmetric oxidative cyclization of alkenes, we found that
oxidation of ortho-alk-1-enylbenzoate with the hypervalent
iodine reagent regio- and diastereoselectively gave 3-alkyl-4-
oxyisochroman-1-one in a practically useful degree of enan-
tiomeric purity (90—98% ee); the isochromanone framework
is a biologically relevant building block of natural prod-
ucts.[13,14] Herein, we report the synthetic utility of such
enantiodifferentiating endo-selective oxylactonizations.
The series of optically active hypervalent iodine (III)
reagents 1–6 employed in this report is shown in Scheme 1.
On the basis of reagents 1 and 2 reported previously,[12] the
structures of the iodine reagents were tuned for improved
enantioselectivity. The X-ray crystallographic structures of 1–
4 showed a typical T-shape orientation around the iodine
center, where the two acetoxy ligands occupied apical
positions (see the Supporting Information).
Table 1: Tosyloxylactonization of 2-ethenylbenzoic acid.[a]
Entry
Ar*I(OAc)2
Yield [%]
ee [%][b]
(8a/9a)
(S)-8a
9a
1
2
3
1
2
4
5
6
66 (93:7)
69 (96:4)
70 (96:4)
74 (86:14)
65 (95:5)
75
90
76
60
97
18(R)
42(R)
22(S)
28(S)
26(S)
4
5[c]
[a] Reaction conditions are given in the Supporting Information. The
acetoxylactonization product 10a was detected in less than 10% yield.
[b] The ee values were determined by HPLC analysis on a chiral stationary
phase. [c] In order to estimate a change in ee value of the product owing
to the crystallization for isolation, the crude mixture was analyzed before
crystallization. The ee value of 8a in the crude mixture was 97% and
agreed with that in the isolated product.
[*] Prof. Dr. M. Fujita, Y. Yoshida, K. Miyata, Prof. Dr. T. Sugimura
Graduate School of Material Science, University of Hyogo
Kohto, Kamigori, Hyogo 678-1297 (Japan)
Fax: (+81)791-58-0115
E-mail: fuji@sci.u-hyogo.ac.jp
Dr. A. Wakisaka
National Institute of Advanced Industrial Science and Technology
Onogawa, Tsukuba, Ibaraki 305-8569 (Japan)
It is remarkable that the oxylactonization proceeds with
endo selectivity[15,16] in addition to the high enantioselectivity.
For elucidation of the mechanism of the endo selectivity and
its synthetic applications, we varied the nucleophile and the
substrate in the oxylactonization. When boron trifluoride
diethyl etherate was employed as an activator in the presence
[**] We are very grateful to Dr. Hiroki Akutsu and Prof. Shin’ichi
Nakatsuji (Hyogo) for X-ray crystallographic analyses and to
Prof. Tadashi Okuyama (Hyogo) for reading this manuscript.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003503.
7068
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Chemie
of acetic acid, an acetate group was introduced onto the
enantioselective oxylactonization product (Table 2). The
acetoxylactonization also preferentially gave d-lactone 10a.
The enantioselectivity depended on the structure of the
(Table 3). An oxy functional group, such as methoxymethyl
(7’b) and hydroxymethyl (7’c) groups in the alkenyl moiety
hardly affected the yield of the d-lactone product. The
cis configuration of the product is consistent with the small
coupling constant (J < 2 Hz) observed for the 1H NMR signal
owing to the benzylic proton, and was confirmed by X-ray
crystallographic analysis of 10b (for further details, see the
Supporting Information). The reaction of these alkenyl
substrates 7’b–d gave higher enantioselectivity than that of
the vinyl substrate 7’a.
Table 2: Acetoxylactonization of 2-ethenylbenzoic acid.[a]
On the basis of the syn selectivity observed, a plausible
reaction mechanism is proposed in Scheme 2. (Diacetoxyio-
do)arene is activated by acid (TsOH or BF3·OEt2). Electro-
Entry
Ar*I(OAc)2
Yield [%]
ee [%][b]
(10a/11a)
(S)-10a
(R)-11a
1
2
3
4
1
2
3
6
73 (97:3)
57 (95:5)
75 (96:4)
68 (92:8)
82
83
86
88
51
54
78
72
[a] Reaction conditions are given in the Supporting Information. [b] The
ee value was determined by GC analysis on a chiral stationary phase.
hypervalent iodine(III) reagent, and the trend in enantiose-
lectivity was similar to that observed in the tosyloxylactoni-
zation. Although the ee value of 10a was slightly lower than
that of tosyloxy lactone 8a, the ee value of the minor product
(g-lactone 11a) was higher than that of 9a. The reaction of
methyl ester substrate 7’a also gave acetoxylactonization
products 10a and 11a, as shown in Table 3, entries 1–3.
Preferential formation of the d-lactone 10b–d was also
observed for the reaction of the alkenyl substrates 7’b–d
Scheme 2. Plausible pathway to explain the stereochemical outcome.
philic addition of the activated iodine(III) reagent to the
alkene may be followed by nucleophilic displacement with
inversion of configuration. Alkyl-l3-iodane B or C undergoes
a second nucleophilic displacement with inversion of config-
uration. These two consecutive substitution steps proceed
with inversion of configuration, and result in the syn selec-
tivity. The initial nucleophilic substitution would then take
place at the benzylic position, where a positive charge may be
localized, and then the intermediate B is generated. The
counteranion of intermediate A may act as the initial
nucleophile, judging from the effective formation of tosyloxy
product 8a despite the presence of a highly nucleophilic
acetate (Table 1). The ensuing participation of the internal
carboxylate (carboxylic acid) leads to the departure of the
aryliodonio group. Preferential formation of the d-lactone is
rationalized by the reaction pathway A!B!8/10.[17] The d-
lactone could also be generated via intermediate C. The
reaction pathway via C is unlikely because reactions of 7a and
7’a gave similar regioselectivity independent of the nucelo-
philicity of the internal carboxylic acid and carboxylate. If the
reaction proceeded via C, the stereochemical purity of the
product would have decreased owing to facial elimination of
aryliodonio group at the benzyl position of the intermediate C
(SN1).[18]
Table 3: Acetoxylactonization of methyl ortho-alk-1-enylbenzoate 7’.[a]
Entry
R,
Ar*I(OAc)2
10
Substrate
Yield [%]
ee [%]
1
2
3
4
H
H
H
7’a
7’a
7’a
7’b
7b
7’b
7’b
7b
7’c
7’c
7’c
7’d
7’d
1
3
6
1
1
3
6
6
1
3
6
1
3
73
64
72
80
84
62
60
84
84
80
57
63
76
75[b]
84[b]
90[b]
84
84
97
97
96
86
94
CH2OMe
CH2OMe
CH2OMe
CH2OMe
CH2OMe
CH2OH
CH2OH
CH2OH
n-C5H11
n-C5H11
5[c]
6
7
8[c]
9
10
11
12
13
96
90
96
The reaction of 2-ethenylbenzoic acid 7a with 1–6
preferentially gave (S)-8a and (S)-10a rather than the (R)-
isomers. For the reaction of ortho-alkenylbenzoate, the
(3S,4S)-isomer was obtained under stereocontrol of these
enantiopure hypervalent iodine(III) reagents, as shown in
[a] Reaction conditions are given in the Supporting Information. Details
of the minor isomers are shown in Table S3. [b] The S enantiomer was
preferentially obtained. [c] 2-((E)-3-Methoxyprop-1-enyl)benzoic acid 7b
was used as the substrate.
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www.angewandte.org 7069

Communications
synthesis of a natural product 13 (see below). According to
and a small signal owing to M+·MeCN was observed: only one
molecule of the solvent coordinates to the iodonium species.
That is, the 1-(methoxycarbonyl)ethoxy side-chain of 1 and 6
prevents acetonitrile molecules from interacting with a
cationic site of the iodonium.[21] These experimental results
strongly suggest that the internal lactate side chain interacts
with the hypervalent iodine moiety even in solution: the most
reasonable structure is illustrated in Figure 1c. Such inter-
action must control the enantiodifferentiating ability of the
hypervalent iodine 1–6.
The utility of this enantioselective oxylactonization was
demonstrated by its application to the crucial step of a concise
asymmetric synthesis of a biologically active compound (ꢀ)-
13, which was recently isolated from Xyris pterygoblephara
and found to act as a selective inhibitor of aromatase
(Scheme 3).[13a,b] The substrate 12 for asymmetric lactoniza-
the mechanism in Scheme 2, these hypervalent iodine
reagents must attack the si face of the alkene substrate.[17]
Similar facial selectivity was observed for tetrahydrofurany-
lation of 4-acyloxybutenes.[12] The high facial selectivity of
these hypervalent iodine reagents suggests that the iodine
atom of reactive site of 1–6 may be strongly affected by
chirality of the lactate moiety.
To understand structural characteristics of the optically
active hypervalent iodine reagent in solution,[19] solvated
clusters of cationic iodonium generated from the reagent was
analyzed using specially designed mass spectrometry.[20]
Electrospray ionization mass spectra of (diacetoxyiodo)ar-
enes were measured in aqueous acetonitrile solution contain-
ing trifluoroacetic acid (Figure 1). In all of these cases, an m/z
Scheme 3. Total synthesis of a biologically active natural product 13.
tion was prepared in 74% yield by a Suzuki coupling reaction
with (E)-hept-1-enylboronic acid. The reaction of 12 with 3
gave the antipodal enantiomer of the natural product (ꢀ)-13
((3S,4S)-(+)-13) in 64% yield and 98% ee. The desired
enantiomer of the natural product, (3R,4R)-(ꢀ)-13, was
obtained by using the enantiomer of 1 (ent-1) and isolated
in 73% yield with 95% ee.
In conclusion, we have developed an efficient route to
optically active 4-oxyisochroman-1-one through oxidative
lactonization of ortho-alkenylbenzoate by using lactate-
derived aryl-l3-iodane 1–6. The oxylactonization proceeds
endo-selectively to give the d-lactone. The stereocontrolled
transformation was applied to asymmetric synthesis of a
biologically active natural product. The easily functionaliz-
able lactate-derived aryl-l3-iodane framework and ready
availability of its chiral source suggest promising future
applications of enantioselective oxidation using hypervalent
iodine reagents.
Figure 1. Electrospray ionization mass spectra of a) 1-(diacetoxyiodo)-
2,4,6-trimethylbenzene, b) 1, and c) 6 in 0.5%(v/v)water-99.5%(v/
v)acetonitrile solution containing trifluoroacetic acid (16 mm). The
peaks marked with D represent clusters composed of acetonitrile and
water molecules. Peaks at m/z=60, 83, 101, and 142 correspond to
H3O+·MeCN, H+·(MeCN)2, H3O+·(MeCN)2, and H3O+·(MeCN)3,
respectively.
Received: June 9, 2010
Published online: August 18, 2010
signal corresponding to the aryl(hydroxy)iodonium ion, M+ =
ArI+(OH), was observed together with H3O+·(MeCN)n (n =
1, 2, 3) in the range m/z < 150. The iodonium ion, solvated by
acetonitrile, M+·(MeCN)n (n = 1, 2), was observed for 1-
(diacetoxyiodo)-2,4,6-trimethylbenzene, which has no oxy-
substituent on the aryl group (Figure 1a): one or two
molecules of the solvent coordinate to the iodonium. In
contrast, the solvation of iodonium by acetonitrile was hardly
observed for 6, which was doubly functionalized by lactate
side-chains (Figure 1c). For the mono-functionalized reagent
1 (Figure 1b), no signal owing to M+·(MeCN)2 was detected,
Keywords: asymmetric synthesis · hypervalent compounds ·
iodine · lactones · oxidation
.
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Angew. Chem. Int. Ed. 2010, 49, 7068 –7071
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