Asymmetric Oxidative Cycloetherification of Naphtholic Alcohols

Article abstract of DOI:10.1002/chem.201700667

The catalytic, enantioselective oxidative cyclization of naphthol-derived carboxylic acids mediated by chiral hypervalent iodine reagents has been studied extensively in the recent past, but analogous reactions of non-carboxylic substrates are yet unknown. This paper describes a catalytic, enantioselective, hypervalent iodine-promoted oxidative cycloetherification reaction of naphtholic alcohols. The new process relies on a variant of the Uyanik–Ishihara catalyst, in which the stereogenic centers have been relocated closer to the iodine atom. The new catalyst design affords optical yields comparable to those available with Uyanik–Ishihara iodides, but chemical yields are sensibly higher, at least with the tests substrates. An even more problematic reaction is the catalytic, enantioselective oxidative cyclization of naphtholic sulfonamides. In this case, the new catalyst affords significantly higher optical inductions than Uyanik–Ishihara iodides. The kinetic resolution of particular naphtholic alcohols is demonstrated. The absolute configuration of a numver of enantioenriched compounds obtained in this study was ascertained by X-ray diffractometry.

Full text of DOI:10.1002/chem.201700667

A Journal of
Accepted Article
Title: Asymmetric Oxidative Cycloetherification of Naphtholic Alcohols
Authors: Marco A Ciufolini, Nikita Jain, and Sanjia Xu
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To be cited as: Chem. Eur. J. 10.1002/chem.201700667
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10.1002/chem.201700667
Chemistry - A European Journal
DOI: 10.1002/anie.200((will be filled in by the editorial staff))
(Asymmetric Catalysis)
Asymmetric Oxidative Cycloetherification of Naphtholic Alcohols**
Nikita Jain, Sanjia Xu and Marco A. Ciufolini*
Significant efforts have been channelled in recent years toward the
enantioselective oxidative cyclization of phenolic, and especially
naphtholic, carboxylic acids (cf. 1  2, Scheme 1). These reactions
rely upon the use of chiral aryl iodide catalysts,[1] which are
oxidized in situ to the I-hypervalent state, typically with MCPBA.[2]
Noteworthy examples have been described by Kita,[3] Fujita,[4] and
Uyanik and Ishihara.[5] Analogous reactions of non-carboxylic acid
substrates appear to be unknown. For instance, the oxidative
cyclization of alcohols 3 is documented only in the racemic series,[6]
although a related bimolecular, enantioselective hydroxylation
reaction has been developed by Quideau and Pouysegu.[7] Herein,
we describe a catalytic, enantioselective variant of the process 34.
on the oxidative cyclization of 3, R=H, promoted by MCPBA
and aryl iodides 5, under conditions similar to those described in the
literature for the cyclization of acids 1. A potential challenge
associated with the transformation 3  4 and related processes is
that such reactions are acid-catalyzed, so that suppression of the
carboxylic functionality may have an adverse effect on rates and
yields. Indeed, alcohol 3, R=H, reacted much more slowly than the
corresponding acid. Furthermore, the reaction was best carried out
in the presence of 20 mol% of chiral iodide instead of the typical 10
mol%. Also, 3, R=H, was a less-than-ideal substrate. Depending on
conditions, nearly one-third of it was lost to oxidation to the
corresponding 1,4-naphthoquinone; moreover, variable quantities of
product 4, R=H, underwent epoxidation of the double bond in the
course of the reaction. No such problems were seen with 6a (Table
1), which was thus used for subsequent work. As seen in Table 1,
best results, in terms of rates and ee's, were achieved with iodide 5b,
R = COMes (entry d): the same iodide of that type that also works
best in the reaction 1  2. However, yields remained in the mid-60's.
Table 1. Asymmetric cyclization of 6a in the presence of catalysts 5
Scheme 1. Ortho-oxidative cyclization of naphtholic substrates.
HO
O
20 mol% 5
OH
O
*
Uyanik-Ishihara iodides (Figure 1) are easily made and modified,
and as such they are especially useful as starting points for the
development of new transformations.[8] Initial forays thus focused
MCPBA[a]
6a
7a
Cl
Cl
entry
catalyst (R)
time (h)
% ee[b]
% yield[c]
5a (COOMe)[d]
5a (COOH)[d]
5a (CONH-Ar1)[d],[e],[f]
5b (COAr1)[d],[e]
5b (COAr2)[e]
5b (COAr3)[e]
5b (Ts)
42
18
32
15
36
33
37
13
29
65
90
64
74
27
27
51
62
65
52
60
46
R
R
a
b
c
d
e
f
I
HN
I
NH
R
O
O
R
O
O
5a
5b
Figure 1. Structure the Uyanik-Ishihara chiral iodides, 5.
g
[a] Conditions: 1.3 equiv MCPBA, –20 °C, 0.02 M in CH2Cl2. [b]
Determined by chiral supercritical fluid chromatography (SFC). [c]
After column chromatography. [d] Known compound: ref. 5. [e] Ar1 =
2,4,6-Me3C6H2; Ar2 = 3,5-(MeO)2C6H3; Ar3 = 2,4,6-(MeO)3C6H2. [f]
Catalysts 5a and 5b promoted the opposite sense of induction.
[]
N. Jain; Dr. Sanjia Xu, Prof. Dr. M. A. Ciufolini, Department
of Chemistry, University of British Columbia, 2036 Main
Mall, Vancouver, BC V6T 1Z1, Canada
Fax: (+) 1 604 822-2710
Uyanik and Ishihara proposed that intramolecular H-bonding, as
per Figure 2, may be a key aspect of catalyst operation.[5b] This led
E-mail: ciufi@chem.ubc.ca
Homepage:
www.chem.ubc.ca/personnel/faculty/ciufolini/index.shtml
[] We gratefully acknowledge support for this research from
the University of British Columbia, the Canada Research
Chair program, and NSERC.
Supporting information for this article is available on the
WWW under http://www.angewandte.org or from the
author.
Figure 2. Working hypothesis for new catalyst design.
1
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10.1002/chem.201700667
Chemistry - A European Journal
to the surmise that relocating the stereocenters closer to the
amide NH group, as seen in 8, might enforce the hydrogen-bonded
conformation and improve catalyst efficiency. New iodides 9 and
10[9] (Scheme 2) were thus evaluated. Catalyst 9 was equivalent to
5b, R = COMes, (66% yield of 7a, 90% ee). In contrast, iodide 10
afforded comparable ee's (93%), but yields were distinctly better
(79%). Subsequent work thus focused on the use of 10. An
extensive survey of reaction conditions[10] confirmed that optimal
results required operation in CH2Cl2 at –20 °C, with 20 mol % of 10
and 1.3 equiv of MCPBA. The rate of addition of MCPBA seemed
to have no significant effect on overall efficiency; however, it was
essential to avoid an excess of the reagent.
that those exhibiting electron-donating ones. This may reflect a
diminished tendency of the complex resulting upon the union of the
substrate with the oxidized form of the catalyst to dissociate: an
event that would erode ee's and induce various side reactions. An X-
ray structural analysis of an appropriate derivative[9] of (+)-7a
revealed that the spirocenter was of R-configuration. It should be
noted that catalyst 5b, R = COMes, afforded the opposite sense of
induction, providing instead (S)-(–)-7a. An analogous X-ray
structural study of a derivative[9] of (+)-7n also revealed this product
to be of R-configuration.
Table 3. Cyclization of alcohol 6a with recycled catalyst 10
HO
O
20 mol% 10
HO
O
20% catalyst
–20 °C
CH2Cl2
OH
O
OH
O
9: 90% ee, 66% yield
10: 93% ee, 79% yield
–20 °C
MCPBA[a]
6a
(R)-7a
CH2Cl2
MCPBA
6a
7a
Cl
Cl
Cl
Cl
I
cycle
time (h)
% ee[b]
% yield[c]
% rec. cat.[c]
O
O
R
O
O
1[d]
2
25
29
27
27
93
93
92
92
79
74
77
78
96
90
95
93
9
R = i-Pr
10 R = tert-Bu
Mes
N
R
N
H
Mes
H
3
4
Scheme 2. Structure and performance of catalysts 9 and 10.
[a] 1.3 equiv MCPBA, –20 °C, substrate conc. = 0.02 M. [b] By chiral
SFC. [c] After column chromatography. [d] Fresh catalyst.
Table 2 summarizes the results obtained in the cyclization of
substrates 6 in the presence of 10 under optimized conditions. As
far as asymmetric induction is concerned, substrates incorporating
electron-withdrawing substituents produced generally better results
Iodide 10 is easily recovered from reaction mixtures by column
chromatography (> 90% yield), and it can be recycled with no loss
of efficacy. Results of typical experiments carried out with recycled
10 appear in Table 3. Further evaluation of 10 involved a
comparison with 5b, R = COMes, in the cyclization of poor
substrate 3, R = H, to 4, R = H, at various temperatures and catalyst
loadings. As far as optical induction goes, iodide 10 retained
efficacy even at rt and 5 mol% loading (Table 4), even though yields
decreased appreciably at higher temperatures. This seems to provide
support for the notion that repositioning the centers of chirality
closer to the H-bonding sites is beneficial for effective induction.
Table 2. Cyclization of alcohols 6 in the presence of catalyst 10
entry
R
time (h)
% ee[b]
% yield[c]
a
b
c
d
e
f
4-Cl
4-Br
25
27
20
134
46
55
18
38
41
21
14
38
13
39
35
93[d]
79
73
Table 4. Relative efficacy of iodidoarene catalysts 10 and 5b (R =
mesitoyl) in the cyclization of 3, R = H, to 4, R = H.
84
4-Me
76
29
10 or 5b[a]
4-C(O)Me
92
15
HO
O
42[e]
41[f]
63[g]
52
mol%
4-C(O)Ph
92
92
OH
O
4-C(O)(4-Tol)
4-Ph
MCPBA
g
h
i
80
75 (98)[h]
3 (R=H)
temp., time[b]
4 (R=H)
4-(4-Tol)
4-(3-MeO-C6H4)
4-(4-MeC(O)-C6H4)
4-(4-F-C6H4)
4-(4-NC-C6H4)
4-(4-MeSO2-C6H4)
4-(4-CF3-C6H4)
4-(3,5-(CF3)2C6H3)
69
73
mol%
T (°C)
time (h)
% ee
10 (5b)[c]
% yield of 4
10 (5b)[d]
j
88
83 (90)[h]
75
10 (5b)
k
l
64
20
20
10
10
10
5
–20
rt
21 (12)
3.5 (2.5)
20.5 (16)
9.0 (5.5)
6.5 (4.5)
8.5 (7.0)
93 (93)
93 (86)
96 (90)
93 (89)
92 (86)
89 (82)
67 (61)
45 (43)
50 (46)
46 (42)
41 (30)
35 (23)
92
71
m
n
o
90
78
90[d] (98)[h]
55
–20
0
93
47
rt
[a] Conditions: 1.3 equiv MCPBA, –20 °C, substrate conc. = 0.02 M.
[b] Determined by chiral SFC. [c] After column chromatography. [d]
Absolute configuration was found to be R (see text). [e] 47% yield
based on RSM. [f] 50% yield based on RSM. [g] 69% yield based on
RSM. [h] ee of material remaining in the mother liquor after a single
recrystallization.
rt
[a] R = 2,4,6-Me3C6H2C(O). [b] Conditions: CH2Cl2, 1.3 equiv
MCPBA, –20 °C, substrate conc. = 0.02 M,. [c] Determined by chiral
SFC. [d] After column chromatography.
2
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10.1002/chem.201700667
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ation with PIFA in CH2Cl2 afforded the expected racemic
products in 40-50% yield (Scheme 4). Product (±)-18 emerged as a
single diastereomer, the NOESY-2D spectrum of which was clearly
in accord with the configuration shown. The reaction of (±)-16 was
almost non-selective, while that of (±)-17 exhibited moderate
selectivity (77:23) in favor of (±)-21. Individual product isomers
proved difficult to separate by standard chromatography.[12] The
relative configuration of each isomeric pair was thus assigned based
on a NOESY-2D study of the mixture (after purification).[9]
Table 5. Oxidative cyclization of substrates 11 mediated by 10.
20 mol% 10
HO
O
–20 °C
OH
O
CH2Cl2
MCPBA[a]
11
12
R
R
entry
R
time (h)
% ee[b]
% yield[c]
The enantioselective variant of the process was studied with
both iodides 9 and 10. These proved to be equally efficient in terms
of optical and chemical yields, but rates were 5 times faster with 9,
which therefore was the catalyst of choice in this case. Reactions
were run to about 50% conversion in order to evaluate the efficiency
of kinetic resolution. As seen in Scheme 5, oxidative cyclization
mediated by 9 proceeded with virtually unchanged levels of
diastereoselectivity, indicating that the chiral catalyst has only a
minor influence on that aspect of the reaction. The cyclization of
(±)-15 proceeded with poor chemical (28%) and optical (34% ee)
yields; also, recovered 15 (40%) was of only 9% ee. The same
reaction of (±)-16 was highly enantioselective (90% ee for each
diastereomer), but diasteroselectivity, chemical yields, and optical
enrichment of unreacted alcohol were unsatisfactory. The absolute
configurations of the various products were not determined.
a
H
Cl
Br
70
60
36
36
36
98
76
74
90
87
61
66
77
50
64
b
c[d]
d[d]
e[d]
4-Ph
4-(4-CF3-C6H4)
[a] 1.3 equiv MCPBA, –20 °C, substrate conc. = 0.02 M. [b]
Determined by chiral SFC. [c] After column chromatography. [d] 1.8
equiv of MCPBA was used.
A drawback of above processes is that reaction times are long.
In an effort to shorten reaction times, we examined the cyclization
of substrates 11 (Table 5), wherein the Thorpe-Ingold effect[11] of
the gem-dimethyl arrangement could promote a rate acceleration.
Contrary to this expectation, slower reaction rates were observed
both with compounds 11 as well as with naphthol 13 (Scheme 3),
which also afforded lower optical and chemical yield relative to 11b.
9 (20 mol%)
O
MCPBA
(0.6 equiv)
H
O
+
(±)-15
15
20 mol% 10
HO
O
CH2Cl2 (0.02 M)
–20 °C, 14.5 h
49% conv. (NMR)
Ph
(+)-18
34% ee
28%
MCPBA (1.3 equiv)
40%
9% ee
OH
41% yield
72% ee
Cl
O
*
O
CH2Cl2 (0.02 M)
13
14
O
–20 °C, 61 h
O
Cl
Cl
as
above
Me
H
H
O
16
+
Me
+
(±)-16
Scheme 3. Oxidative cyclization of 13 mediated by catalyst 10.
9.3 h
55% conv.
(NMR)
36%
8% ee
19
90% ee
20
90% ee
Cl
Cl
dr = 59 : 41
33%
These results suggest that unfavorable steric interactions within
the substrate-catalyst complex override the Thorpe-Ingold effect.
However, they also raise the question of whether one could harness
such interactions to influence the diastereoselectivity of the
cyclization of naphthols carrying a single substituent on the
hydoxypropyl chain, or even to induce kinetic resolution. The latter
issues were addressed with (±)-15-17, which upon oxidative cycliz-
Scheme 5. Enantioselective oxidative cyclization of rac-15 and 16.
The behavior of (±)-17 (Scheme 6) was more interesting. As in
the previous cases, diastereoselectivity was almost unaffected (83:17
vs. 77:23), but yields were higher (52%), ee's were excellent (94%
for the major diastereomer, (R,R)-21; 98% for the minor one, (R,S)-
22), and recovered alcohol (+)-(S)-17 was enriched to 79% ee.
R
O
2'
PIFA
CH2Cl2
3'
H
HO
(±)-18
single
diast.
40%
1'
O
OH
Ph
0.02 M
rt
H
(±)-15 R = 3'-Ph
(±)-16 R = 1'-Me
(±)-17 R = 2'-i-Pr
9 (20 mol%)
Cl
nOe
Cl
O
MCPBA
(0.6 equiv)
HO
(+)-(S)-17
79% ee
44% (NMR)
35% isolated
O
OH
(±)-17
+
Me
H
O
O
CH2Cl2
0.02 M
+
H
(±)-20
(minor)
(±)-19
Me
Cl
O
H
–20 °C, 13.5 h
56% conv.
H
(major)
dr = 55:45
43%
Cl
nOe
nOe
Cl
O
O
H
i-Pr
O (S)
O (R)
O
dr = 83:17
52% yield
+
(R)
(R)
i-Pr
H
i-Pr
H
O
(±)-22
(minor)
O
(±)-21
+
(R,R)-21
(major)
94% ee
(+)-(R,S)-22
(minor)
98% ee
(major )
Cl
Cl
H
i-Pr
H
H
dr = 77:23
49%
Cl
Cl
nOe
nOe
Scheme 6. Oxidative cyclization of (±)-17 mediated by catalyst 9.
Scheme 4. Diastereoselective oxidative cyclization of (±)-15-17.
3
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10.1002/chem.201700667
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The absolute configuration of these materials was determined as
follows. Recovered (+)-17 of 79% ee was used in a second round of
oxidative cyclization mediated by 9 run to 43% conversion. This
operation returned residual (+)-17 of 96% ee. An X-ray study of a
derivative[9] revealed that the compound was of (S)-configuration;
therefore, (R)-(–)-17 is the fast-reacting enantiomer. This implies
that the absolute configuration of enantioenriched 21 is (R,R), as
shown. Oxidative cyclization of (+)-17 of 96% ee in the presence of
9 afforded (+)-22 of 99% ee. This product was identical to the
major enantiomer of the minor diastereomer produced in the first
experiment. Therefore, the configuration of the carbon atom
bearing the i-Pr group in (+)-22 is (S), and that of the spirocenter
must be (R). This is consistent with the sense of enantioinduction
observed with simpler substrates.
alcohol 17 is best achieved with chiral iodide 9. Compounds 9-10
embody a structural modification of the Uyanik-Ishihara iodide 5b.
The new catalyst design may also be beneficial for the
enantioselective oxidative cyclization of naphtholic sulfonamides
such as 23. Additional efforts in this area are ongoing and pertinent
results will be described in due course.
Received: ((will be filled in by the editorial staff))
Published online on ((will be filled in by the editorial staff))
Keywords: asymmetric cycloetherification, asymmetric catalysis,
hypervalent iodine, oxidative amidation, phenols
A preliminary picture of the preferences of catalysts 9-10 thus
emerges as follows. The iodides have an insignificant effect on
diastereoselectivity, which remains largely under substrate control.
Kinetic resolution of the substrate is possible with alcohols of the
type 17, wherein a substituent is present at C-2'. These compounds
react with fair diastereoselectivity and excellent optical induction.
The sense of enantioinduction of the catalyst is unaffected by the
presence of the C-2' substituent.
On a final note, iodide 10 was briefly evaluated in the
enantioselective oxidative cyclization of a naphtholic sulfonamide:
another undocumented process[13] that appears to be even more
problematic than the analogous reaction of alcohols 3. So far as
optical induction is concerned, iodide 10 appears to be advantageous
relative to 5b, R = mesitoyl, in the oxidative cyclization of 23
(Table 6). An X-ray analysis of material recrystallized to 96% ee
revealed that (+)-24 obtained with 10 was of R-configuration, while
(–)-24 obtained with 5b, R = mesitoyl, was of S-configuration.[9]
[1] Reviews: a) A. M. Harned, Tetrahedron Lett. 2014, 55, 4681; b) A. Parra,
S. Reboredo, Chem. Eur. J. 2013, 19, 17244; c) H. Liang, M. A.
Ciufolini, Angew. Chem. Int. Ed. 2011, 50, 11849; Angew. Chem.
2011, 123, 12051; d) M. Ngatimin, D. W. Lupton, Aust. J. Chem.
2010, 63, 653; e) M. Ochiai, K. Miyamoto, Eur. J. Org. Chem. 2008,
4229.
[2] Review: R. D. Richardson, T. Wirth, Angew. Chem. Int. Ed. 2006, 45,
4402; Angew. Chem. 2006, 118, 4510, as well as references 3-7.
[3] a) T. Dohi, A. Maruyama, N. Takenaga, K. Senami, Y. Minamitsuji, H.
Fujioka, S. B. Caemmerer, Y. Kita, Angew. Chem. Int. Ed. 2008, 47,
3787; Angew. Chem. 2008, 120, 3847; b) T. Dohi, Y. Kita, Chem.
Commun. 2009, 2073; c) T. Dohi, N. Takenaga, T. Nakae, Y. Toyoda,
M. Yamasaki, M. Shiro, H. Fujioka, A. Maruyama, Y. Kita, J. Am.
Chem. Soc. 2013, 135, 4558.
[4] a) M. Fujita, Y. Yoshida, K. Miyata, A. Wakisaka, T. Sugimura, Angew.
Chem. Int. Ed. 2010, 49, 7068; Angew. Chem. 2010, 122, 7222. See
also: b) M. Shimogaki, M. Fujita, T. Sugimura, Eur. J. Org. Chem.
2013, 7128.
[5] a) M. Uyanik, T. Yasui, K. Ishihara, Angew. Chem. Int. Ed. 2010, 49,
2175; Angew. Chem. 2010, 122, 2221; b) M. Uyanik, T. Yasui, K.
Ishihara, Angew. Chem. Int. Ed. 2013, 52, 9215; Angew. Chem. 2013,
125, 9385.
Table 6. Relative efficacy of iodides 10 and 5b, R = MesCO, in the
cyclization of sulfonamide 23
[6] a) M. Uyanik, N. Sasakura, M. Kuwahata, Y. Ejima, K. Ishihara, Chem.
Lett. 2015, 44, 381-383; b) D. Sarkar, M. K. Ghosh, N. Rout, Org.
Biomol. Chem. 2016, 14, 7883, and literature cited therein.
[7] a) C. Bosset, R. Coffinier, P. A. Peixoto, M. El Assal, K. Miqueau, J.-M.
Sotiropoulos, L. Pouysegu, S. Quideau, Angew. Chem. Int. Ed. 2014,
53, 9860; Angew. Chem. 2014, 126, 10018; b) R. Coffinier, M. El
Assal, P. A. Peixoto, C. Bosset, K. Miqueau, J.-M. Sotiropoulos, L.
Pouysegu, S. Quideau, Org. Lett. 2016, 18, 1120.
HO
O
chiral iodide
(20 mol%)
NHMs
N
*
Ms
–20 °C
MCPBA[a]
23
24
entry
iodide
selectivity
% ee[b]
% yield[c]
[8] Examples: a) H. Wu, Y.-P. He, L. Xu, D.-Y. Zhang, L.-Z. Gong, Angew.
Chem., Int. Ed. 2014, 53, 3466; b) D.-Y. Zhang, L. Xu, H. Wu, L.-Z.
Gong, Chem. Eur. J. 2015, 21, 10314.
a
b
5b[d]
10
(–)-(S)-24
(+)-(R)-24
46
67
20
20
[9] See Supporting Information for details.
[a] Conditions: 20 mol% chiral iodide, 1.3 equiv MCPBA, –20 °C,
CH2Cl2, substrate conc. = 0.02 M, 10-15h. [b] Determined by chiral
HPLC. [c] After column chromatography. [d] R = MesCO.
[10] Details will be provided in a forthcoming full paper.
[11] a) R. M. Beesley, C. K. Ingold, J. F. Thorpe, J. Chem. Soc., Trans.,
1915, 107, 1080; b) S. M. Bachrach, J. Org. Chem., 2008, 73, 2466.
[12] In contrast, excellent separation was attained by SFC (Supporting
Information).
[13] This reaction is known only in the racemic series: N. Jain, M. A.
Ciufolini, Synlett 2015, 26, 631.
.
In summary, the enantioselective oxidative cyclization of non-
carboxylic naphtholic substrates, such as alcohols 3, 6, 11, 13
appears to be best carried out with chiral iodide 10, while that of
4
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
(Asymmetric Catalysis)
Nikita Jain, Sanjia Xu, Marco A.
Ciufolini* __________ Page – Page
Asymmetric Oxidative
Cycloetherification of Naphtholic
Alcohols
A catalytic, enantioselective oxidative cycloetherification reaction of naphtholic
alcohols is described.
5
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