Asymmetric synthesis of 2-Substituted dihydrobenzofurans and 3-hydroxydihydrobenzopyrans through the enantioselective epoxidation of O-silyl-protected ortho-allylphenols

Article abstract of DOI:10.1002/adsc.201000505

The Shi-type epoxidation of O-tert-butyldiphenylsilyl (TBDPS) protected o-allylphenols serves as an efficient strategy to construct the dihydrobenzofurans and dihydrobenzopyrans in up to 97% ee. This methodology led to the enantioselective synthesis of (+)-marmesin, (-)-(3′R)-decursinol, and (+)-lomatin.

Full text of DOI:10.1002/adsc.201000505

FULL PAPERS
DOI: 10.1002/adsc.201000505
Asymmetric Synthesis of 2-Substituted Dihydrobenzofurans and
3-Hydroxydihydrobenzopyrans through the Enantioselective
Epoxidation of O-Silyl-Protected ortho-Allylphenols
Hang Jiang,a Takaya Sugiyama,a Akinari Hamajima,a and Yasumasa Hamadaa,*
a
Graduate School of Pharmaceutical Sciences, Chiba University, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
Fax : (+81)-43-290-2987; phone: (+81)-43-290-2987; e-mail: hamada@p.chiba-u.ac.jp
Received: June 29, 2010; Revised: October 20, 2010; Published online: December 15, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000505.
Abstract: The Shi-type epoxidation of O-tert-butyldi- sis of (+)-marmesin, (À)-(3’R)-decursinol, and (+)-
phenylsilyl (TBDPS) protected o-allylphenols serves lomatin.
as an efficient strategy to construct the dihydroben-
zofurans and dihydrobenzopyrans in up to 97% ee. Keywords: dihydrobenzofurans; dihydrobenzopyr-
This methodology led to the enantioselective synthe- ans; lomatin; marmesin; Shi-type epoxidation
Introduction
zopyrans through the enantioselective epoxidation of
O-silyl-protected o-allylphenols using the Shi-type
2-Substituted dihydrobenzofurans 1 are important asymmetric epoxidation as the key step.
structural motifs, which are often found in a variety of
As illustrated in Scheme 1, the 2-substituted dihy-
natural products and exhibit a wide range of biologi- drobenzofurans 4 and 3-hydroxydihydrobenzopyrans
cal activities (Figure 1).[1] Although a number of 5 can be constructed from the same epoxide 3, which
methods have been developed for the preparation of would be obtained from the enantioselective epoxida-
the dihydrobenzofurans,[2–6] a simple and reliable tion of the corresponding ortho-allylphenols. The
method for the construction of optically active dihy- starting ortho-allylphenols can be prepared by the
drobenzofurans is scarce and remains to be devel- direct alkylation of phenols or the Claisen rearrange-
oped. Previous investigations from this laboratory ment of O-allylphenyl ethers.
have demonstrated the highly enantioselective synthe-
sis of angelmarin (2),[7] a dihydrobenzofuran natural
product bearing a strong activity against PANC-1 Results and Discussion
cancer cells under a nutrient-starved environment,
using our developed asymmetric synthesis as a key Our synthesis commenced with the preparation of the
step.[8–10] As an extention of this synthesis, we now de- starting ortho-allylphenols. For the ortho-3-methylbut-
scribe the successful asymmetric synthesis of 2-substi- 2-enylation, the Claisen rearrangement of a corre-
tuted dihydrobenzofurans and 3-hydroxydihydroben- sponding O-2-methylbut-3-en-2-yl aryl ether is the
most efficient. The required rearrangement precursor
can be prepared by the palladium-catalyzed direct al-
Figure 1. Dihydrobenzofurans.
Scheme 1. Synthetic route.
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Scheme 3. Reaction conditions of epoxidation
Scheme 2. Preparation of ortho-prenylphenols.
lylation of a phenol, i.e., Kaihoꢁs procedure.[11] Table 2. Reaction conditions of epoxidation.a)
Scheme 2 and Table 1 summarize the synthesis of
Entry
Alkene
Ketone
(equiv.)
Conditions
Yield
[%][b]
%
ortho-allylphenols. The reaction of the corresponding
phenols with a slight excess amount of 2-(2-methyl)-
but-3-enyl 2-methylpropyl carbonate (7) in the pres-
ence of tetrakis(triphenylphosphine)palladium (1–
1.5 mol%) in tetrahydrofuran at room temperature
for 1.5–7.5 h smoothly proceeded to give allyl ethers
8a–e in 89–98% yields. The exposure of 8a–e to re-
fluxing conditions in toluene for 24 h–2 d furnished
the ortho-allylphenols 9a–e in 67–85% yields. In the
case of 8b, the regioselective control of the Claisen re-
arrangement was difficult and an equal amount of the
products, 9ba and 9bb, was obtained in 44% and 42%
yields, respectively (entry 2).
G
ee[c]
1
2
3
10a
10b
10b
13 (0.3)
13 (0.3)
14 (0.15)
À108C, 4 h
08C, 3 h
08C, 23 h
28
27
88
21
77
97
[a]
The reaction was carried out using 13 or 14 in the pres-
ence of (n-Bu)4NHSO4 (0.04 equiv.), oxone (1.63 equiv.),
and potassium carbonate (2.4 equiv.) in CH3CN-DMM
buffer.
Isolated yield of two steps.
Enantiomeric excess of 12 determined by HPLC.
[b]
[c]
The asymmetric epoxidation conditions of ortho-al- tioselectivity were poor. While the reaction of the
lylphenols were optimized using ortho-prenylcoumar- silyl-protected phenol 10b provided the stable epox-
ins 10a and 10b as shown in Scheme 3 and Table 2. ide 11b and the deprotection of the epoxide with
When the protection-free phenol 10a was epoxidated tetra-n-butylammonium fluoride (TBAF) occurred
using the Shi ketone 13 at À108C for 4 h, the inter- with in situ cyclization to afford the benzofuran 12 in
mediately formed epoxide 11a spontaneously cyclized 27% yield with 77% ee (entry 2). The yield was still
and directly formed benzofuran 12, columbianetin, in unsatisfactory. Finally, the use of Shiꢁs other ketone
28% yield with 21% ee (entry 1). The yield and enan- 14,[12] designed for less reactive substrates, maximized
Table 1. Preparation of ortho-prenylphenols.
Entry
Substrate
Conditions of (A)
Yield of 8
Time of (B)
Yield of 9
1
2
3
4
5
6a
6b
6c
6d
6e
1 mol%, 7.5 h
1 mol%, 3 h
1.5 mol%, 3 h
1.3 mol%, 3 h
1 mol%, 1.5 h
89%
92%
98%
94%
92%
24 h
2 d
13 h
24 h
24 h
85%
44%,[a] 42%[b]
85%
70%
67%
[a]
Yield of 9ba.
Yield of 9bb.
[b]
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Asymmetric Synthesis of 2-Substituted Dihydrobenzofurans and 3-Hydroxydihydrobenzopyrans
Table 3. Asymmetric synthesis of dihydrobenzofurans.[a]
Entry
Substrate
Product
Yield [%][b]
% ee[c]
1
2
3
4
5
6
7
8
15a
15b
15c[d]
15d
15e
15f
15g
15h
15i
17a
17b
17c[e]
17d
17e
17f
17g
17h
17i
95
38
85
96
84
89
96
94
86
83
84
83
96
39
92[f]
40
91
96
89
90
97
97
9
10
11
12
15j
15k
15l
17j
17k
17l
97
96 (68)[g]
[a]
The reaction was carried out by two-step sequence: (1)
the epoxidation of 15 using 14 (0.15 equiv.) in the pres-
ence of (n-Bu)4NHSO4 (0.04 equiv.), oxone (1.63 equiv.),
and potassium carbonate (2.4 equiv.) in CH3CN-DMM-
buffer and (2) the conversion of 16 to 17 by the treat-
ment with TBAF (1.2 equiv.) in THF.
Isolated yield of two steps.
Enantiomeric excess of 17 was determined by HPLC.
The ratio of trans/cis is 97:3.
[b]
[c]
[d]
[e]
[f]
Diastereomeric ratio=96:4.
Enantiomeric excess of the major diastereomer.
The enantiomeric excess in parenthesis is for the TBS de-
rivative of 15l.
[g]
the yield and enantioselectivity to 88% and 97% ee,
respectively (entry 3). The absolute configuration of
12 was established by comparison to the known com-
pound in the literature.[13] It is interesting to note, in
contrast to Shiꢁs conditions, that the Shi ketone 14 is
not only highly reactive and enantioselective, but is
also robust and we were able to decrease the catalytic
amount to 15 mol%.
Scheme 4. Asymmetric synthesis of dihydrobenzofurans.
Using the optimized reaction conditions, we set out ity and in 95% yield (entry 1). The disubstituted
to explore its generality and scope. Table 3 and olefin 15c (trans/cis=96/4) was also a good substrate
Scheme 4 summarize the synthesis of dihydrobenzo- for this asymmetric epoxidation and furnished the di-
furans from various ortho-allylphenols, which were hydrobenzofuran 17c with a 96/4 diastereomeric ratio
prepared as described above or by the reported pro- and 92% ee (major diastereomer) and in 85% yield
cedures.[14] The silyl-protected phenols 15a–l were pre- (entry 3). However, the reaction of the monosubsti-
pared by the reaction of the corresponding phenols tuted and tetrasubstituted olefins 15b and 15d result-
with tert-butylchlorodiphenylsilane and imidazole. ed in fair enantioselectivities (entries 2 and 4). For all
The reaction of 15a–l was carried out by using the the trisubstituted substrates, excellent enantioselectiv-
ketone 14 (15 mol%) and oxone (1.63 mol equiv.) in ities of up to 97% ee were obtained (entries 5–12). To
the presence of tetra-n-butylammonium hydrogen sul- demonstrate this methodology, we chose (+)-marme-
fate and potassium carbonate in 1:2 acetonitrile/dime- sin (17l), a naturally occurring dihydrobenzofuran. In
thoxymethane (v/v) and buffer solution (pH 6) at this case, the effect of the silyl protecting groups was
08C. The exposure of the resulting epoxides 16a–l to briefly examined and the bulkiness was found to
a slightly excess amount of TBAF in THF provided affect the enantioselectivity. The sequential reaction
dihydrobenzofurans 17a–l. The enantiomeric excess of 15l with a bulky protecting group furnished (+)-
was determined by the HPLC analysis of 17a–l. First, marmesin (17l) with a high enantiomeric purity (96%
the substituent effects of the allyl moiety were exam- ee) and in 83% yield (entry 12). The specific optical
ined. The reaction of ortho-prenylphenol 15a, a tri- rotation and the spectroscopic data of our synthetic
substituted substrate, provided the corresponding di- sample matched those of the reported values.[15] On
hydrobenzofuran 17a with a very high enantioselectiv- the other hand, the tert-butyl dimethylsilyl derivative
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Scheme 5. Synthesis of (À)-(3’R)-decursinol (19).
Table 4. Reaction conditions of (À)-(3’R)-decursinol (19).
crown-6-TFA at À408C for 19 h followed by TsOH at
À508C for 22 h furnished (+)-lomatin (22) with 96%
ee in 70% yield (two steps) as shown in Scheme 6, of
which the physical values were identical to those of
the natural (+)-lomatin.[18,19]
Entry A
Ratio of
X
T
B
Ratio of
18:17l
[8C]
19:17l
1
2
3
17 h
2 h
20 min >99/1
94/6
>98/2
0.1 0
0.1 À50 5 d >98/2
À50 19 h >99/1
6 h 92/8
1
Conclusions
The Shi-type epoxidation of the O-TBDPS-protected
of 15l provided a decreased enantioselectivity of 68% ortho-allylphenols serves as an efficient method to
ee. construct dihydrobenzofurans and dihydrobenzopyr-
Next, we investigated the formation of a dihydro- ans with up to 97% ee. This methodology led to the
benzopyran, (À)-(3’R)-decursinol (19), using 16l as enantioselective synthesis of (+)-marmesin, (À)-
shown in Scheme 5 and Table 4.[16,17] The direct trans- (3’R)-decursinol, and (+)-lomatin.
formation of 16l under several conditions, such as hy-
drogen chloride-dioxane, trimethylsilyl triflate-di-
chloromethane, and potassium fluoride-18-crown-6-
TsOH, failed to afford 19 and resulted in the ring-
Experimental Section
cleavage product 20. However, TBAF-trifluoroacetic
General Procedure for the Synthesis of o-Allyl-
acid (TFA) was promising and provided 18 as a major
phenols
product together with 19 in a 9:1 ratio. These results
To a stirred solution of a phenol (0.31 mmol) and isobutyl 2-
methylbut-3-en-2-yl carbonate (101.4 mg, 0.46 mmol, 85%
purity, 1.5 equiv.) in THF (2 mL) at room temperature was
were still unsatisfactory. Therefore, we decided to
employ a two-step procedure for this transformation.
Although the chemoselective removal of the TBDPS
group was difficult for the in situ cycilization, a com-
bination of potassium fluoride (KF, 2 equiv.)-18-
crown-6 (2.2 equiv.)-TFA (2 equiv.) in tetrahydrofuran
was very effective, affording 19 with 96% ee in 67%
yield (two steps). For the TBS-protected 11b, low
temperature was necessary to avoid the benzofuran
added
tetrakis(triphenylphosphine)palladium
(3.6 mg,
0.003 mmol, 1 mol%) and the mixture was stirred at 238C
for 1.5–7.5 h. The reaction mixture was diluted with ethyl
acetate and the whole was washed with brine, dried over
sodium sulfate, filtered, and concentrated under vacuum.
The residue was purified by silica gel column chromatogra-
phy to give an O-allyl ether, which was used for next reac-
formation. Finally, the treatment of 11b with KF-18- tion. A solution of the O-allyl ether in toluene was heated
Scheme 6. Synthesis of (+)-lomatin (22).
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Asymmetric Synthesis of 2-Substituted Dihydrobenzofurans and 3-Hydroxydihydrobenzopyrans
to reflux for 13 h–2 d. After cooling the reaction mixture to
room temperature, the mixture was concentrated under
vacuum. The residue was purified by silica gel column chro-
matography to give the ortho-allylphenol.
Na]+: 229.1204; HPLC (CHIRALCEL OD-H, n-hexane/i-
PrOH=92/8, flow rate=0.5 mLminÀ1): retention time=
12.9 min (major), 15.2 min (minor).
(R)-1-[(S)-5-tert-Butyl-2,3-dihydrobenzofuran-2-yl]etha-
nol (17c): Yield: 85%; diastereomeric ratio=96:4, 92% ee
(major isomer); colorless oil; [a]2D3: +17.5 (c 0.56, CHCl3)
for 92% ee; 1H NMR (400 MHz, CDCl3): d=1.23 (d, J=
6.4 Hz, 3H), 1.29 (s, 9H), 1.97 (brs, 1H), 3.08 (dd, J=15.6,
9.6 Hz, 1H), 3.24 (dd, J=15.6, 8.8 Hz, 1H), 4.17 (m, J=6.4,
3.2 Hz, 1H), 4.69 (td, J=8.8, 3.6 Hz, 1H), 6.70 (d, J=
8.4 Hz, 1H), 7.13 (dm, J=8.4 Hz, 1H), 7.21 (s, 1H);
13C NMR (100 MHz, CDCl3): d=17.7, 29.4, 31.7, 34.2, 68.1,
86.6, 108.3, 122.1, 124.6, 126.5, 143.7, 157.2; IR (KBr): n=
3420, 2960, 1493, 1362, 1266, 1235, 1174, 1119, 992, 898, 814,
730 cmÀ1; HRMS (FAB): m/z=220.1466, calcd for C14H20O2
[M]+: 220.1463; HPLC (CHIRALCEL OD-H, n-hexane/i-
PrOH=98/2, flow rate=0.5 mLminÀ1): retention time=
20.1 min (minor), 21.6 min (major).
(S)-2-(5-tert-Butyl-2-methyl-2,3-dihydrobenzofuran-2-yl)-
propan-2-ol (17d): Yield: 96%; 40% ee; colorless oil; [a]2D4:
+2.5 (c 0.82, CHCl3) for 40% ee; 1H NMR (400 MHz,
CDCl3): d=1.23 (s, 3H), 1.29 (s, 9H), 1.36 (s, 3H), 1.39 (s,
3H), 2.04 (brs, 1H), 2.75 (d, J=15.6 Hz, 1H), 3.50 (d, J=
15.6 Hz, 1H), 6.68 (d, J=8.4 Hz, 1H), 7.12 (d, J=8.4 Hz,
1H), 7.18 (s, 1H); 13C NMR (100 MHz, CDCl3): d=22.6,
24.2, 24.8, 31.7, 34.2, 37.7, 74.2, 93.7, 108.6, 122.2, 124.6,
126.7, 143.3, 156.4; IR (KBr): n=3437, 2962, 1492, 1363,
1252, 1176, 1113, 1061, 957, 897, 849, 814 cmÀ1; HR-MS
(FAB): m/z=248.1788, calcd. for C16H24O2 [M]+: 248.1776;
HPLC (CHIRALPAC AD, n-hexane/i-PrOH=90/10, flow
rate=0.3 mLminÀ1): retention time=13.3 min (major),
14.9 min (minor).
General Procedure for the Asymmetric Synthesis of
2-Substituted Dihydrobenzofurans through the
Enantioselective Epoxidation of O-Silyl-protected
ortho-Allylphenols
An O-silyl-protected ortho-allylphenol (0.1 mmol) and
ketone 14 (9.1 mg, 0.03 mmol) were dissolved in acetoni-
trile/dimethoxymethane (1.98 mL, 1:2 v/v). A pH 6 buffer
solution (0.36 mL) and tetra-n-butylammonium hydrogen
sulfate (1.4 mg, 0.004 mmol) were slowly added with stirring,
and the mixture was cooled to 08C. The flask was equipped
with two syringe pumps; one of them was filled with a solu-
tion of Oxone (100.2 mg, 0.163 mmol) in pH 6 buffer solu-
tion (0.63 mL), and the other one with a solution of K2CO3
(33.2 mg, 0.24 mmol) in water (0.63 mL). The two solutions
were added dropwise over a 2 h period. The solution was
stirred at 08C for 24 h. The crude material was quenched by
addition of water (3 mL) and hexane (3 mL). The reaction
mixture was extracted with hexane (5 mLꢂ2). The com-
bined organic extracts were washed with brine, dried over
sodium sulfate, filtered, and concentrated under vacuum to
give a crude epoxide. The residue was used in next step
without any purification.
To a solution of the crude epoxide in THF (1 mL) at 08C
was added TBAF (0.12 mL, 0.12 mmol, 1.0M in THF) and
then the mixture was allowed to gradually warmed to room
temperature. After being stirred for 4 h, the reaction was
quenched with water. The residue was extracted with ethyl
acetate. The organic layer was washed with brine, dried over
sodium sulfate, filtered, and concentrated under vacuum.
The residue was purified by silica gel column chromatogra-
phy to give a dihydrobenzofuran.
(S)-2-(5-tert-Butyl-2-methyl-2,3-dihydrobenzofuran-2-yl)-
propan-2-ol (17a): Yield: 95%; 96% ee; colorless oil; [a]2D2:
+42.4 (c 0.97, CHCl3) for 96% ee; 1H NMR (400 MHz,
CDCl3): d=1.21 (s, 3H), 1.29 (s, 9H), 1.34 (s, 3H), 3.14 (m,
2H), 4.58 (t, J=9.2 Hz, 1H), 6.70 (d, J=8.4 Hz, 1H), 7.12
(d, J=8.4 Hz, 1H), 7.19 (d, J=0.8 Hz, 1H); 13C NMR
(100 MHz, CDCl3): d=23.9, 26.3, 30.8, 31.7, 34.2, 71.7, 89.3,
108.2, 121.9, 124.6, 126.7, 143.5, 157.2; IR (KBr): n=3429,
2961, 1734, 1616, 1541, 1494, 1362, 1295, 1267, 1230, 1175,
1152, 1119, 963, 882, 862, 814, 731, 688 cmÀ1; HR-MS
(FAB): m/z=234.1616, calcd. for C15H22O2 [M]+: 234.1620;
HPLC (CHIRALPAC AD, n-hexane/i-PrOH=85/15, flow
rate=0.3 mLminÀ1): retention time=14.7 min (minor),
16.2 min (major).
(S)-tert-Butyl 2-(2-hydroxypropan-2-yl)-2,3-dihydrobenzo-
furan-6-ylcarbamate (17e): Yield: 84%; 91% ee; white solid;
mp 127–128.58C (n-hexane/ethyl acetate); [a]2D3: +12.0 (c
1
1.075, CHCl3) for 91% ee; H NMR (400 MHz, CDCl3): d=
1.18 (s, 3H), 1.30 (s, 3H), 1.50 (s, 9H), 2.20 (brs, 1H), 3.07
(d, J=9.2 Hz, 2H), 4.59 (t, J=9.2 Hz, 1H), 6.71 (brs, 1H),
6.75 (dd, J=8.0, 1.2 Hz, 1H), 6.94 (brs, 1H), 7.00 (d, J=
8.0 Hz, 1H); 13C NMR (100 MHz, CDCl3): d=23.8, 26.0,
28.3, 30.1, 71.8, 80.4, 89.8, 100.4, 110.8, 121.6, 124.7, 138.3,
152.7, 160.2; IR (KBr): n=3284, 2976, 1715, 1605, 1541,
1500, 1419, 1364, 1233, 1154, 1109, 1052, 962, 889, 853,
772 cmÀ1
;
HR-MS (FAB): m/z=293.1619, calcd. for
C16H23NO4 [M]+: 293.1627; HPLC (CHIRALPAC AD, n-
hexane/i-PrOH=85:15, flow rate=0.3 mLminÀ1): retention
time=36.4 min (minor), 54.2 min (major).
(S)-tert-Butyl 2-(2-hydroxypropan-2-yl)-2,3-dihydrobenzo-
furan-4-ylcarbamate (17f): Yield: 89%; 96% ee; yellow oil;
[a]2D2: +43.7 (c 1.35, CHCl3) for 96% ee; 1H NMR
(400 MHz, CDCl3): d=1.22 (s, 3H), 1.34 (s, 3H), 1.52 (s,
9H), 3.06 (m, 2H), 4.63 (t, J=8.8 Hz, 1H), 6.24 (brs, 1H),
6.52 (d, J=8.0 Hz, 1H), 7.08 (t, 8.0 Hz, 1H), 7.29 (d, J=
8.0 Hz, 1H); 13C NMR (100 MHz, CDCl3): d=24.1, 26.0,
28.3, 28.6, 71.7, 80.7, 89.3, 104.5, 111.9, 116.2, 128.8, 135.0,
152.5, 160.0; IR (KBr): n=3447, 3281, 2979, 2925, 1689,
1605, 1533, 1446, 1366, 1291, 1229, 1157, 1094, 1061, 979,
892, 872, 772, 753 cmÀ1; HR-MS (FAB): m/z=293.1604,
calcd. for C16H23NO4 [M]+: 293.1627; HPLC (CHIRALPAC
AD, n-hexane/i-PrOH=85:15, flow rate=0.3 mLminÀ1): re-
tention time=22.0 min (minor), 23.9 min (major).
(S)-(5-tert-bBtyl-2-methyl-2,3-dihydrobenzofuran-2-yl)me-
thanol (17b): Yield: 79%; 39% ee; colorless oil; [a]2D4: +17.8
(c 0.29, CHCl3) for 39% ee; 1H NMR (400 MHz, CDCl3):
d=1.29 (s, 9H), 2.11 (brs, 1H), 3.00 (dd, J=15.6, 7.6 Hz,
1H), 3.23 (dd, J=15.6, 9.2 Hz, 1H), 3.73 (dd, J=11.6,
6.4 Hz, 1H), 3.84 (d, J=11.6 Hz, 1H), 4.89 (m, 1H), 6.71 (d,
J=8.4 Hz, 1H), 7.14 (dm, J=8.4 Hz, 1H), 7.21 (m, 1H);
13C NMR (100 MHz, CDCl3): d=31.4, 31.7, 34.2, 65.0, 83.1,
108.6, 122.1, 124.8, 126.1, 143.7, 156.9; IR (KBr): n=3394,
2953, 1492, 1362, 1266, 1233, 1174, 1119, 1047, 814, 729 cmÀ1
HR-MS (FAB): m/z=229.1210, calcd. for C13H18O2Na [M+
;
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(S)-tert-Butyl 2-(2-hydroxypropan-2-yl)-2,3-dihydrobenzo-
furan-7-ylcarbamate (17g): Yield: 96%; 89% ee; colorless
oil; [a]2D2: +15.0 (c 1.355, CHCl3) for 89% ee; 1H NMR
(400 MHz, CDCl3): d=1.22 (s, 3H), 1.36 (s, 3H), 1.52 (s,
9H), 3.18 (m, 2H), 4.61 (t, J=8.8 Hz, 1H), 6.57 (brs, 1H),
6.82 (m, 1H), 7.71 (brs, 1H); 13C NMR (100 MHz, CDCl3):
d=23.9, 26.2, 28.3, 31.3, 71.7, 80.6, 90.0, 117.8, 118.9, 121.1,
122.4, 126.7, 148.1, 152.7; IR (KBr, cmÀ1): n=3291, 2979,
1718, 1627, 1540, 1440, 1389, 1363, 1305, 1239, 1150, 1085,
984, 944, 864, 754, 729; HR-MS (FAB): m/z=293.1634,
calcd. for C16H23NO4 [M]+: 293.1627; HPLC (CHIRALPAC
AD, n-hexane/i-PrOH=85:15, flow rate=0.3 mLminÀ1): re-
tention time=17.3 min (minor), 18.7 min (major).
(S)-tert-Butyl 2-(2-hydroxypropan-2-yl)-2,3-dihydrobenzo-
furan-5-ylcarbamate (17h): Yield: 94%; 90% ee; colorless
solid; mp 118.5–119.58C (n-hexane/ethyl acetate); [a]2D3:
+36.6 (c 1.37, CHCl3) for 90% ee; 1H NMR (400 MHz,
CDCl3): d=1.20 (s, 3H), 1.32 (s, 3H), 1.50 (s, 9H), 3.12 (m,
2H), 4.58 (t, J=8.8 Hz, 1H), 6.41 (brs, 1H), 6.67 (d, J=
8.4 Hz, 1H), 6.91 (dd, J=8.8, 2.4 Hz, 1H), 7.31 (brs, 1H);
13C NMR (100 MHz, CDCl3): d=23.9, 26.0, 28.3, 30.9, 71.7,
80.2, 89.5, 108.8, 117.0, 119.3, 127.7, 131.3, 153.3, 155.7; IR
(KBr): n=3368, 2974, 2926, 1714, 1617, 1539, 1497, 1438,
1367, 1289, 1227, 1163, 1119, 1048, 1027, 967, 942, 875, 819,
800, 756 cmÀ1; HR-MS (FAB): m/z=293.1619, calcd. for
C16H23NO4 [M]+: 293.1627; HPLC (CHIRALPAC AD, n-
hexane/i-PrOH=85:15, flow rate=0.3 mLminÀ1): retention
time=27.8 min (minor), 36.1 min (major).
1H), 7.11 (d, J=8.8 Hz, 1H), 7.30 (t, J=8.0 Hz, 1H), 7.47
(t, J=8.0 Hz, 1H), 7.59 (d, J=8.0 Hz, 1H), 7.67 (d, J=
8.8 Hz, 1H), 7.80 (d, J=8.4 Hz, 1H); 13C NMR (100 MHz,
CDCl3): d=23.9, 25.9, 29.6, 71.9, 90.0, 111.7, 118.7, 122.6,
122.9, 126.6, 128.6, 128.9, 129.1, 130.6, 156.9; IR (KBr): n=
3416, 2974, 2932, 1631, 1599, 1577, 1520, 1465, 1374, 1321,
1240, 1206, 1150, 1065, 965, 858, 806, 768, 743 cmÀ1; HR-MS
(FAB): m/z=228.1148, calcd. for C15H16O2 [M]+: 228.1150;
HPLC (CHIRALPAC AD, n-hexane/i-PrOH=85:15, flow
rate=0.3 mLminÀ1): retention time=21.0 min (minor),
23.5 min (major).
Synthetic (+)-marmesin (17l): Yield: 83%; 96% ee; white
solid; mp 180.5–181.58C (n-hexane/ethyl acetate); [a]2D1:
+22.9 (c 0.885, CHCl3) for 96% ee {lit.15a [a]D23: +21.7 (c 0.9,
CHCl3), lit.15b [a]D21: +20.3 (c 1.2, CHCl3)}; 1H NMR
(400 MHz, CDCl3): d=1.24 (s, 3H), 1.38 (s, 3H), 3.24 (m,
2H), 4.74 (t, J=8.8 Hz, 1H), 6.20 (d, J=9.6 Hz, 1H), 6.72
(s, 1H), 7.22 (s, 1H), 7.59 (d, J=9.2 Hz, 1H); 13C NMR
(100 MHz, CDCl3): d=24.3, 26.0, 29.3, 71.5, 91.1, 97.7,
111.9, 112.5, 123.3, 125.1, 143.7, 155.4, 161.5, 163.1; IR
(KBr): n=3476, 2977, 2930, 1698, 1629, 1569, 1487, 1446,
1402, 1365, 1309, 1267, 1227, 1183, 1130, 1000, 960, 946, 860,
834, 818, 754, 726 cmÀ1; HR-MS (FAB): m/z=247.0959
calcd. for C14H15O4 [M+H]+: 247.0970; HPLC (CHIRAL-
PAC
AD,
n-hexane/i-PrOH=75:25,
flow
rate=
0.5 mLminÀ1): retention time=15.5 min (minor), 28.3 min
(major).
(S)-2-(7-Chloro-2,3-dihydrofuroACTHNUGTRNEUNG[2,3-c]pyridin-2-yl)propan-
Synthetic (À)-(3’R)-Decursinol (19)
2-ol (17i): Yield: 86%; 97% ee; colorless solids; mp 140–
1418C (n-hexane/ethyl acetate); [a]2D2: +27.2 (c 0.715,
18-Crown-6 (120.0 mg, 0.454 mmol), trifluoroacetic acid
(47.1 mg, 0.413 mmol), and potassium fluoride (24.0 mg,
0.413 mmol) were dissolved in THF (4 mL). Epoxide 16l
(100 mg, 0.206 mmol, 96% ee) was added in one portion at
room temperature. After being stirred at room temperature
for 20 min, the reaction was quenched by addition of water
(5 mL). The reaction mixture was extracted with ethyl ace-
tate, washed with brine (5 mL), dried over sodium sulfate,
filtered, and concentrated under reduced pressure to give
crude epoxide 18 that was used in next step without further
purification; yield: 103.0 mg. 1H NMR (400 MHz, CDCl3):
d=1.39 (s, 3H), 1.50 (s, 3H), 2.80 (m, 1H), 3.07 (m, 2H),
6.22 (d, J=9.2 Hz, 1H), 6.91 (s, 1H), 7.24 (s, 1H), 7.61 (d,
J=9.6 Hz, 1H), 8.25 (brs, 1H).
The epoxide 18 (0.206 mmol) was dissolved in toluene
(4 mL). The solution was cooled to À508C, and p-toluene-
sulfonic acid monohydrate (39.2 mg, 0.206 mmol) was
added. After being stirred at À508C for 19 h, the reaction
was quenched by addition of water (5 mL). The whole was
extracted with ethyl acetate, washed with brine (5 mL),
dried over sodium sulfate, filtered, and concentrated under
reduced pressure. The residue was purified by silica gel
column chromatography (n-hexane/ethyl acetate=1/1) to
give (À)-decursinol (19) as a white solids; yield: 33.9 mg
(67%); mp 180–180.58C (n-hexane/ethyl acetate) (lit.17 mp
180.5–181.58C); [a]1D8: À10.8 (c 1.015, CHCl3) for 96% ee
{lit.17 [a]2D3: À11 (c 0.7, CHCl3)}; 1H NMR (400 MHz,
CDCl3): d=1.37 (s,3H), 1.40 (s, 3H), 2.34 (brs, 1H), 2.84
(dd, J=16.8, 6.0 Hz, 1H), 3.11 (dd, J=16.8, 4.8 Hz, 1H),
3.88 (dd, J=6.0, 4.8 Hz, 1H), 6.19 (d, J=9.6 Hz, 1H), 6.75
(s, 1H), 7.17 (s, 1H), 7.57 (d, J=9.2 Hz, 1H); 13C NMR
(100 MHz, CDCl3): d=22.0, 25.0, 30.6, 68.9, 78.2, 104.6,
1
CHCl3) for 97% ee; H NMR (400 MHz, CDCl3): d=1.24 (s,
3H), 1.40 (s, 3H), 3.24 (dd, J=16.8, 9.6 Hz, 1H), 3.35 (dd,
J=16.8, 8.4 Hz, 1H), 4.74 (t, J=9.2 Hz, 1H), 7.08 (dd, J=
4.8, 0.4 Hz, 1H), 7.92 (d, J=4.8 Hz, 1H); 13C NMR
(100 MHz, CDCl3): d=24.3, 26.1, 31.0, 71.4, 89.9, 119.5,
132.0, 138.5, 141.5, 152.9; IR (KBr): n=3337, 2977, 1572,
1458, 1420, 1380, 1321, 1240, 1209, 1183, 1148, 1067, 1039,
949, 921, 869, 834, 777 cmÀ1; HR-MS (FAB): m/z=214.0642,
calcd. for C10H13O2NCl [M+H]+: 214.0635; HPLC (CHIR-
ALPAC AD, n-hexane/i-PrOH=85:15, flow rate=
0.3 mLminÀ1): retention time=24.9 min (minor), 27.4 min
(major).
(S)-2-(2,3-DihydronaphthoACHTNUTRGNE[UNG 1,2-b]furan-2-yl)propan-2-ol
(17j): Yield: 83%; 97% ee; colorless oil; [a]2D1: +5.3 (c 0.65,
1
CHCl3) for 97% ee; H NMR (400 MHz, CDCl3): d=1.26 (s,
3H), 1.42 (s, 3H), 3.33 (m, 2H), 4.82 (t, J=9.6 Hz, 1H),
7.27 (d, J=19.2 Hz, 1H), 7.34 (d, J=18.4 Hz, 1H), 7.42 (m,
2H), 7.80 (dd, J=7.2, 3.2 Hz, 1H), 7.96 (dd, J=9.2 , 2.0 Hz,
1H); 13C NMR (100 MHz, CDCl3): d=23.9, 26.0, 31.4, 71.9,
89.9, 120.09, 120.18, 120.21, 121.2, 122.8, 125.2, 125.4, 127.8,
133.8, 154.6; IR (KBr): 3420, 2973, 2926, 1734, 1594, 1575,
1519, 1466, 1441, 1398, 1375, 1280, 1260, 1233, 1170, 1067,
1042, 1004, 941, 876, 800, 772, 742 cmÀ1; HR-MS (FAB):
m/z=228.1157, calcd. for C15H16O2 [M]+: 228.1150; HPLC
(CHIRALPAC AD, n-hexane/i-PrOH=85:15, flow rate=
0.3 mLminÀ1): retention time=17.0 min (minor), 20.7 min
(major).
(S)-2-(1,2-DihydronaphthoACHTNUTRGNE[UNG 2,1-b]furan-2-yl)propan-2-ol
(17k): Yield: 84%; 97% ee; colorless oil; [a]2D2: +79.3 (c
1
0.835, CHCl3) for 97% ee; H NMR (400 MHz, CDCl3): d=
1.27 (s, 3H), 1.39 (s, 3H), 3.42 (m, 2H), 4.80 (t, J=9.2 Hz,
160
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Adv. Synth. Catal. 2011, 353, 155 – 162

Asymmetric Synthesis of 2-Substituted Dihydrobenzofurans and 3-Hydroxydihydrobenzopyrans
112.8, 113.0, 116.6, 129.0, 143.2, 154.0, 156.5, 161.5; IR References
(KBr): n=3447, 1698, 1625, 1561, 1390, 1134, 1070, 820,
750 cmÀ1
;
HR-MS (FAB): m/z=247.0966, calcd. for
[1] For recent examples, see: a) H. Min, M. Aye, T. Tanigu-
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[2] For the enantioselective synthesis using the Sharpless
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C14H15O4 [M+H]+: 247.0970; HPLC (CHIRALPAC AD, n-
hexane/i-PrOH=90:10, flow rate=1.0 mLminÀ1): retention
time=29.6 min (minor), 38.2 min (major).
Synthetic (+)-Lomatin (22)
Epoxide 11b (158.0 mg, 0.434 mmol, 97% ee) and 18-crown-
6 (252.5 mg, 0.955 mmol) were dissolved in THF (4 mL). Tri-
fluoroacetic acid (99.0 mg, 0.869 mmol) was slowly added
with stirring, and the mixture was cooled to À408C. Potassi-
um fluoride (50.5 mg, 0.869 mmol) was added in one pot.
The solution was stirred at À408C for 19 h. The reaction
was quenched by addition of water (5 mL). The reaction
mixture was extracted with ethyl acetate, washed with brine
(5 mL), dried over sodium sulfate, filtered, and concentrated
under vacuum to give crude 21 that was used for next step
without any purification; yield: 164.7 mg. 1H NMR
(400 MHz, CDCl3): d=1.40 (s, 3H), 1.56 (s, 3H), 2.50 (dd,
J=15.2, 10.4 Hz, 1H), 3.09 (dd, J=10.4, 2.0 Hz, 1H), 3.90
(dd, J=15.2, 1.6 Hz, 1H), 6.25 (d, J=9.6 Hz, 1H), 6.92 (d,
J=8.8 Hz, 1H), 7.30 (d, J=8.8 Hz, 1H), 7.66 (d, J=9.6 Hz,
1H), 8.10 (brs, 1H).
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The epoxide 21 (164.7 mg, 0.434 mmol) was dissolved in
toluene (4 mL). The solution was cooled to À508C, and p-
toluenesulfonic acid monohydrate (82.6 mg, 0.434 mmol)
was added in one portion. After being stirred at À508C for
22 h, the reaction was quenched by addition of water
(5 mL). The reaction mixture was extracted with ethyl ace-
tate, washed with brine (5 mL), dried over sodium sulfate,
filtered, and concentrated under reduced pressure. The resi-
due was purified by silica gel column chromatography (n-
hexane/ethyl acetate=1/1) to give (+)-lomatin 22 as a color-
less solid; yield: 75.2 mg (0.305 mmol, 70.3%); 96% ee; mp
182–1838C (n-hexane/ethyl acetate) (lit.17 mp 182.5–
183.58C); [a]1D7: +49.1 (c 0.375, EtOH) for 99.2% ee {lit.17
1
[a]2D3: +52 (c 0.4, EtOH)}; H NMR (400 MHz, CDCl3): d=
1.36 (s, 3H), 1.42 (s, 3H), 2.22 (d, J=6.0 Hz, 1H), 2.98 (dd,
J=17.6, 5.2 Hz, 1H), 3.15 (dd, J=17.6, 4.8 Hz, 1H), 3.93
(dd, J=10.8, 5.2 Hz, 1H), 6.23 (d, J=9.2 Hz, 1H), 6.79(d,
J=8.8 Hz, 1H), 7.25 (d, J=8.4 Hz, 1H), 7.63 (d, J=9.6 Hz,
1H); 13C NMR (100 MHz, CDCl3): d=22.1, 24.6, 25.8, 68.3,
78.1, 107.5, 112.1, 112.3, 114.4, 126.6, 144.0, 153.5, 156.4,
161.5; IR (KBr): n=3492, 1695, 1600, 1493, 1227, 1125,
1070, 827, 760 cmÀ1; HR-MS (FAB): m/z=247.0973, calcd.
for C14H15O4 [M+H]+: 247.0970; HPLC (CHIRALPAC AD,
n-hexane/i-PrOH=75:25, flow rate=0.5 mLminÀ1): reten-
tion time=13.6 min (minor), 16.2 min (major).
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Acknowledgements
This work was financially supported in part by Special Funds
for Education and Research (Development of SPECT
Probes for Pharmaceutical Innovation) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 155 – 162