Synthesis of 2,5-dihydrobenzo[b]oxepines and 5,6-dihydro-2H-benzo[b] oxocines based on a '[3+3] cyclization-olefin-metathesis' strategy

Article abstract of DOI:10.1016/j.tetlet.2005.10.132

Functionalized 2,5-dihydrobenzo[b]oxepines and 5,6-dihydro-2H-benzo[b] oxocines were prepared based on a '[3+3] cyclization-olefin-metathesis' strategy.

Full text of DOI:10.1016/j.tetlet.2005.10.132

Tetrahedron
Letters
Tetrahedron Letters 47 (2006) 113–116
Synthesis of 2,5-dihydrobenzo[b]oxepines
and 5,6-dihydro-2H-benzo[b]oxocines based on a
Ô[3+3] cyclization-olefin-metathesisÕ strategy
Van Thi Hong Nguyen,a Esen Bellura and Peter Langerb,c,
*
aInstitut fu¨r Chemie und Biochemie der Ernst-Moritz-Arndt-Universita¨t Greifswald, Soldmannstr. 16, D-17487 Greifswald, Germany
bInstitut fu¨r Chemie, Universita¨t Rostock, Albert-Einstein-Str. 3a, 18059 Rostock, Germany
cLeibniz-Institut fu¨r Organische Katalyse an der Universita¨t Rostock e. V. (IfOK), Albert-Einstein-Str. 29a, 18059 Rostock, Germany
Received 22 July 2005; revised 25 October 2005; accepted 26 October 2005
Available online 11 November 2005
Abstract—Functionalized 2,5-dihydrobenzo[b]oxepines and 5,6-dihydro-2H-benzo[b]oxocines were prepared based on a Ô[3+3] cycli-
zation-olefin-metathesisÕ strategy.
ꢀ 2005 Elsevier Ltd. All rights reserved.
2,3,4,5-Tetrahydrobenzo[b]oxepines are of pharmaco-
logical relevance and occur in a number of natural prod-
ucts, such as heliannuol C and D1 or plumbagic acid
lactone.2 Their eight-membered ring analogs, 3,4,5,6-
tetrahydro-2H-benzo[b]oxocines, are found, for exam-
ple, in heliannuol A and K,1,3 helianane,4 and protosap-
panine B.5 2,5-Dihydrobenzo[b]oxepines are present, for
example, in heliannuol B1 and in the radulanins A, H,
and L isolated from Haliclona fascigera and Radula vari-
abilis, respectively.6 The 5,6-dihydro-2H-benzo[b]oxo-
cine core structure occurs, for example, in heliannuol
G and H,4 specionine,7 and sophoroside A.7 Some years
ago, Snieckus and co-worker reported the synthesis
of benzene-fused oxygen heterocycles by application of
a Ôdirected-ortho-metalation (DoM)-olefin-metathesisÕ
strategy.8 In recent years, a number of natural product
syntheses based on RCM have been reported.9 Herein,
we report a new and convenient synthesis of 2,5-
dihydrobenzo[b]oxepines and 5,6-dihydro-2H-benzo[b]-
oxocines based on a Ô[3+3] cyclization-olefin-metathesisÕ
strategy. Our approach relies on the regioselective
assembly of the benzene moiety by [3+3] cyclizations
of 1,3-bis(trimethylsilyloxy)hepta-1,3,6-trienes and 1,3-
bis(trimethylsilyloxy)octa-1,3,7-trienes with 3-silyloxy-
alk-2-en-1-ones to give functionalized salicylates.10,11
The latter were transformed into the desired products
by O-allylation and subsequent ring-closing metathesis
(RCM).
OH
HO
O
Ph
OH
O
Heliannuol B
Radulanin A
OH
HO
O
O
Helianane
Heliannuol H
The 1,3-bis-silyl enol ethers 5a,b were prepared accord-
ing to a known procedure (Scheme 1).12,13 The TiCl4
mediated [3+3] cyclization of 5a with 1,1,3,3-tetrameth-
oxypropane afforded the allyl-substituted salicylate 6a
(Table 1). Allylation of the hydroxy group afforded
the allylic ether 7a, which was transformed into the
desired 2,5-dihydrobenzo[b]oxepine 8a by RCM using
GrubbsÕ catalyst 9.14–16 Likewise, the 5,6-dihydro-2H-
benzo[b]oxocine 8b was prepared from 1,3-bis-silyl enol
ether 5b. Application of the Mitsunobu reaction for
O-allylation was not successful.
Keywords: Cyclizations; Heterocycles; Medium-sized rings; Ring-clos-
ing metathesis; Silyl enol ethers.
*
Corresponding author. Tel.: +49 381 498 6410; fax: +49 381 498
6412; e-mail: peter.langer@uni-rostock.de
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.10.132

114
V. T. H. Nguyen et al. / Tetrahedron Letters 47 (2006) 113–116
O
O
Me3SiO
()n
OSiMe3
OEt
R1
O
O
OEt
Br
i
R2
R3
1
()n
i
()n
5a,b
OEt
()n
+
Me3SiO
R1
O
3a,b
OH
+
R3
2a,b (n = 1, 2)
EtO
O
R2
ii
6c-i
10a-d
Me3SiO
()n
O
Me3SiO
()n
OSiMe3
OEt
PCy3
Ru CHPh
PCy3
Cl
Cl
iii
ii
OEt
5a,b
iv
4a,b
1
R
1
9
R
R2
R3
()n
OMe OMe
OMe
R2
R3
()n
O
iii
MeO
O
EtO
O
EtO
O
()n
()n
O
v
8c-i
7c-i
OH
Scheme 2. Synthesis of 8c–i. Reagents and conditions: (i) TiCl4
(1.0 equiv), CH2Cl2, ꢀ78 ! 20 ꢁC; (ii) H2C@CH–CH2Br (1.5 equiv),
NaH (2.0 equiv), TBAI (2.0 equiv), THF, 0 ! 20 ꢁC, 8–12 h; (iii) 9
(5 mol %), CH2Cl2 (1.5 equiv), 20 ꢁC, 6–8 h.
EtO
O
EtO
O
6a,b
7a,b
vi
()n
PCy3
Ru CHPh
PCy3
Cl
Cl
Table 2. Products and yields
O
O
6–8
n
R1
R2 R3 % (6)a % (7)a % (8)a
EtO
9
c
d
e
f
1
1
1
1
2
2
2
Me
Me
Et
H
Me 44
85
95
78
77
76
82
96
90
93
91
80
76
91
70
8a,b (n = 1, 2)
Me Me 52
H
Et 31
Me 43
Scheme 1. Synthesis of 8a,b. Reagents and conditions: (i) (1) LDA
(2.3 equiv), THF, 0 ꢁC, 1 h, (2) 2a,b, ꢀ78 ! 20 ꢁC; (ii) Me3SiCl, NEt3,
toluene, 20 ꢁC, 24 h; (iii) (1) LDA, THF, ꢀ78 ꢁC, 1 h, (2) Me3SiCl,
20 ꢁC, ꢀ78 ! 20 ꢁC; (iv) TiCl4, CH2Cl2, ꢀ78 ! 20 ꢁC; (v) H2C@CH–
CH2Br (1.5 equiv), NaH (2.0 equiv), TBAI (2.0 equiv), THF, 0 ꢁC, 24 h,
0 ! 20 ꢁC, 8–12 h; (vi) 9 (5 mol %), CH2Cl2 (1.5 equiv), 20 ꢁC, 6–8 h.
C6H4(CH2)2
C6H4(CH2)2
g
h
i
Me
Et
Me Me 47
H
Et 50
Me 33
a Yields of isolated products.
Table 1. Products and yields
6–8
n
% (6)a
% (7)a
% (8)a
a
b
1
2
45
53
86
84
99
75
O
a Yields of isolated products.
O
EtO
8f
O
EtO
O
8i
The TiCl4 mediated [3+3] cyclization of 5a with silyl
enol ethers 10a–c, prepared from pentane-2,4-dione, 3-
methylpentane-2,4-dione, and heptane-3,5-dione, affor-
ded the allyl-substituted salicylates 6c–e, which were
transformed into the 2,5-dihydrobenzo[b]oxepines 8c–e
(Scheme 2 and Table 2). The tetracyclic 2,5-dihydro-
benzo[b]oxepine 8f was prepared from 10d, which is
available from 2-acetyltetralone. The 5,6-dihydro-2H-
benzo[b]oxocines 8g–i were prepared from 1,3-bis-silyl
enol ether 5b.
Figure 1.
structures of tetracyclic 2,5-dihydrobenzo[b]oxepine 8f
and 5,6-dihydro-2H-benzo[b]oxocine 8i are given below
for clarity (Fig. 1).
In summary, we have reported a new regioselective
synthesis of functionalized 2,5-dihydrobenzo[b]oxepines
and 5,6-dihydro-2H-benzo[b]oxocines based on a Ô[3+3]
cyclization-ring-closing-metathesisÕ strategy.
The [3+3] cyclizations (to give salicylates 6a–i)
proceeded in 31–52% yield; the yields are similar to
those reported for related cyclizations11 and are not
decreased by the presence of the additional alkenyl
moiety in the 1,3-bis-silyl enol ether. Migration of the
olefin functionality (to form cyclic enol ethers) was not
observed during the ring-closing metathesis step.17 The
Acknowledgments
Financial support from the Ministry of Education of
Vietnam (scholarship for V.T.H.N.), from the DAAD

V. T. H. Nguyen et al. / Tetrahedron Letters 47 (2006) 113–116
115
12. Molander, G. A.; Cameron, K. O. J. Am. Chem. Soc.
1993, 115, 830.
(scholarship for E.B.), from the state of Mecklenburg-
Vorpommern (Landesforschungsschwerpunkt ÔNeue
Wirkstoffe und ScreeningverfahrenÕ), from the BASF
AG and from the Deutsche Forschungsgemeinschaft is
gratefully acknowledged.
´
13. Langer, P.; Eckardt, T.; Saleh, N. N. R.; Karime, I.;
Muller, P. Eur. J. Org. Chem. 2001, 3657.
¨
14. General procedure for the synthesis of 6a–i: To a CH2Cl2
solution of 5a,b and 10a–d or 1,1,3,3-tetramethoxypro-
pane was dropwise added TiCl4 at ꢀ78 ꢁC under argon
atmosphere. The solution was stirred at ꢀ78 ꢁC for 30 min
and was subsequently allowed to warm to 20 ꢁC during
18 h. To the solution was added a saturated aqueous
solution of NaHCO3. The organic layer was separated and
aqueous layer was extracted with CH2Cl2 (4 · 100 mL).
The combined organic layers were dried (Na2SO4),
filtered, and the filtrate was concentrated in vacuo. The
residue was purified by column chromatography (silica
gel, n-hexane/EtOAc).
References and notes
1. Macias, F. A.; Molinillo, J. M. G.; Varela, R. M.; Torres,
A.; Fronczek, F. R. J. Org. Chem. 1994, 59, 8261.
2. Dinda, B.; Das, S. K.; Hajra, A. K.; Bhattacharya, A.; De,
K.; Chel, G.; Achari, B. Ind. J. Chem. Sect. B 1999, 38,
577.
3. (a) Macias, F. A.; Varela, R. M.; Torres, A.; Molinillo, J.
M. G.; Fronczek, F. R. Tetrahedron Lett. 1993, 34, 1999;
(b) Takabatake, K.; Nishi, I.; Shindo, M.; Shishido, K. J.
Chem. Soc., Perkin Trans. 1 2000, 1807; (c) Macias, F. A.;
Varela, R. M.; Torres, A.; Molinillo, J. M. G. J. Nat.
Prod. 1999, 62, 1636.
4. Crews, P.; Harrison, B. J. Org. Chem. 1997, 62, 2646.
5. (a) Nagai, M.; Nagumo, S. Chem. Pharm. Bull. 1987, 35,
3002; (b) Oh, S. R.; Kim, D. S.; Lee, I. S.; Jung, K. Y.;
Lee, J. J.; Lee, H.-K. Planta Med. 1998, 64, 456.
6. (a) Asakawa, Y.; Toyota, M.; Takemoto, T. Phytochem-
istry 1978, 17, 2005; (b) Asakawa, Y.; Takeda, R.; Toyota,
M.; Tsunematsu, T. Phytochemistry 1981, 20, 858; (c)
Asakawa, Y.; Hashimoto, T.; Takikawa, K.; Tori, M.;
Ogawa, S. Phytochemistry 1991, 30, 235; (d) Yamaguchi,
S.; Furihata, K.; Miyazawa, M.; Yokoyama, H.; Hirai, Y.
Tetrahedron Lett. 2000, 41, 4787; (e) Breuer, M.; Leeder,
G.; Proksch, P.; Budzikiewicz, H. Phytochemistry 1986,
25, 495; (f) McCormick, S.; Robson, K.; Bohm, B.
Phytochemistry 1986, 25, 1723.
Synthesis of ethyl 3-allyl-2-hydroxybenzoate (6a): Starting
with 5a (1.570 g, 5.0 mmol), TiCl4 (0.945 g, 5.0 mmol),
1,1,3,3-tetramethoxypropane (0.821 g, 5.0 mmol), and
CH2Cl2 (10 mL), 6a was isolated by column chromato-
graphy (n-hexane/EtOAc = 20:1) as a colorless oil (0.464
g, 45%). 1H NMR (CDCl3, 300 MHz): d = 1.41 (t,
J = 7.2 Hz, 3H, OCH2CH3), 3.44 (d, 2H, CH2@CHCH2,
J = 6.6 Hz), 4.42 (q, J = 7.1 Hz, 2H, OCH2CH3), 5.05–
5.12 (m, 2H, CH2@CHCH2), 5.95–6.08 (m, 1H,
CH2@CHCH2), 6.82 (t, J = 7.8 Hz, 1H, Ar), 7.31 (dd,
J = 6.0 Hz, 1.5 Hz, 1H, Ar), 7.75 (dd, J = 6.3 Hz, 1.7 Hz,
1H, Ar), 11.14 (s, 1H, OH). 13C NMR (CDCl3, 75 MHz):
d = 14.2 (CH3), 33.6, 61.4 (CH2), 112.2 (C), 115.8 (CH2),
118.6, 127.9 (CH), 128.5 (C), 135.6, 136.2 (CH), 159.6,
170.6 (C). IR (KBr, cmꢀ1): m ¼ 3139 (m, br), 2983 (m),
~
1672 (s), 1614 (m), 1149 (s), 1303 (s), 1248 (s), 1150 (s),
1025 (s), 760 (s). UV–vis (CH3CN, nm): kmax (loge) =
209.3 (4.47), 242.4 (3.92), 309.7 (3.63). MS (EI, 70 eV): m/z
(%) = 206 (M+, 38), 160 (43), 132 (100), 103 (34), 77 (41),
51 (16). Anal. Calcd for C12H14O3: C, 69.88; H, 6.79.
Found: C, 69.31; H, 7.04.
7. (a) Nohara, T.; Kinjo, J.; Furusawa, J.; Sakai, Y.; Inoue,
M. Phytochemistry 1993, 33, 1207; (b) Shirataki, Y.;
Tagaya, Y.; Yokoe, I.; Komatsu, M. Chem. Pharm. Bull.
1987, 35, 1637.
8. For the synthesis of 2,5-dihydrobenzo[b]oxepines and 5,6-
dihydro-2H-benzo[b]oxocines based on a ÔDOM–RCMÕ
strategy, see: (a) Stefinovic, M.; Snieckus, V. J. Org. Chem.
Synthesis of ethyl 3-allyl-2-hydroxy-4,6-dimethylbenzoate
(6c): Starting with 5a (1.257 g, 4.0 mmol), TiCl4 (0.760 g,
4.0 mmol), 10a (0.680 g, 4.0 mmol), and CH2Cl2 (10 mL),
6c was isolated after chromatography (silica gel, n-hexane/
1
EtOAc = 20:1) as a colorless oil (0.338 g, 44%). H NMR
1998, 63, 2808; (b) Furstner, A.; Ackermann, L. Chem.
(CDCl3, 300 MHz): d = 1.42 (t, 3H, CH3CH2O,
J = 7.1 Hz), 2.25 (s, 3H, CH3), 2.49 (s, 3H, CH3), 3.43
(m, 2H, CH2@CHCH2, J = 1.5 Hz), 4.42 (q, 2H,
CH3CH2O, J = 7.1 Hz), 4.89–4.99 (m, 2H, CH2@CHCH2,
J = 15.5, 10.4, 1.7 Hz), 5.93 (m, 1H, CH2@CHCH2,
J = 5.2, 1.5 Hz), 6.55 (s, 1H, CH, Ar), 11.75 (s, 1H,
OH). 13C NMR (CDCl3, 75 MHz): d = 14.2, 19.6, 23.9
(CH3), 30.1, 61.4 (CH2), 109.8 (C), 114.4 (CH2), 123.8 (C),
124.7, 135.7 (CH), 138.5, 143.6, 160.8, 172.2 (C). IR (KBr,
cmꢀ1): ~m ¼ 2979 (m), 2934 (m), 1654 (s), 1616 (m), 1449
(m), 1396 (s), 1268 (s), 1173 (s), 1031 (m), 847 (m). UV–vis
(CH3CN, nm): kmax (loge) = 216.7 (4.41), 253.2 (3.98),
315.6 (3.57). MS (EI, 70 eV): m/z (%) = 234 (M+, 29), 188
(34), 173 (24), 160 (53), 145 (27), 114 (9), 91 (11), 28 (100).
The exact molecular mass m/z = 234.1256 2 ppm for
C14H18O3 was confirmed by HRMS (EI, 70 eV). All
products gave correct spectroscopic data and correct
elemental analyses and/or high resolution mass data.
15. General procedure for the synthesis of 7: To a mixture of
NaH and of nBu4NI were simultaneously added a THF
solution of 6 and of 3-bromoprop-1-ene at 0 ꢁC under
argon atmosphere. The pale yellow colored solution was
stirred at 0 ꢁC and was allowed to warm to 20 ꢁC within 8–
12 h. The solvent was removed in vacuo and the residue
was purified by column chromatography (silica gel,
n-hexane/EtOAc = 20:1) to give the product as a color-
less oil. Synthesis of ethyl 3-allyl-2-(allyloxy)-4,6-
¨
Commun. 1999, 95; see also: (c) Visser, M. S.; Heron, N.
M.; Didiuk, M. T.; Sagal, J. F.; Hoveyda, A. H. J. Am.
Chem. Soc. 1996, 118, 4291.
9. For the synthesis of heliannuol D, see: (a) Sabui, S. K.;
Venkateswaran, R. V. Tetrahedron Lett. 2004, 45, 2047;
for (ꢀ)-heliannuol C: (b) Kamei, T.; Shindo, M.; Shishido,
K. Tetrahedron Lett. 2003, 44, 8505; for (ꢀ)-heliannuol A,
see: (c) Kishuku, H.; Shindo, M.; Shishido, K. Chem.
Commun. 2003, 350; for pterulones, see: (d) Kahnberg, P.;
Lee, C. W.; Grubbs, R. H.; Sterner, O. Tetrahedron 2002,
58, 5203; for the synthesis of pterulones, see: (e) Gruijters,
B. W. T.; van Veldhuizen, A.; Weijers, C. A. G. M.;
Wijnberg, J. B. P. A. J. Nat. Prod. 2002, 65, 558.
10. For a review of 1,3-bis-silyl enol ethers, see: Langer, P.
Synthesis 2002, 441.
11. For [3+3] cyclizations of 1,3-bis-silyl enol ethers, see: (a)
Chan, T.-H.; Brownbridge, P. J. Am. Chem. Soc. 1980,
102, 3534; (b) Brownbridge, P.; Chan, T.-H.; Brook, M.
A.; Kang, G. J. Can. J. Chem. 1983, 61, 688; for
cyclizations with 2-acetyl-1-silyloxybut-1-en-3-one, see:
(c) Dede, R.; Langer, P. Tetrahedron Lett. 2004, 45,
9177; for the synthesis of dibenzo[b,d]pyran-6-ones, see:
(d) Nguyen, V. T. H.; Langer, P. Tetrahedron Lett. 2005,
46, 1013; for cyclizations with 1,1-diacylcyclopropanes,
see: (e) Bose, G.; Nguyen, V. T. H.; Ullah, E.; Lahiri, S.;
Go¨rls, H.; Langer, P. J. Org. Chem. 2004, 69, 9128.

116
V. T. H. Nguyen et al. / Tetrahedron Letters 47 (2006) 113–116
dimethylbenzoate (7c): Starting with 6c (0.285 g, 1.22
argon, the solution was exposed to air, and concentrated
in vacuo. The residue was purified by column chromato-
graphy (silica gel, n-hexane/EtOAc = 25:1) to give the
product as a colorless oil. The product should be
stored at ca 0 ꢁC. Synthesis of ethyl 8,6-dimethyl-2,5-
dihydrobenzo[b]oxepine-9-carboxylate (8c): Starting with
7c (0.260 g, 0.95 mmol), 9 (0.039 g, 0.047 mmol, 5 mol %,
dissolved in 2 mL of CH2Cl2) and CH2Cl2 (18 mL), 8c was
isolated as a colorless oil (0.210 g, 90%). 1H NMR
(CDCl3, 300 MHz): d = 1.38 (t, 3H, CH3CH2O,
J = 7.1 Hz), 2.26 (s, 3H, CH3), 2.27 (s, 3H, CH3), 3.44
(m, 2H, CH2, J = 1.9 Hz), 4.39 (q, 2H, CH3CH2O,
J = 7.1 Hz), 4.63 (m, 2H, CH2, J = 1.5 Hz), 5.46 (m, 1H,
CH, J = 5.2 Hz, 1.4 Hz), 5.83 (m, 1H, CH), 6.76 (s, 1H,
Ar). 13C NMR (CDCl3, 75 MHz): d = 14.3, 18.9, 19.8
(CH3), 25.7, 60.9, 71.4 (CH2), 124.9 (CH), 126.1 (C),
127.5, 127.8 (CH), 133.5 (2C), 136.6, 152.2, 168.1 (C). IR
(KBr, cmꢀ1): ~m ¼ 3438 (w), 2979 (s), 2930 (s), 1726 (s),
1609 (s), 1456 (s), 1387 (m), 1293 (s), 1270 (s), 1223 (m),
1149 (s), 1091 (s), 1046 (s), 862 (m), 653 (w). UV–vis
(CH3CN, nm): kmax (loge) = 205.8 (4.25), 263.1 (3.62),
301.8 (3.27). MS (EI, 70 eV): m/z (%) = 247 ([M+1]+, 9),
246 (M+, 77), 201 (92), 200 (100), 199 (47), 183 (35), 173
(23), 172 (63), 171 (24), 157 (53), 129 (41), 128 (31), 114
(21), 45 (18), 31 (41). Anal. Calcd for C15H18O3 (246.302):
C, 73.15; H, 7.36. Found: C, 73.03; H, 6.96.
mmol), NaH (0.059 g, 2.44 mmol), nBu4NI (0.797 g,
2.44 mmol), 3-bromoprop-1-ene (0.220 g, 1.83 mmol),
and THF (15 mL), 7c was isolated as a colorless oil
1
(0.285 g, 85%). H NMR (CDCl3, 300 MHz): d = 1.37 (t,
3H, CH3CH2O, J = 7.1 Hz), 2.24 (s, 3H, CH3), 2.26 (s,
3H, CH3), 3.42 (m, 2H, CH2@CHCH2Ar, J = 1.8 Hz),
4.37 (q, 2H, CH3CH2O, J = 7.1 Hz), 4.39 (m, 2H, CH2@
CHCH2O, J = 1.5 Hz), 4.86–4.93 (m, 2H, CH2@
CHCH2Ar, J = 15.3, 10.2, 1.8 Hz), 5.34–5.41 (m, 2H,
CH2@CHCH2O, J = 10.4, 15.5, 1.6 Hz), 5.88–5.93 (m,
1H, CH2@CHCH2Ar, J = 5.4 Hz), 5.99–6.04 (s, 1H,
CH2@CHCH2O, J = 5.3 Hz), 6.80 (s, 2H, Ar). 13C
NMR (CDCl3, 75 MHz): d = 14.3, 19.0, 19.4 (CH3),
30.5, 61.1, 76.0, 115.2, 116.8 (CH2), 126.8 (C), 127.8
(CH), 128.9 (C), 133.9 (CH), 134.0 (C), 136.1 (CH), 140.1,
154.4, 168.6 (C). IR (KBr, cmꢀ1): m ¼ 3081 (m), 2981 (s),
~
2928 (s), 2870 (m), 1726 (s), 1642 (m), 1609 (m), 1666 (m),
1453 (s), 1411 (s), 1297 (s), 1270 (s), 1150 (s), 1108 (s), 1067
(s), 1041 (s), 992 (s), 918 (s), 864 (m). UV–vis (CH3CN,
nm): kmax (loge) = 202.9 (4.59), 276.4 (3.89). MS (EI,
70 eV): m/z (%) = 275.5 ([M+1]+, 11), 274.5 (M+, 62), 233
(24), 232 (26), 229 (36), 228 (17), 201 (12), 188 (19), 187
(100), 173 (20), 161 (34), 160 (36), 159 (31), 145 (18), 91
(16), 41 (20), 29 (11). Anal. Calcd for C17H22O3 (274.355):
C, 74.42; H, 8.08. Found: C, 74.41; H, 7.91.
16. General procedure for the synthesis of 8: To a CH2Cl2
solution of 7 was added a CH2Cl2 solution of 9 under
argon atmosphere. After stirring for 6–8 h at 20 ꢁC under
17. For the combination of RCM with isomerizations to give
enol ethers, see: Sutton, A. E.; Seigal, B. A.; Finnegan, D.
F.; Snapper, M. L. J. Am. Chem. Soc. 2002, 124, 13390.