632
OH
O
References and Notes
H
H
H
1
4779, and referenses cited there in.
Co(CO)3
Co(CO)3
a
b
O
O
O
2
For examples: a) P. A. Wender, C. D. Jesudason, H. Nakahira,
in.
H
H
H
OHC
OTIPS 19
21
O
H
c
O
O
O
3
K. Tanino, F. Kondo, T. Shimizu, M. Miyashita, Org. Lett.
O
+
O
H
O
4
5
C. W. Jefford, A. W. Sledeski, P. Lelandais, J. Boukouvalas,
HO
HO
22 91% from 19
23 57%
H
6
7
Isolation of furanether B: R. Battaglia, M. De Bernardi, G.
Total synthesis of furanether B: a) M. E. Price, N. E. Schore,
d,e
O
O
furanether B
H
76%
Scheme 7. Total synthesis of Furanether B. Reagents and
conditions: (a) (NH4)2Ce(NO3)6 (12 equiv), CH3CN-H2O, rt,
30 min; (b) NaBH4 (5 equiv), THF-H2O, rt, 30 min then HCl
(aq), rt, 30 min; (c) DIBAL (1.8 equiv), CH2Cl2, ¹78 °C, 10 min
then HCl (aq); (d) Tf2O (1.5 equiv), 2,6-di-tert-butylpyridine (3
equiv), CH2Cl2 ¹78 °C, 45 min; (e) LiBHEt3 (2.1 equiv), THF,
rt, 15 min.
8
9
2341. b) K. Tanino, T. Shimizu, M. Miyama, I. Kuwajima,
10 G. A. Olah, S. C. Narang, B. G. B. Gupta, R. Malhotra, J.
The stage was set for conversion of the dicobalt acetylene
complex moiety to a maleic anhydride derivative. Surprisingly,
treatment of the alcohol 19 with an excess amount of CAN led to
formation of aldehyde 21 having an oxygen bridge in the
cycloheptane ring (Scheme 7). The result indicates that the
secondary hydroxy group of the substrate underwent trans-
annular oxidative coupling with the enol silyl ether moiety.12
The unexpected reaction allowed us to accomplish the total
synthesis in fewer steps.
Thus, the reaction of the aldehyde 21 with NaBH4 followed
by workup under acidic conditions gave rise to hydroxy lactone
22 as a mixture of regioisomers in high overall yield.
Conversion of the unsaturated lactones to the furan ring was
achieved by DIBAL reduction. Finally, the primary hydroxy
group of 23 was removed via reduction of the corresponding
triflate with LiBHEt3. The spectral data of the synthetic
compound were identical with those of furanether B.
In conclusion, a novel synthetic method for functionalized
cycloheptanone derivatives was developed on the basis of a
[5 + 2] cycloaddition reaction using a dicobalt hexacarbonyl
propargyl cation species and a silyloxyallene. The reaction
proceeds through an intermolecular addition followed by an
intramolecular conjugate addition. The method can be applied
for constructing functionalized hydroazulene skeletons, and a
new total synthesis of furanether B was achieved through the
reactions involving transformation of dicobalt acetylene com-
plex into the maleic anhydride derivative and the key oxidative
transannular ether ring formation.
11 The experimental procedure for the [5 + 2] cycloaddition
reaction: To a mixture of siloxyallene 12 (3.8 g, 9.3 mmol)
and cobalt complex 1 (7.2 g, 11 mmol) in CH2Cl2 (22 mL)
was added a 0.96 M hexane solution of EtAlCl2 (19.3 mL,
18.6 mmol) at 0 °C. After stirring for 30 min, the reaction was
quenched with a saturated aqueous solution of Rochelle salt.
The mixture was stirred vigorously for 30 min and separated.
The aqueous layer was extracted with ether and the combined
organic layer was dried over MgSO4. Concentration under
reduced pressure followed by flash column chromatography
using neutral silica gel afforded 6.3 g of a mixture of
cycloadducts 16 and 17. The mixture was dissolved in
CH2Cl2 (41 mL), and to this was added triethylamine
(6.8 mL, 49 mmol) followed by TMSOTf (4.4 mL, 25 mmol)
at 0 °C. After stirring for 30 min, a saturated aqueous solution
of NaHCO3 was added, and the mixture was separated. The
aqueous layer was extracted with ether, and the combined
organic layer was dried over MgSO4. Concentration under
reduced pressure followed by purification by flash column
chromatography afforded 5.4 g (69% from 12) of enol silyl
ether 18 and 1.1 g (16% from 12) of cycloadduct 17. Cobalt
complex 18: 1H NMR (500 MHz, CDCl3): ¤ 6.28 (s, 1H),
5.30 (d, J = 8.0 Hz, 1H), 4.43 (d, J = 17.2 Hz, 1H), 3.31 (d,
J = 17.2 Hz, 1H), 3.20 (d, J = 9.7 Hz, 1H), 3.14 (d,
J = 9.7 Hz, 1H), 2.85 (td, J = 8.0, 4.6 Hz, 1H), 1.64 (dd,
J = 14.3, 8.0 Hz, 1H), 1.47 (dd, J = 14.3, 4.6 Hz, 1H), 1.14
(sept, J = 6.3 Hz, 3H), 1.06 (d, J = 6.3 Hz, 18H), 1.04 (s,
6H), 0.24 (s, 9H). 13C NMR (125 MHz, CDCl3): ¤ 199.95
(3C), 199.63 (3C), 147.30, 136.78, 116.88, 115.96, 95.24,
87.81, 44.12, 37.89, 34.27, 29.09, 27.33, 27.30, 24.14, 17.64
(6C), 11.79 (3C), 0.24 (3C). HRMS (EI) m/z; calcd for
C31H45O8Si2Co2I: 846.0362, found: 846.0316.
This work was partially supported by the Global COE
Program (Project No. B01: Catalysis as the Basis for Innovation
in Materials Science) and Grant-in-Aid for Scientific Research
on Innovative Areas (Project No. 2105: Organic Synthesis Based
on Reaction Integration) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
12 A similar cyclization reaction of an alcohol having an
allylsilane moiety was reported: S. R. Wilson, C. E. Augelli-
Chem. Lett. 2010, 39, 630-632
© 2010 The Chemical Society of Japan