An easily assembled highly active ruthenium initiator for olefin metathesis

Article abstract of DOI:10.1055/s-2004-817777

A practical and easily assembled synthesis of new ruthenium alkylidene precatalyst 8 is described, which exhibits excellent metathesis activity in ring-closing and ring-opening cross metathesis processes.

Full text of DOI:10.1055/s-2004-817777

LETTER
667
An Easily Assembled Highly Active Ruthenium Initiator for Olefin Metathesis
An
Easily
A
ssemb
i
led Hig
c
hly Active
o
R
uthenium
I
l
nitiator
e
for
O
lefin
Metat
B
hesis uschmann, Hideaki Wakamatsu, Siegfried Blechert*
Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany
Fax +49(30)31423619; E-mail: blechert@chem.tu-berlin.de
Received 22 October 2003
reactivity. Given the substitution pattern of 4 we set out to
Abstract: A practical and easily assembled synthesis of new ruthe-
nium alkylidene precatalyst 8 is described, which exhibits excellent
metathesis activity in ring-closing and ring-opening cross meta-
thesis processes.
investigate the introduction of less elaborate substituents
R on the benzylidene moiety that could also lead to cata-
lysts with enhanced activity, without requiring expensive
and multistep syntheses, which would detract from cata-
lyst utility. However the synthesis of corresponding ana-
logues of 2 with either a methyl or a second alkyl moiety
as R proved to be non-trivial. These results indicate that
the presence of steric bulk adjacent to the isopropoxy
group also seems to be critical and therefore not every de-
sired substituent in the catalyst is accessible. We then set
out to synthesize complex 8, a 3-methoxy analogue of 2.
The alkoxy functionality is of particular interest because
it represents a functional handle with regard to immobili-
zation.
Key words: catalysis, metathesis, ring-closing, ring-opening, ru-
thenium
Since olefin metathesis has emerged as a powerful tool for
carbon-carbon bond formation,1 much effort has been de-
voted to the development of new efficient and practical
olefin metathesis catalysts. It is in this context that ruthe-
nium complexes, bearing N-heterocyclic carbene (NHC)
ligands such as 12 have attracted much attention, due to
their high activity and remarkable air/water stability. The
introduction of a phosphine-free variant incorporating a
chelating o-isopropoxybenzylidene ligand (catalyst 2) has
enabled catalyst recovery by column chromatography.3
This catalyst possesses improved general reactivity to-
wards electron deficient alkenes4 and is readily amenable
to immobilizations.5 We have found that the superior sta-
bility of 2 comes hand in hand with longer reaction times.
Our investigations aimed at the preparation of highly ac-
tive and stable ruthenium-initiators recently led to the dis-
covery of a variant 3 which possesses similar shelf-
stability but a remarkable activity that surpasses that of 2
and even 1 (Figure 1).6
Herein we report the synthesis of 8 and its activity in ring-
closing metathesis (RCM) and ring-opening cross meta-
thesis (ROM-CM).
2-Isopropoxy-3-methoxybezaldehyde (6) was synthe-
sized from commercially available o-vanillin (5) through
alkylation using sodium hydride and isopropyl bromide in
DMF. Subsequent Wittig olefination delivered styrene 7.
The bright green ruthenium complex 8 was prepared by
the reaction of 1 with two equivalents of 7 in the presence
of CuCl as a phosphane scavenger in CH2Cl2, and was iso-
lated by flash chromatography (FC) on silica gel
(Scheme 1).7 Precatalyst 8 has a comparable air/moisture
stability to 2.
MesN
NMes
CHO
OH
CHO
a
Cl
Cl
Ru
MesN
NMes
Oi-Pr
MesN
NMes
Ph
Cl
Cl
OMe
OMe
Ru
i-PrO
i-PrO
Cl
Cl
Ru
5
6
PCy3
i-PrO
b
R
1
2 R = H
4 R = Ph
3
MesN
NMes
Cl
Cl
c
Ru
Figure 1 Ruthenium precatalysts.
Oi-Pr
i-PrO
MeO
OMe
This enhanced reactivity prompted us to undertake a more
systematic study on the effect of the substituent R on the
o-isopropoxybenzylidene moiety. Apparently steric ef-
fects of R in 4 seemed to be responsible for its improved
8
7
Scheme 1 Reagents and conditions a) 5 (1.0 Equiv), NaH (1.2
equiv), DMF (0.5 M), 0 °C to r.t., then i-PrBr (1.5 equiv), 50 °C, 36
h, 67%; b) t-BuOK (2.0 equiv), Ph3PCH3Br (2.0 equiv), Et2O (0.1 M),
0 °C, 10 min, then 6 (1.0 equiv), 5 min, 98%; c) 1 (1.0 equiv), 7 (2.0
equiv), CuCl (1.1 equiv), CH2Cl2 (0.01 M), 40 °C, 1 h, 81%.
SYNLETT 2004, No. 4, pp 0667–0670
0
9.
0
3.
2
0
0
4
Advanced online publication: 10.02.2004
DOI: 10.1055/s-2004-817777; Art ID: G28503ST
© Georg Thieme Verlag Stuttgart · New York

668
N. Buschmann et al.
LETTER
The potential of 8 was first tested in the RCM of tosyl- An enhanced activity of 8, compared to 1 is obvious from
amide 9 (Scheme 2).8 To our delight, in a matter of min- an examination of the results. For example, the RCM of
utes complete conversion of 9 was observed in CH2Cl2 at 16 to 17 (Table 1, entry 8) proceeded smoothly using 0.5
room temperature, using as little as 1 mol% of 8. The ac- mol% of 8 with an isolated yield of 74% after 10 minutes.
tivity of complex 8 in this reaction was therefore signif- In contrast only 8% of 17 (Table 1, entry 7) could be
icantly higher than that of 2 and 1 under identical reaction detected by HPLC when the same reaction was promoted
conditions.6a
by 1 under identical conditions. Another conspicuous ex-
ample is the conversion of 13 to dihydropyridinone 14 in
93% yield (Table 1, entry 4) after FC and 20 minutes re-
action time using only 0.1 mol% of 8, whereas under iden-
tical conditions 1 gave only 5% conversion (Table 1, entry
3) detected by HPLC analysis. Interestingly, terminal
acrylamide 15 proved to be less reactive. We propose that
the formation of a more stable ethylidene propagating
species from substrate 13 accounts for its fast conversion
to 14. However it must be noted that similar yields were
achieved in RCM reactions by using catalyst 1 and in-
creased reaction times (up to 14 h).
Ranges of substrates were then examined, encompassing
various functional groups and ring sizes and the results are
presented in Table 1. All reactions were carried out at
room temperature in CH2Cl2 (0.05 M) under an inert at-
mosphere in the presence of 0.1–0.5 mol% of the pre-cat-
alysts 1 and 8. After 10–20 minutes, the reaction mixtures
were quenched by using ethyl vinyl ether.
1 mol % cat.
CH2Cl2 (0.01M)
r.t., 10 min
yield [%]
cat.
Ts
N
Ts
N
1
2
4
8
44
13
>99
98
In addition, the application of 8 in ROM-CM processes
was also tested (Scheme 3).9 Oxanorbornene derivative
20 was ring-opened in the presence of 2 equivalents of
various functionalized alkenes of general type 21 at room
temperature.
9
10
Scheme 2 RCM of 9 using 1, 2 and 8.
Table 1 RCM Using 1, 4 and 8
Entry
Substrate
Product
Catatlyst (mol%)
Time (min)
Yield (%)
1d
2d
E
E
1 (0.5)
4 (0.5)
8 (0.5)
20
10
20
43b
99b
85b
E
E
12
11
3
4
1 (0.1)
8 (0.1)
20
20
5c
Bn
N
Bn
N
O
O
O
93a
14
14
13
15
5
6
1 (0.1)
8 (0.1)
20
20
1c
Bn
N
24c
7
8
1 (0.5)
8 (0.5)
10
10
8c
Ts
N
Ts
N
74a
17
19
16
18
9
10
1 (0.5)
8 (0.5)
20
20
24a
88a
O
O
a Isolated yields after column chromatography.
b Conversions were determined by 1H NMR spectroscopy.
c Conversions were determined by HPLC.
d E = CO2Et.
Synlett 2004, No. 4, 667–670 © Thieme Stuttgart · New York

LETTER
An Easily Assembled Highly Active Ruthenium Initiator for Olefin Metathesis
669
Acknowledgment
O
We would like to thank the ‘Fonds der Chemischen Industrie’ and
the Graduiertenkolleg ‘Synthetische, mechanistische und reaktions-
technische Aspekte von Metallkatalysatoren’ for financial support.
OTBDMS
OTBDMS
8
OTBDMS
OTBDMS
R
O
+
20
21
22
References
R
(1) (a) Connon, S. J.; Blechert, S. Angew. Chem. Int. Ed. 2003,
42, 1900; Angew. Chem. 2003, 115, 1944. (b) Trnka, T. M.;
Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18. (c) Fürstner,
A. Angew. Chem. Int. Ed. 2000, 39, 3012; Angew. Chem.
2000, 112, 3140. (d) Grubbs, R. H.; Chang, S. Tetrahedron
1998, 54, 4413. (e) Schuster, M.; Blechert, S. Angew.
Chem., Int. Ed. Engl. 1997, 36, 2036; Angew. Chem. 1997,
109, 2124.
(2) (a) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett.
1999, 1, 953. (b) Chatterjee, A. K.; Grubbs, R. H. Org. Lett.
1999, 1, 1751.
(3) (a) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A.
H. J. Am. Chem. Soc. 2000, 112, 8168. (b) Gessler, S.;
Randl, S.; Blechert, S. Tetrahedron Lett. 2000, 41, 9973.
(4) (a) Randl, S.; Gessler, S.; Wakamatsu, H.; Blechert, S.
Synlett 2001, 430. (b) Randl, S.; Connon, S. J.; Blechert, S.
Chem. Commun. 2001, 1796.
(5) (a) Randl, S.; Buschmann, N.; Connon, S. J.; Blechert, S.
Synlett 2001, 1547. (b) Yao, Q. Angew. Chem. Int. Ed. 2000,
39, 3896; Angew. Chem. 2000, 112, 4060. (c) Kingsbury, J.
S.; Garber, S. B.; Giftos, J. M.; Gray, B. L.; Okamoto, M.
M.; Farrer, R. A.; Fourkas, J. T.; Hoveyda, A. H. Angew.
Chem. Int. Ed. 2001, 40, 3898; Angew. Chem. 2001, 113,
4251.
Scheme 3 ROM-CM using catalyst 8.
Allyl trimethylsilane 21a proved to be the best substrate
for the ROM-CM processes (Table 2). After only 5 min-
utes using 0.05 mol% of 8 nearly quantitative conversion
1
was observed by H NMR spectroscopy. In general, low
amounts of catalyst were required for these processes and
a high functional group tolerance could be demonstrated.
Even sulfide 21d reacted, although higher catalyst load-
ings and longer reaction times were required.10
In summary, we have developed a practical and in partic-
ular easily accessible precatalyst 8, which exhibits excel-
lent metathesis activity, surpassing that of 1 and 2.
Reactions with 8 are slightly slower in comparison to
those with 4 (for example, see formation of 10 and 12).
However, 8 is easier to prepare and has a better shelf sta-
bility which is important for practical purposes. o-Substi-
tution next to the isopropoxy group is effective for high
activity up to the present our work, however, its detail is
not clear yet.11 The same effect has recently been used
successfully by Hoveyda et al.12 In addition to an ortho-
effect, electronic effects can influence the reactivity of
styrene ether complexes. Studies in this field have been
published by Grela,13 Hoveyda12 and by us.14 On the other
hand, development of highly active immobilized catalyst
will be realized by the present work. Further develop-
ments of homogeneous and heterogeneous ruthenium
metathesis initiator having excellent activity and stability
are in progress in our laboratory.
(6) (a) Wakamatsu, H.; Blechert, S. Angew. Chem. Int. Ed.
2002, 41, 794; Angew. Chem. 2002, 114, 832.
(b) Wakamatsu, H.; Blechert, S. Angew. Chem. Int. Ed.
2002, 41, 2403; Angew. Chem. 2002, 114, 2509.
(7) Procedure for the Preparation of Catalyst 8: To a solution
of 7 (136 mg, 0.71 mmol) in CH2Cl2 (35 mL) was added
CuCl (38 mg, 0.39 mmol) and 1 (300 mg, 0.35 mmol) under
an atmosphere of N2. The reaction mixture was stirred for
1 h at 40 °C, concentrated, dissolved in a minimal volume of
CH2Cl2 and passed through a Pasteur pipette containing a
plug of cotton, and then concentrated. The residue was
Table 2 ROM-CM Results
Entry
1
CM-substrate (CH2=CHCH2R) Product
Catalyst (mol%)
Time (min)
5
Yield (%)
22a
8 (0.05)
98a
83b
TMS
21
2
22b
8 (0.5)
5
99a
72b
O
21b
3
4
22c
8 (0.5)
8 (3)c
5
99a
51b
O
O
21c
22d
14 h
86a
70b
Si
O
S
21d
a Conversions were determined by 1H NMR spectroscopy.
b Isolated yields after column chromatography.
c Reaction was carried out at reflux.
Synlett 2004, No. 4, 667–670 © Thieme Stuttgart · New York

670
N. Buschmann et al.
LETTER
purified by column chromatography on silica gel (hexane/
MTBE 2:1) to give 8 (188 mg, 81%) as a green solid. IR
(ATR): n = 3482 (w), 2971 (s), 2918 (s), 1701 (m), 1607 (w),
1574 (s), 1475 (s), 1445 (s), 1267 (ss), 1106 (s), 852 (w), 759
(w) cm–1. 1H NMR (500 MHz, CD2Cl2): d = 1.15 [d,
J = 6 Hz, 6 H, OCH(CH3)2], 2.40 (br s, 18 H, CH3-Mes),
3.78 (s, 3 H, OCH3), 4.14 (s, 4 H, H-2), 5.69 [qsep, J = 6,
6 Hz, 1 H, OCH(CH3)2], 6.54 (d, J = 8 Hz, 1 H, CH-Ar), 6.88
(dd, J = 8, 8 Hz, 1 H, CH-Ar), 7.06 (br s, 4 H, CH-Mes), 7.15
(d, J = 8 Hz, 1 H, CH-Ar), 16.51 (s, 1 H, H-3). 13C NMR
(126 MHz, CD2Cl2): d = 18.4 (CH3, Mes), 20.2 (CH3, Mes),
20.9 [CH3, OCH(CH3)2], 21.6 [CH3, OCH(CH3)2], 51.5 (br,
CH2, C-2), 56.3 (CH3, OCH3), 80.5 [CH, OCH(CH3)2],
114.1 (CH, Ar), 115.4 (CH, Ar), 123.4 (CH, Ar), 129.2 (br,
CH, Mes), 138.8 (Cq), 138.9 (Cq), 139.7 (Cq), 139.9 (Cq),
147.6 (Cq), 149.6 (Cq), 210.6 (Cq, C-2), 297.6 (CH, C-3).
LRMS (FAB): m/z (%) = 656 (2) [M+], 578 (6), 541 (2), 441
(5), 405 (34) 307 (37), 147 (22), 109 (23), 91 (42), 69 (78),
55 (100). Anal. Calcd for C32H40N2O2Cl2Ru: C, 58.53; H,
6.14; N, 4.27. Found: C, 58.18; H, 5.90; N, 4.51.
5.42–5.53 (m, 1.5 H, H-7 and H-8), 5.65 (dt, J = 15, 7 Hz,
0.5 H, H-8, E), 5.86 (m, 1 H, H-2). 13C NMR (125.8 MHz,
CDCl3): d = –5.6, –5.6, –5.5 [CH3, OSi(CH3)2], 18.1, 18.1
[Cq, OSiC(CH3)3], 21.9 (CH2), 25.8 [CH3, OSiC(CH3)3],
26.3 (CH2), 29.9, 29.9 (CH, C-12), 43.0, 43.5, (CH2), 48.9,
49.0, 49.1, 49.8 (CH, C-4 and C-5), 60.5, 60.6, 60.8, 60.9
(CH2, C-13 and C-14), 76.3, 81.8, 82.2, 82.3 (CH, C-3 and
C-6), 115.3, 115.5 (CH2, C-1), 131.0, 131.0, 131.8, 131.9
(CH, C-7 and C-8), 139.4, 139.6 (CH, C-2). LRMS (EI):
m/z (%) = 482 (<1) [M+], 425 (42), 189 (31), 147 (67), 89
(67), 73 (100). HRMS: m/z calcd for C26H50O4Si2 [M+]:
482.3248; found: 482.3259. Anal. Calcd for C26H50O3Si2: C,
64.67; H, 10.44. Found: C, 64.63; H, 10.21.
Compound 22c: IR (ATR): n = 2954 (s), 2929 (s), 2857 (s),
1644 (w), 1255 (s), 1090 (s), 837 (ss) cm–1. 1H NMR (500
MHz, CDCl3, two diastereomers 1:1, E/Z = 4:1): d = 0.04 [br
s, 12 H, OSi(CH3)2], 0.88 [br s, 18 H, OSiC(CH3)3], 2.14–
2.23 (m, 2 H), 2.59–2.63 (m, 1 H), 2.79 (dt, J = 4, 4 Hz, 1 H),
3.14 (m, 1 H), 3.34–3.45 (m, 1 H), 3.63–3.85 (m, 5 H), 3.99–
4.49 (m, 3 H), 4.59 (br t, J = 6 Hz, 1 H), 5.07 (br d,
(8) General Procedure for RCM: The acyclic 11, 13, 15, 16,
18 diene was dissolved in dry CH2Cl2 (0.05 M) at r.t., 8 (0.1–
0.5 mol%) was added and the solution was stirred for 10–20
min at r.t. Subsequently the reaction mixture was quenched
using freshly distilled ethyl vinyl ether and concentrated.
The crude product was purified by FC on silica gel (hexane/
EE or hexane/MTBE), respectively.
(9) General Procedure for ROM-CM: The Oxanorbornene
derivative 20 was dissolved in dry CH2Cl2 (0.05 M) at r.t., 2
equiv of the desired cross metathesis partner 21a–d and 8
(0.05–3.00 mol%) were added and the solution was stirred
for 5 min at r.t. (21d: 14 h at reflux). Subsequently, the
reaction mixture was quenched using freshly distilled ethyl
vinyl ether and concentrated. The crude product 22a–d15 was
purified by FC on silica gel (hexane/EE or hexane/MTBE).
(10) For an example of RCM using sulfide substrates see:
Spagnol, G.; Heck, M.-P.; Nolan, S. P.; Mioskowski, C. Org.
Lett. 2002, 4, 1767.
(11) (a) For example, the catalyst, which is substituted in alkyl
group instead of methoxy group, could not be isolated: Zaja,
M.; Blechert, S. unpublished results. (b) Mechanism of
phosphine based ruthenium metathesis catalysts, see:
Sanford, M. S.; Ulman, M.; Grubbs, R. H. J. Am. Chem. Soc.
2001, 123, 749. (c) Sanford, M. S.; Love, J. A.; Grubbs, R.
H. J. Am. Chem. Soc. 2001, 123, 6543. (d) Love, J. A.;
Sanford, M. S.; Day, M. W.; Grubbs, R. H. J. Am. Chem.
Soc. 2003, 125, 10103.
J = 10 Hz, 0.2 H), 5.11 (dd, J = 10, 4 Hz, 0.8 H), 5.22 (br d,
J = 17 Hz, 0.2 H), 5.24 (br d, J = 17 Hz, 0.8 H), 5.54–5.79
(m, 2 H), 5.84 (m, 1 H, H-2). 13C NMR (125.8 MHz, CDCl3):
d = –5.6, –5.5, –5.5, –5.5, [CH3, OSi(CH3)2], 18.1, 18.2 [Cq,
OSiC(CH3)2], 25.8, 25.8, 25.9, 26.0 [CH3, OSiC(CH3)2],
44.3, 44.4, 44.4 (CH2, C-12), 48.9, 49.0, 49.1, 49.7, 49.9,
50.6, 50.7, 50.8 (CH, C-4, C-5 and C-11), 60.2, 60.6, 60.7,
60.7, 60.8, 61.0, 61.1, 61.3 (CH2, C-13 and C-14), 67.0, 67.1,
70.6, 70.6, 70.8, 71.3, 71.3, 71.4 (CH2, C-9 and C-10), 76.1,
76.7, 81.1, 81.3, 81. 7, 82.3, 82.4, 82.7(CH, C-3 and C-6),
115.0, 115.5, 115.7 (CH2, C-1), 127.5, 127.5,128.6, 128.7,
133.8, 133.9, 134.4, 134.4 (CH, C-7 and C-8), 139.2, 139.4,
139.7 (CH, C-2). LRMS (EI): m/z (%) = 455 (8) [M+ – (tert-
Bu)], 189 (20), 147 (48), 131 (100), 89 (84), 73 (95). HRMS:
m/z calcd for C23H43O5Si2 [M+ – (tert-Bu)]: 455.2649; found:
455.2651. Anal. Calcd for C27H52O5Si2: C, 63.23; H, 10.22.
Found: C, 63.10; H, 9.86.
Compound 22d: IR (ATR): n = 2954 (s), 2928 (s), 2857 (s),
1644 (w), 1472 (m), 1252 (s), 1087 (s), 836 (ss) cm–1. 1H
NMR (500 MHz, CDCl3, E/Z = 1/1): d = 0.03 [s, 3 H,
OSi(CH3)2], 0.04 [s, 3 H, OSi(CH3)2], 0.04 [s, 3 H,
OSi(CH3)2], 0.05 [s, 3 H, OSi(CH3)2], 0.12 (s, 3 H, H-10),
0.14 (s, 1.5 H, H-10), 0.15 (s, 1.5 H, H-10), 0.89 [s, 9 H,
OSiC(CH3)3], 0.90 [s, 4.5 H, OSiC(CH3)3], 0.90 [s, 4.5 H,
OSiC(CH3)3], 1.58–1.66 (m, 1.5 H, H-9), 1.73 (dd, J = 9,
2 Hz, 0.5 H, H-9), 2.09–2.23 (m, 5 H, H-4, H-5 and H-13),
2.62 (t, J = 7 Hz, 2 H, H-12), 3.63–3.80 (m, 6 H, H-11, H-14
and H-15), 4.19 (t, J = 8 Hz, 0.5 H, H-6), 4.27 (dt, J = 6,
7 Hz, 1 H, H-3), (dd, J = 8, 7 Hz, 0.5 H, H-6), 5.08 (dd,
J = 10, 4 Hz, 1 H, H-1, Z), 5.23 (ddt, J = 17, 8, 2 Hz, 1 H, H-
1, E), 5.34–5.41 (m, 1 H, H-7), 5.56 (dt, J = 10, 9 Hz, 0.5 H,
H-8, Z), (dt, J = 16, 8 Hz, 0.5 H, H-8, E), 5.87 (dddd, J = 17,
10, 6, 2 Hz, 1 H, H-2). 13C NMR (125.8 MHz, CDCl3): d =
–5.5, –5.5, –5.5 [CH3, OSi(CH3)2], –2.4, –2.4, 2.3, 2.3 (CH3,
C-10), 16.1, 16.1 (CH3, C-13), 18.1, 18.2 [Cq, OSiC(CH3)3],
18.9, 22.4 (CH2, C-9), 25.8, 25.9, 25.9 [CH3, OSiC(CH3)3],
36.3, 36.3 (CH2, C-12), 49.0, 49.0, 49.1, 50.1 (CH, C-4 and
C-5), 60.5, 60.6, 60.9, 61.0 (C-14 and C-15), 62.4, 62.4
(CH2, C-11), 76.3, 82.0, 82.0, 82.1 (CH, C-3 and C-6),
115.1, 115.2 (CH2, C-1), 127.7, 128.2 (CH, C-8), 129.7,
130.6 (CH, C-7), 139.7, 139.9 (CH, C-2). LRMS (EI): m/z
(%) = 574 (<1) [M+], 223 (3), 189 (4), 149 (100), 133 (6),
89(8), 75 (23). HRMS: m/z calcd for C28H58O4SSi3 [M+]:
574.3364; found: 574.3371.
(12) Van Veldhuizen, J. J.; Gillingham, D. G.; Garber, S. B.;
Kataoka, O.; Hoveyda, A. H. J. Am. Chem. Soc. 2003, 125,
12502.
(13) (a) Grela, K.; Kim, M. Eur. J. Org. Chem. 2003, 963.
(b) Grela, K.; Trynowski, M.; Bieniek, M. Tetrahedron Lett.
2002, 963.
(14) Zaja, M.; Connon, S. J.; Dunne, A. M.; Rivard, M.;
Buschmann, N.; Jiricek, J.; Blechert, S. Tetrahedron 2003,
59, 6545.
(15) Selected spectroscopic data:
Compound 22b: IR (ATR): n = 2954 (s), 2928 (s), 2857 (s),
1720 (s), 1643 (w), 1472 (m), 1255 (s), 1089 (s), 836 (ss)
cm–1. 1H NMR (500 MHz, CDCl3, E/Z = 1:1): d = 0.02–0.06
[m, H, OSi(CH3)2], 0.88 [m, 18 H, OSiC(CH3)3], 2.11–2.54
(m, 8 H, H-4, H-, H-9, H-10 and H-12), 3.62–3.77 (m, 2 H,
H-13 and H-14), 4.19 (dd, J = 7, 7 Hz, 0.5 H, H-6), 4.26 (m,
1 H, H-3), 4.61 (dd, J = 7 Hz, 0.5 H, H-6), 5.09 (br dd,
J = 10, 5 Hz, 1 H, H-1, Z), 5.23 (d, J = 17 Hz, 1 H, H-1, E),
Synlett 2004, No. 4, 667–670 © Thieme Stuttgart · New York