Stereoselective synthesis of allyl vinyl ethers from silyl enol ethers

Full text of DOI:10.1021/jo960289w

2262
J . Org. Chem. 1996, 61, 2262-2263
Table 1. The results on the reaction of these iodo acetals
with butyllithium in DME are also shown in Table 1.
Ster eoselective Syn th esis of Allyl Vin yl
Eth er s fr om Silyl En ol Eth er s
The addition of butyllithium (2.4 equiv) to a solution
of 2a A, 2a B, or 2a C in DME at -78 °C gave allyl vinyl
ether 3a A, 3a B, or 3a C at a yield of 66%, 92%, or 60%,
respectively.10,11 Surprisingly, the tert-butyldimethylsi-
loxy group was selectively eliminated prior to the allyloxy
group, and only a small amount of silyl enol ether 1a was
obtained (<9% yield).12 Treatment of 2bD (erythro/threo
) 99/1) with butyllithium in DME at -78 °C gave allyl
vinyl ether 3bD (E/Z ) 96/4, 76% yield) along with silyl
enol ether (18%, 1c/1b ) 92/8). The addition of HMPA
(4 equiv to n-BuLi) increased the yield of 3bD at the
expense of 1c/1b (3%), although E stereoselectivity
decreased slightly (E/Z ) 90/10). Furthermore, treat-
ment of 2cD (erythro/threo ) 4/96) with butyllithium in
DME at -78 °C gave 1-(allyloxy)-1-decene (3cD) (Z/E )
86/14, 79% yield) in addition to 1-siloxy-1-decene (19%,
1c/1b ) 55/45). The addition of HMPA improved both
the yield of 3cD and its Z stereoselectivity. Thus, (Z)-
1-(allyloxy)-1-decene (3cD) (Z/E ) 96/4, 81% yield) was
obtained selectively along with a small amount of 1c/1b
(9%, 1c/1b ) 84/16). Similar results were obtained from
2d E and 2eE. Trisubstituted allyl alkenyl ethers 3fE
and 3gE were also prepared from 1f and 1g, respectively,
in good yield.13
Katsuya Maeda, Hiroshi Shinokubo,
Koichiro Oshima,* and Kiitiro Utimoto*
Department of Material Chemistry, Faculty of Engineering,
Kyoto University, Sakyo-ku, Yoshida, Kyoto 606-01, J apan
Received February 13, 1996
Claisen rearrangement of allyl vinyl ethers is one of
the most powerful tools for stereoselective carbon-carbon
bond formation.1 The stereoselective formation of the
enol ether moiety is the key to diastereocontrol over the
newly created chiral centers of the product. However,
acyclic enol ether Claisen rearrangement shows no
significant diastereoselectivity because of the lack of
stereoselective synthesis of the enol ether moiety.2 Allyl
vinyl ethers are typically prepared by either mercury-
or acid-catalyzed vinyl ether exchange with allylic alco-
hols or by Wittig-type alkenation reactions from carbonyl
precursors.3 However, the stereoselectivity in these
reactions is usually low.4 We report here the stereose-
lective synthesis of allyl vinyl ethers from silyl enol ethers
through mixed iodo acetals.
Mixed iodo acetals were very easily prepared stereo-
selectively5 by the addition of a solution of silyl enol ether
in dichloromethane to a stirred heterogeneous mixture
of N-iodosuccinimide (NIS) and a primary or secondary
allylic alcohol in dichloromethane at -78 °C.6,7 While
(Z)-1-(tert-butyldimethylsiloxy)-1-decene (1b)8 selectively
gave erythro-1-(allyloxy)-1-(tert-butyldimethylsiloxy)-2-
iododecane (2bD) (erythro/threo ) 99/1)9 in 94% yield, (E)-
silyl enol ether 1c8 selectively gave threo-1-(allyloxy)-1-
siloxy-2-iododecane (2cD) (threo/erythro ) 96/4) in 77%
yield upon treatment with (E)-2-hexen-1-ol and NIS.
Even the tertiary allylic alcohol linalool gave 2a C in 42%
yield by this method. The results are summarized in
The solvent plays a critical role in the distribution of
the products. A complete change in the course of the
reactions was observed when hexane was used as a
solvent in place of DME. For instance, treatment of
erythro 2bD with butyllithium in hexane at -78 °C gave
1-(tert-butyldimethylsiloxy)-1-decene 1c as the sole prod-
uct with high stereoselectivity (E/Z ) >99/<1). A trace
of allyl vinyl ether 3bD (<5%) could be detected in the
(10) A typical procedure is as follows. Butyllithium (1.6 M hexane
solution, 1.5 mL, 2.4 mmol) was added to a solution of 2a A (0.42 g, 1.0
mmol) in DME (6 mL) at -78 °C. After being stirred for 0.5 h, the
resulting mixture was poured into saturated aqueous NaHCO3 and
extracted with hexane (10 mL × 2). The combined organic layers were
dried over Na2SO4 and concentrated in vacuo. The residual oil was
applied to an alumina (ICN alumina B activity III) column to give 3a A
(0.11 g) at a yield of 66% along with (tert-butyldimethylsiloxy)ethene
(8%).
(11) The conversion of halo ether or halo acetal to alkene or alkenyl
ether with n-BuLi or t-BuLi has been reported in the total synthesis
of natural products. Wender, P. A.; Keenan, R. M.; Lee, H. Y. J . Am.
Chem. Soc. 1987, 109, 4390. Ireland, R. E.; Ha¨bich, D.; Norbech, D.
W. J . Am. Chem. Soc. 1985, 107, 3271.
(12) It is not clear why the siloxy group was eliminated prior to the
allyloxy group. Selective elimination of the siloxy group may have
occurred because silanol has a higher acidity than alcohols. Bassindale,
A. R.; Taylor, P. G. In The Chemistry of Organic Silicon Compounds;
Patai, S., Rappoport, Z., Eds.; J ohn Wiley & Sons, Ltd.: New York,
1989; Part 1, Chapter 12, pp 809-838.
(13) We applied this new approach to the diastereoselective forma-
tion of adjacent quaternary carbon atoms. The organoaluminium-
promoted Claisen rearrangement14 of these ethers has been examined.
For instance, treatment of a solution of 3fE or 3gE in dichloromethane
with i-Bu3Al at 25 °C gave the corresponding 4-alken-1-ol with
contiguous quaternary carbon centers.15
(1) Recent reviews: (a) Bartlett, P. A. Tetrahedron 1980, 36, 3. (b)
Gajewski, J . J . Hydrocarbon Thermal Isomerizations; Academic
Press: New York, 1981. (c) Lutz, R. P. Chem. Rev. 1984, 84, 205. (d)
Hill, R. K. In Asymmetric Synthesis; Morrison, J . D., Ed.; Academic
Press: New York, 1984; Vol. 3, p 503. (e) Murray, A. W. Org. React.
Mech. 1986, 429; 1987, 457. (f) Moody, C. J . Adv. Heterocycl. Chem.
1987, 42, 203. (g) Ziegler, F. E. Chem. Rev. 1988, 88, 1423. (h)
Kallmerten, J .; Wittman, M. D. Stud. Nat. Prod. Chem. 1989, 3, 233.
(i) Blechert, S. Synthesis 1989, 71. (j) Wipf, P. In Comprehensive
Organic Synthesis; Trost, B. M., Fleming, I., Paquette, L. A., Eds.;
Pergamon Press: New York, 1991; Vol. 5, Chapter 7.2, p 827.
(2) Sugihara, M.; Yanagisawa, M.; Nakai, T. Synlett 1995, 447.
Ziegler, F. E. Acc. Chem. Res. 1977, 10, 227.
(3) The Tebbe reagent has been used to prepare allyl vinyl ether.
Kinney, W. A.; Coghlan, M. J .; Paquette, L. A. J . Am. Chem. Soc. 1985,
107, 7352.
(4) The selective preparation of (Z)-alkenyl ethers with a RCHBr2-
Zn-TiCl4 system has been reported previously. Okazoe, T.; Takai, K.;
Oshima, K.; Utimoto, K. J . Org. Chem. 1987, 52, 4410.
(5) Thiem, J .; Karl, H.; Schwentner, J . Synthesis 1978, 696.
(6) For a related procedure, see: Stork, G.; Sher, P. M. J . Am. Chem.
Soc. 1986, 108, 303.
(7) The use of (trimethylsiloxy)ethene in place of (tert-butyldimeth-
ylsiloxy)ethene gave an R-iodocarbonyl compound upon treatment with
NIS in the presence of alcohols. Reaction of trimethylsilyl enol ethers
with NCS has been reported to give R-chloro ketones. Hambly, G. F.;
Chan, T. H. Tetrahedron Lett. 1986, 27, 2563.
(8) The stereoselective synthesis of silyl enol ethers has been studied
extensively. Brownbridge, P. Synthesis 1983, 1. Silyl enol ether 1a was
prepared according to the reported procedure. J ung, M. E.; Blum, R.
B. Tetrahedron Lett. 1977, 3791. (Z)-Isomer 1b was generated from
decanal with t-BuMe2SiOTf and triethylamine. Mander, L. N.; Sethi,
S. P. Tetrahedron Lett. 1984, 25, 5953. (E)-Isomer 1c was generated
with t-BuMe2SiCl and DBU. Taniguchi, Y.; Inanaga, J .; Yamaguchi,
M. Bull. Chem. Soc. J pn. 1981, 54, 3229.
(14) Takai, K.; Mori, I.; Oshima, K.; Nozaki, H. Bull. Chem. Soc.
J pn. 1984, 57 446. Nonoshita, K.; Banno, H.; Maruoka, K; Yamamoto,
H. J . Am. Chem, Soc. 1990, 112, 316. Maruoka, K.; Saito, S.;
Yamamoto, H. J . Am. Chem. Soc. 1995, 117, 1165.
(15) Gilbert, J . C.; Kelley, T. A. Tetrahedron 1988, 44, 7587.
(16) The yield of the product could not be determined because of its
volatility.
(9) For the nomenclature of threo and erythro, see: Noyori, R.;
Nishida, H. J . Am. Chem. Soc. 1981, 103, 2106.
0022-3263/96/1961-2262$12.00/0 © 1996 American Chemical Society

Communications
J . Org. Chem., Vol. 61, No. 7, 1996 2263
Ta ble 1. P r ep a r a tion a n d Rea ction of Mixed Iod o Aceta ls
silyl enol ether 1
R1
acetal 2
yield (%)
alkenyl ether 3
R2
allylic alcohol (ROH)
erythro/threo
yield (%)
E/Z
1a
1a
1a
1b
1c
1d
1e
1b
1b
1f
H
H
H
H
H
H
H
A
B
C
D
D
E
E
B
F
2a A
2a B
2a C
2bD
2cD
2d E
2eE
2bB
2bF
2fE
92
66
42
94
77
89
69
65
91
79
79
3a A
3a B
3a C
3bD
3cD
3d E
3eE
3bB
3bF
3fE
66
92
60
76
81a
86
77a
76
74
94
88
n-C8H17
H
CH3
99/1
4/96
99/1
15/85
99/1
99/1
98/2
4/96
96/4
4/96
92/8
24/76
91/9
91/9
94/6
8/92
n-C8H17
H
CH3
H
H
CH3
C2H5
H
n-C8H17
n-C8H17
C2H5
CH3
E
E
1g
2gE
3gE
a
HMPA (4 equiv to n-BuLi) was used as a cosolvent. See text.
Sch em e 1
Sch em e 2
of threo 2cD with butyllithium gives (E)-1-(allyloxy)-1-
alkene 3bD selectively through transition state A. Using
the same argument, erythro 2bD exclusively gives (E)-
1-(siloxy)-1-alkene 1c. On the other hand, in DME,
1-(allyloxy)-1-siloxy-2-iodo species 2b D and 2cD could
undergo the stereoselective anti elimination of the siloxy
and iodine moieties (Figure 1, C). In polar solvents such
as DME and DME-HMPA, lithium is surrounded by
solvents and cannot be coordinated by the siloxy or
allyloxy group, as shown in A and B.
F igu r e 1.
reaction mixture. Thus, (Z)-1-(tert-butyldimethylsiloxy)-
1-decene (1b) could be converted exclusively into the (E)-
isomer 1c through 1-(allyloxy)-1-(tert-butyldimethylsiloxy)-
2-iododecane (2bD). On the other hand, threo 2cD
afforded (E)-1-(allyloxy)-1-decene (3bD) exclusively with-
out contamination by siloxy-1-decene 1c upon treatment
with n-BuLi in hexane. Therefore, both (E)-3bD and (Z)-
3cD could be prepared selectively by changing solvents
from the same mixed iodo acetal 2cD (Scheme 1).
We are tempted to assume the following reaction
mechanism. In hexane, the reaction might proceed via
the syn periplanar transition state of an E2 reaction
(Figure 1, A or B, 2cD: R ) n-C8H17, R′ ) n-PrCHdCH-
CH2). Coordination of the siloxy group to lithium triggers
the attack of iodine by the butyl anion. Severe steric
repulsion between the siloxy group and the alkyl group
makes transition state Β less favorable. Thus, treatment
In conclusion, (1) in DME, stereospecific anti-elimina-
tion of the siloxy and iodine moieties proceeded. Thus,
employment of erythro mixed acetal led to the formation
of (E)-alkenyl ether. Conversely, reaction of threo isomer
gave the (Z)-alkenyl ether. (2) In hexane, syn-elimination
took place preferentially. Elimination of the elements
of alcohol proceeded to give (E)-silyl enol ether from
erythro mixed acetal, while reaction of threo isomer
afforded only the (E)-alkenyl ether (Scheme 2).
Su p p or tin g In for m a tion Ava ila ble: Characterization
data (4 pages).
J O960289W