736
J . Org. Chem. 1997, 62, 736
in turn, dictated by use of a bulky silylating agent. Thus,
Sh or t P r ep a r a tion of
diene 1 was prepared4 by heating a solution of croton-
aldehyde, triethylamine, and tert-butyldimethylsilyl tri-
flate in dichloromethane for 36 h at reflux. The product
was obtained in 91% yield as a single isomer after
distillation. The enolate was generated from 1 over a 6
h period in DME. Replacing DME with THF gave only
a small amount of the enolate even at long reaction times
(24 h). Enolate acylation proceeded smoothly when
freshly distilled acid chloride was used. Diene 3 was
obtained as a 97:3 mixture of E:Z isomers (GC) in 63%
overall yield from crotonaldehyde. HPLC with a chiral
solid phase5 showed only a trace amount of the enanti-
omer (>99% ee). A detailed experimental follows.
(S)-(E)-1-(O-Meth ylm a n d eloxy)bu ta d ien e
Barry M. Trost,* Louis S. Chupak, and Thomas Lu¨bbers
Department of Chemistry, Stanford University,
Stanford, California 94305
Received September 20, 1996
Diene 3 is a synthetically useful and theoretically
interesting compound. Synthetically, high levels of dia-
stereoselectivity have been observed in Diels-Alder
reactions using 3.1 Thus, 3 provides an alternative to
the more common motif of placing the chiral auxiliary
in the dienophile. The ability to control asymmetry
occurs despite the flexibility of the ester linkage and the
distance between the forming and existing chiral centers.
These characteristics have prompted theoretical studies2
to probe the roles and relative importance of the ester
conformation and π-stacking in the transition state.
Such studies, along with empirical observations, have led
to a new model for asymmetric induction that does not
invoke π-π interactions between the phenyl and dienyl
groups.
Exp er im en ta l Section
(E)-1-(ter t-Bu t yld im et h ylsiloxy)b u t a d ien e (1). Freshly
distilled crotonaldehyde (5.03 g, 71.8 mmol) and triethylamine
(9.95 g,98.3 mmol) were dissolved in dichloromethane (30 mL).
To this solution was added tert-butyldimethylsilyl triflate (15
mL, 65.3 mmoL) dropwise at 0 °C. The resulting red solution
was heated at reflux for 36 h. The room temperature solution
was diluted with Et2O (120 mL) and extracted with cold NaHCO3
(saturated aqueous, 2 × 50 mL) and brine (5 mL). The organic
phase was dried over MgSO4 and the solvent removed in vacuo.
The residue was distilled (12 mmHg, 76-80 °C) to give the title
compound as a colorless oil (10.96 g, 59.5 mmol, 91%).
(S)-O-Meth ylm a n d eloyl Ch lor id e (2). Thionyl chloride (15
mL) was distilled directly into a flask containing (S)-methoxy-
phenylacetic acid (3.0 g, 18 mmol). The resulting solution was
aged at room temperature for 1 h, and the volatiles were
removed in vacuo. Kugelrohr distillation (∼5 mmHg, 110 °C)
of the residue gave the title compound (2.4 g, 72%) as a colorless
oil (stored at 4 °C and used within 12 h after distillation).
(S)-(E)-1-(O-Meth ylm a n d eloxy)bu ta d ien e (3). To (E)-1-
(tert-Butyldimethylsiloxy)butadiene (1) (0.54 g, 2.9 mmol) in
DME (5 mL) at room temperature was added methyllithium in
ether (1.4 M, 2.1 mL, 2.9 mmol). The solution was stirred for 6
h or until more than 90% of the enol ether was consumed (aliquot
quenched and GC with tetradecane as internal standard). The
cloudy solution was transferred via cannula to a 0 °C THF (2
mL) solution of acid chloride (0.88 g, 4.8 mmol) and warmed to
room temperature. The resulting clear solution became cloudy
after being stirred 5 min. After 15 min, water (10 mL) and ether
(30 mL) were added. The phases were separated, and the
organic layer was extracted with NaOH (10% aqueous, 3 × 10
mL) and brine (1 × 10 mL). The organic phase was dried over
MgSO4, and the solvent was removed in vacuo. The resulting
yellow oil was chromatographed with EtOAc:hexane (3:97) to
give the title compound as a colorless oil (0.44 g, 2.0 mmol, 69%),
Despite the usefulness of diene 3, a simple preparation
has not been available. Any useful preparation of diene
3 must control the olefin geometry and proceed without
racemization of the mandelate. The existing route1
proceeds in seven steps from the cycloadduct of cyclo-
pentadiene and maleic anhydride. The length of this
preparation is probably responsible for the lack of routine
application of diene 3. Here we present a simple two-
step synthesis of 3 from crotonaldehyde with complete
control of stereochemistry.
It was recognized that lithium enolates can be gener-
ated by the action of methyllithium on the corresponding
silyl enol ethers without loss of the olefin geometry.3
Quenching the enolate corresponding to 1 with acid
chloride 2 would provide diene 3 with retention of the
enol ether geometry. The silyl enol ether geometry is,
[R]28 +7.91° (c ) 2.17, CH2Cl2).5
D
Ack n ow led gm en t. We thank the National Science
Foundation for their generous support of our programs.
J O9618031
(1) (a) Trost, B. M.; O’Krongly, D.; Belletire, J . L. J . Am. Chem. Soc.
1980, 102, 7595-7596. (b) Siegel, C.; Thornton, E. R. Tetrahedron:
Asymmetry 1991, 2, 1413-1428.
(4) (a) Prepared according to the following modified method: Mander,
L. N.; Sethi, S. P. Tetrahedron Lett. 1984, 25, 5953-5956. (b) Full
characterization: Kozlowski, M. C.; Bartlett, P. A. J . Am. Chem. Soc.
1991, 113, 5897-5898 (supplementary material).
(5) Chiralpak AD column using 99.9:0.1 heptane-2-propanol as
eluent. Note that the observed rotation is significantly lower than the
reported rotation (see ref 1a), even though the chiral HPLC column
indicates >99% ee. We have no explanation for this discrepancy.
(2) (a) Maddaluno, J . F.; Gresh, N.; Giessner-Prettre, C. J . Org.
Chem. 1994, 59, 793-802. (b) Tucker, J . A.; Houk, K. N.; Trost, B. M.
J . Am. Chem. Soc. 1990, 112, 5465-5471. (c) Siegel, C.; Thornton, E.
R. Tetrahedron Lett. 1988, 29, 5225-5228.
(3) (a) House, H. O.; Czuba, L. J .; Gall, M.; Olmstead, H. D. J . Org.
Chem. 1969, 34, 2324-2336. (b) Stork, G.; Hudrlik, P. F. J . Am. Chem.
Soc. 1968, 90, 4464-4465.
S0022-3263(96)01803-8 CCC: $14.00 © 1997 American Chemical Society
Trost, Barry M.
