Coupling of silyl ether analogue 31B and carbene complex
1 afforded only silyl ether 32 as a 2:1 mixture of diastere-
omers (unassigned).13
for the synthesis of 4-alkylidenecyclopentenones; however,
the synthesis of 5-alkylidenecyclopentenones by this method
is not viable due to competing net hydrogenation of the
exocyclic double bond and/or unusual reaction processes
resulting from oxygen coordination. The leaving groups
required for the elimination step of the 4-alkylidenecyclo-
pentenone synthesis vary depending upon the stability of the
alkylidene substituent; however, propargylic acetates appear
to uniformly undergo the critical â-elimination event. The
synthesis of rare 4,5-dialkylidene-2-cyclopentenones can be
effected by coupling of cyclopropylcarbene complexes with
enyne-propargyl alcohol derivatives.
The coupling of propargyl alcohol derivatives with cy-
clopropylcarbene complexes appears to be a general method
(9) Procedure. To a 99/1 mixture of toluene/water (30 mL) at reflux were
added a solution of carbene complex 1 (136 mg, 0.49 mmol) and
phenylpropargyl acetate (19B) (85 mg, 0.49 mmol) in toluene (20 mL) over
a 2 h period. After the addition was complete, the solution was refluxed an
additional 24 h and then cooled to room temperature. The resulting green
suspension was filtered through Celite, and the solvent was removed on a
rotary evaporator. Final purification was achieved by flash chromatography
on silica gel using 4/1 hexane/ethyl acetate as the eluent. A single fraction
(64 mg, 65%) identified as alkylidenecyclopentenone 20 was obtained. 1H
NMR (CDCl3): δ 7.28 (m, 5 H), 5.77 (d, 1 H, J ) 1.6 Hz), 5.57 (d, 1 H,
J ) 1.6 Hz), 5.13 (t, 1 H, J ) 1.6 Hz), 4.13 (t, 1 H, J ) 1.6 Hz), 3.96 (s,
3 H). 13C NMR (CDCl3): δ 200.9, 181.2, 143.3, 137.4, 128.5, 128.2, 127.0,
111.5, 105.4, 58.4, 55.4. IR (neat): 1695, 1574 cm-1. Anal. Calcd for
C13H12O2: C, 78.02; H, 6.04. Found: C, 77.85; H, 6.02. For the preparation
of carbene complex 1, see ref 2.
Acknowledgment. We thank the donors of the Petroleum
Research Fund, administered by the American Chemical
Society, and New Mexico State University for financial
support of this research.
(10) Kascheres, A.; Kascheres, C.; Braga, A. C. H. J. Org. Chem. 1993,
58, 1702-1703.
Supporting Information Available: Complete experi-
mental procedures for the synthesis of carbene complexes
11A and 11B, 16C, and 22 and complete experimental
procedures for successful alkylidenecyclopentenone synthe-
ses in Schemes 3-6 and the reactions in Schemes 7 and 8.
This material is available free of charge via the Internet at
(11) (a) Semmelhack, M. F.; Jeong, N.; Lee, G. R. Tetrahedron Lett.
1990, 31, 609-610. (b) For possibly related studies involving molybdenum,
see: Harvey, D. F.; Lund, K. P.; Neil, D. A. J. Am. Chem. Soc. 1992, 112,
8424-8434. (c) In some cases, the propargyl oxygen substituent does not
affect the annulation process. Hsung, R. P.; Wulff, W. D. J. Am. Chem.
Soc. 1994, 116, 6449-6450.
(12) Pulley, S. R.; Carey, J. P. J. Org. Chem. 1998, 63, 5275-5279.
(13) Anhydrous solvent was used to suppress the direct protonation-
net reduction of the cyclopentadienide intermediate. A low yield is
anticipated under these conditions (see refs 1 and 2).
OL990002S
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Org. Lett., Vol. 1, No. 1, 1999