SHORT PAPER
1737
A Direct Preparation of 6-Methylene-2-cyclohexenones
D
i rec
t
Preparat ion
e
of 6-Methy
o
lene-2-cyclo
r
hexenon
g
e
s
e A. Kraus,* Junwon Kim
Department of Chemistry, Iowa State University, Ames, IA 50011, USA
E-mail: gakraus@iastate.edu
Received 10 June 2003; revised 5 February 2004
After stirring for 24 h at r.t., the reaction mixture was diluted with
CH2Cl2 (5 mL) and 1 N HCl (6 mL). After 10 min at r.t., a 2 N so-
lution of NaOH was slowly added to the aqueous phase (until pH
14). The aqueous phase was extracted with CH2Cl2 (4 × 10 mL) and
the combined organic layers were dried (MgSO4) and concentrated
in vacuo to give amine 2, which was dissolved in anhyd MeOH (1
M solution). MeI (1.75mmol) was added slowly with stirring at 0
°C. The resulting solution was warmed to r.t. and stirred for 1 h to
give the ammonium salt as a white precipitate. After removing the
solvent and the excess MeI, the crude salt was dissolved in H2O (10
mL) and Et2O (10 mL). To the solution of salt was added NaHCO3
Abstract: The reaction of enol silyl ethers with Eschenmoser’s salt,
followed by methylation and elimination, provides ready access to
6-methylene-2-cyclohexenones.
Key words: Eschenmoser’s salt, 6-methylene-2-cyclohexenones,
enol silyl ethers
In the course of studying tandem reactions on dienones,
we needed substituted 6-methylene cyclohexenones 1a–
c.1 There are few literature reports of compounds of this (4.5 mmol) and the reaction was stirred vigorously for 2 h at r.t. The
aqueous layer was extracted with Et2O (3 × 20 mL) and the com-
bined organic layers were dried (MgSO4) and concentrated in vac-
uo. Further purification by short path column chromatography on
silica gel (hexane–Et2O = 10:1–5:1) gave compound 1 as a colorless
volatile oil.
class. Wiberg and coworkers prepared 1c by the reaction
of ketene with 1,4-bicyclo[2.2.0]hexene.2 Reddy reported
the preparation of 1b as a by-product in a transition metal
mediated rearrangement.3
Initially, we tried both the Gras methylenation reaction4
and the McMurry methylenation reaction5 with cyclohex-
enone. Although we had employed the latter reaction suc-
cessfully with substituted cyclohexanones, neither
reaction provided 1a. We eventually discovered that the
reaction of enol silyl ethers of cyclohexenone with Es-
chenmoser’s salt6 followed by methylation and elimina-
tion afforded good overall yields of the desired products.
The overall yields from 3a, 3b,7 and 3c were 65%, 54%
and 49%, respectively (Scheme 1).
6-Methylene Cyclohexenone (1a)
Rf = 0.35 (hexane–Et2O, 2:1).
1H NMR (400 MHz, CDCl3): d = 7.06 (dt, J = 10, 4 Hz, 1 H), 6.15
(d, J = 10 Hz, 1 H), 5.96 (d, J = 1.2 Hz, 1 H), 5.29 (d, J = 1.2 Hz, 1
H), 2.75 (t, J = 6.4 Hz, 2 H), 2.46 (m, 2 H).
13C NMR (100 MHz, CDCl3): d = 188.7, 150.7, 142.9, 130.5, 120.1,
31.0, 26.4.
HRMS (EI): m/z [M]+ calcd for C7H8O: 108.0575; found: 108.0577.
2-Methyl-6-methylene Cyclohexenone (1b)
Dienones 1a–c can be prepared in a one-pot reaction from
the corresponding enol silyl ethers.8 The ready availability
of these novel cross-conjugated dienones will make pos-
sible new pathways to complex bridged- and spiro sys-
tems.
Rf = 0.62 (hexane–Et2O, 2:1).
1H NMR (400 MHz, CDCl3): d = 6.81 (br s, 1 H), 5.95 (s, 1 H), 5.25
(d, J = 1.6 Hz, 1 H), 2.71 (t, J = 6.4 Hz, 2 H), 2.40 (dd, J = 4.4, 2
Hz, 2 H), 1.85 (d, J = 0.8 Hz, 3 H).
13C NMR (100 MHz, CDCl3): d = 189.5, 145.8, 143.5, 136.9, 119.7,
31.7, 26.3, 16.4.
HRMS (EI): m/z [M]+ calcd for C8H10O: 122.0732; found:
122.0773.
General Experimental Procedure
To a cooled solution (0 °C) of Eschenmoser’s salt (1.3 mmol) in
CH2Cl2 (0.4 M solution) was added the silyl enol ether (1 mmol).
Scheme 1
SYNTHESIS 2004, No. 11, pp 1737–1738
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Advanced online publication: 22.07.2004
DOI: 10.1055/s-2004-829162; Art ID: M02803SS
© Georg Thieme Verlag Stuttgart · New York
Kraus, George A.
