B. W. Freer et al. / Tetrahedron 59 (2003) 7385–7388
7387
Scheme 2.
isolating a very small quantity of a product C H O which
2
3.1. Reaction of desoxybenzoin with acetophenone
9 24 2
we identified as the diketone 3 by comparison with an
3
authentic specimen prepared from desoxybenzoin 4 and
1
3.1.1. In the presence of oxygen. A solution of potassium
hydroxide (2 g, 35.7 mmol) in ethanol (100 ml) was added
to a solution of desoxybenzoin (5 g, 25.5 mmol) and
acetophenone (3 g, 25.0 mmol) in the same solvent
(50 ml) and the mixture was kept for one week at room
temperature with free access of air and occasional stirring.
The resulting solid was collected, washed with ethanol and
then with water to remove potassium compounds and
crystallised from ethanol to give racemic (2R,4R)-, (2S,4S)-
3-benzoyl-1,2,4,5-tetraphenylpentane-1,5-dione 1a and 1b
(1.516 g, 2.98 mmol; 23%) as colourless monoclinic
,3-diphenylpropenone 10.
The formation of the above products appears to be a
consequence of the initial ready autoxidation of desoxy-
benzoin 4 in alkaline solution (Scheme 1) to form the
alkylperoxide ion 5 which can lose hydroxide with the
production of benzil 6. Reaction of this with acetophenone
gives the enedione 7 which finally undergoes conjugate
addition of desoxybenzoin 4 to form the triketone 1. The
alternative mode of decomposition of the alkylperoxide ion
2
5
, possibly via the dioxetan 8 shown in Scheme 2, provides
crystals mp 198–1998C (lit., 199–2008C) [Found: C,
þ
benzaldehyde 9 and a benzoate ion. Reaction of the former
with acetophenone produces the enone 10 which undergoes
conjugate addition of desoxybenzoin to give the diketone 3.
Support for these pathways comes from the high (75%)
yield of the triketone 1 which results from the alkali-induced
reaction of benzil with acetophenone and desoxybenzoin in
85.15; H, 5.52%, (MþH) (ES), 509.0. C H O requires
3
6 28 3
þ
490.1953. C H O requires M, 490.1953]; m/z 490
C, 85.01; H, 5.55%, (MþH), 509.2. Found: (M2H O) ,
2
3
6 26 2
(0.2%, M2H O), 403 (2, M2PhCO), 386 (3, 403-OH),
2
313 (9, M2PhCHCOPh), 300 (7, M2C H O), 105 (100,
1
5 12
PhCO), and 77 (23, Ph); lmax/nm 251 (log 1 4.538);
21
3
the absence of oxygen and from the ready reaction of the
enone 10 with desoxybenzoin under similar conditions to
give the diketone 3.
nmax/cm
1670 (aromatic CvO); dH 5.00 (1H, d,
J¼10 Hz, CHCHPh), 5.44 (1H, d, J¼10 Hz, CHCHPh),
5.83 (1H, dd, J¼10, 10 Hz, CHCHCH), 6.80–7.13 (8H, m,
ArH), 7.13–7.53 (13H, m, ArH) and 7.73–7.91 (4H, m,
ArH). Acidification of the aqueous washings afforded
benzoic acid (1.69 g, 8.2 mmol), mp 1218C, identical with
an authentic specimen.
The shape of the triketone 1 coupled with its ease of
preparation make it of potential interest as a hub in the
formation of discotic liquid crystals.
Concentration of the ethanolic mother-liquors gave a solid
which on being subjected to repeated TLC using chloroform
3
. Experimental
gave 1,2,3,5-tetraphenylpentane-1,5-dione
0.04 mmol) identical (MS, NMR) with an authentic speci-
3 (5 mg,
NMR spectra were measured in CDCl at 400 MHz on a
3
3
þ
Varian Unity Inova or at 250 MHz on an A. C. Bruker 250
instrument using, respectively, residual CHCl (7.27 ppm)
men mp 194–1968C (lit., 1898C) [Found: M (EI), 404.
þ
Calcd for C29
405.6. Calcd for C29
284 (4, M2PhCOCH
PhCHvCHPh), 105 (100, PhCO) and 77 (25, Ph); d
(1H, dd, J¼3.5, 16.4 Hz) and 3.10 (1H, dd, J¼10.2,
16.4 Hz) (both CHCH
CO), 4.26 (1H, ddd, J¼3.5, 10.2,
10.8 Hz, CHCHCH
), 5.05 (1H, d, J¼10.8 Hz,
PhCOCHCH), 6.88–7.36 (14H, m, ArH), 7.36–7.42 (2H,
H
24
O
: M, 404.2. Found: (MþH) (ES),
3
2
and tetramethylsilane as internal standards. FTIR spectra
were measured with an ATI Mattson Genesis spectropho-
tometer for KBr discs and UV absorption spectra with a
Perkin–Elmer Lambda 15 for methanolic solutions. Mass
spectra were obtained by EI at 70 eV with an AEI MS30
spectrometer or by ES using a Finnigan Navigator. TLC was
H
25
O
2
: M, 405.2]; m/z 404 (0.4%, M),
), 196 (13, PhCOCH Ph), 180 (25,
2.92
3
2
H
2
2
performed using Merck Kieselgel GF254
.