Addition of Acyclic 1,2-Diones to Acetylenic Esters
FULL PAPER
DMAD (3) (198 mg, 1.3953 mmol) at ꢀ108C. The reaction mixture was
allowed to attain room temperature and was then stirred for 12 h. Re-
moval of the solvent followed by column chromatography as described in
the general procedure afforded product 9 as a viscous yellow liquid
(43 mg, 16%). 1H NMR: d=3.86 (s, 6H), 2.24 ppm (s, 6H); 13C NMR:
d=189.5, 165.1, 134.6, 44.0, 28.5 ppm; IR (KBr): n˜ =2967, 1743, 1682,
bond formation in these reactions. It was also shown that
the reaction works well with pyridine, 4-dimethylaminopyri-
dine (DMAP) and 3-methoxypyridine. A detailed study
with various solvents has shown that the reaction works well
in anhydrous 1,2-dimethoxyethane. It is noteworthy that the
reaction occurs by means of an unprecedented benzoyl/acyl
migration. The structural, electronic, energetic and mecha-
nistic details of the reaction are also unravelled by theoreti-
cal calculations, which strongly supported the exclusive for-
mation of the cis isomer, relative to 1,2-diaroyl maleates.
The negatively charged carbon atom of the transient species,
cis-zwitterion 14, was identified as the reactive centre that
upon coordination to the dione gives the fairly stable inter-
mediate product 16. This means that the transformation of
the transient species 14 to the stable intermediate 16 drives
the reaction in the forward direction. As we can see from
mechanism 1 (Figure 7), once 16 is formed, its decomposi-
tion to 14 and dione is less-favoured relative to the forma-
tion of 17 as the former process is endergonic, whereas the
later one is exergonic. Furthermore, the forward reaction
from 16 is more facile than the backward reaction. The ste-
reochemistry of the product 19 is thus mainly decided at the
beginning, when 16 is formed from the cis zwitterion. Al-
1564, 1441, 1351, 1132 cmꢀ1
228.0634 [M+]; found: 228.0663.
; HRMS (EI): m/z: calcd for C10H12O6:
Di-tert-butyl (2Z)-2,3-dibenzoylbut-2-enedioate (11): Pyridine (20 mol%)
was added to a solution of benzil 6 (100 mg, 0.4762 mmol) and di-tert-
butyl acetylenedicarboxylate 10 (129 mg, 0.5714 mmol) at ꢀ108C. The re-
action mixture was allowed to attain room temperature and was then
stirred for 12 h. Removal of the solvent followed by column chromatog-
raphy afforded product 11 as a crystalline white solid (110 mg, 48%).
M.p.: 145–1478C (recrystallized from
a CH2Cl2/hexane mixture);
1H NMR: d=8.01 (d, J=7.32 Hz, 4H), 7.58–7.51 (m, 6H), 1.14 ppm (s,
18H); 13C NMR: d=191.2, 161.8, 142.2, 136.0, 133.7, 128.9, 128.7,
27.3 ppm; IR (KBr): n˜ =2943, 1751, 1672, 1579, 1443, 1406, 1126 cmꢀ1
HRMS (EI): m/z: calcd for C26H28O6: 436.1886 [M+]; found: 436.1904.
;
Acknowledgements
We thank the Council of Scientific and Industrial Research (CSIR) and
Department of Science and Technology (DST), New Delhi for financial
assistance. We also thank S. Mathew for the spectral data and S. Viji for
all the mass and elemental analysis.
ꢀ
though the rotation of the central C C bond in 18 can give
rise to a trans isomer of 19, the rotation itself requires an ac-
tivation energy of 10.0 kcalmolꢀ1 and, therefore, the pre-
dominant formation of the cis isomer 19 is expected. The
solvent effect plays a major role in the facile formation of
the products as there is a substantial reduction in the activa-
tion barrier in all steps of the reaction, which is mainly at-
tributed to the increased stability of the highly charge-sepa-
rated transition states in the solvent phase.
[1] a) O. Diels, K. Alder, Liebigs Ann. Chem. 1932, 498, 16; b) R. M.
Angew. Chem. 1967, 79, 389; Angew. Chem. Int. Ed. Engl. 1967, 6,
423.
[2] V. Nair, C. Rajesh, A. U. Vinod, S. Bindu, A. R. Sreekanth, J. S.
[7] V. Nair, A. R. Sreekanth, N. Abhilash, A. T. Biju, B. Remadevi,
[8] V. Nair, B. Remadevi, N. Vidya, R. S. Menon, N. Abhilash, N. P.
Experimental Section
General information: Melting points were recorded on a Buchi melting
point apparatus and are uncorrected. NMR spectra were recorded at 300
(1H) and 75 MHz (13C), respectively, on a Bruker Avance DPX-300 MHz
NMR spectrometer. Chemical shifts are reported (d) relative to TMS
(1H) and CDCl3 (13C) as the internal standards. Coupling constants (J)
are reported in hertz (Hz). Mass spectra were recorded under EI/HRMS
(at 5000) resolution by using an Auto Spec. mass spectrometer. IR spec-
tra were recorded on Bomem MB Series FTIR spectrophotometer. Ele-
mental analyses were performed on Perkin–Elmer 2400 Elemental Ana-
lyzer.
[10] Indirect evidence for the cis configuration of the zwitterion was ac-
crued from literature reports,[1,3] which show that the zwitterion 1
adds to another molecule of acetylenic ester to form a second zwit-
terion, which then cyclizes to form 4H-quinolizine. Evidently the ad-
dition product from a transoid zwitterion would not undergo such a
cyclization.
[11] a) J. M. Seminario, P. Politzer, Modern Density Functional Theory. A
Tool for Chemistry, Elsevier, Amsterdam, 1995; b) K. N. Houk in
1,3-Dipolar Cycloaddition Chemistry (Ed:. A. Padwa), Vol. 2, Wiley,
New York, 1984; c) C. F. Christian, T. Takeya, M. J. Szymanski,
Badri, D. Willenbring, Tantillo, D. J. Gervay-Hague, J. Org. Chem.
[12] a) L. R. Domingo, M. Arnꢂ, R. Contreras, P. Pꢃrez, J. Phys. Chem.
A 2002, 106, 952; b) L. R. Domingo, Eur. J. Org. Chem. 2004, 23,
d) M. J. Aurell, L. R. Domingo, P. Pꢃrez, R. Contreras, Tetrahedron
Dimethyl (2Z)-2,3-dibenzoylbut-2-enedioate (7): Pyridine (20 mol%) was
added to a solution of benzil 6 (100 mg, 0.4762 mmol) and DMAD (3)
(82 mg, 0.5714 mmol) at ꢀ108C. The reaction mixture was allowed to
attain room temperature and was then stirred for 8 h. The solvent was
distilled off in vacuo by using a rotary evaporator, followed by column
chromatography as described in the general procedure to afford the
product 7 as a white crystalline solid (101 mg, 62%). M.p.: 138–1408C
1
(recrystallized from a CH2Cl2/hexane mixture); H NMR: d=7.80 (d, J=
7.34 Hz, 4H), 7.56 (t, J=7.46 Hz, 2H), 7.41 (d, J=7.78 Hz, 4H),
3.81 ppm (s, 6H); 13C NMR: d= 189.9, 163.7, 141.6, 135.3, 134.2, 129.5,
128.7, 53.3 ppm; IR (KBr): n˜ =2961, 1754, 1666, 1592, 1448, 1430,
1132 cmꢀ1; elemental analysis calcd (%) for C20H16O6: C 68.18, H 4.58;
found: C 68.32, H 4.52.
Dimethyl (2Z)-2,3-diacetylbut-2-enedioate (9): Pyridine (20 mol%) was
added to a solution of 2,3-butanedione (8) (100 mg, 1.1628 mmol) and
Chem. Eur. J. 2008, 14, 5851 – 5860
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