reduction of the corresponding acyl chloride) with p-toluidine
(90%). FVP of 8 at 650 °C gave a quantitative yield of a single
rearranged pyridone 9 (Scheme 3) whose structure was
confirmed by X-ray crystallography‡ (Fig. 2). Assuming that
there is little significant difference in the thermodynamic
properties of the acetyl and aldimine substituents, the pyridone
ring system appears to be overwhelmingly favoured thermo-
dynamically with respect to the related pyrone.
relative to external nitromethane). Reversing this sequence gave
an authentic sample of the labelled pyridone 11 (dN 2198.19).
FVP of 10 at 750 °C, conditions under which the pyrones were
completely equilibrated, gave just 4% of the automerised
species 11 as shown by the small peak at dN 2198.19 in the 15
N
NMR spectrum of the pyrolysate (Scheme 4). This result
nevertheless establishes for the first time that thermal electro-
cyclic ring opening of the 2-pyridone system is a viable, albeit
high energy, process. The rearrangement was much more
pronounced (37%) at 850 °C, but the 15N NMR spectrum was
confused by many other peaks due to decomposition prod-
ucts.
Ar
Ar
O
N
N
Me
N
Me
O
Ar
•
H
H
We are grateful to British Petroleum International Ltd. for a
Research Studentship (to G. A. C.) and to Dr D. Reed for the 15
N
H
Me
•
Me
O
8
O
Me
NMR spectra, which were obtained through the S.E.R.C. (now
E.P.S.R.C.) supported University of Edinburgh high-field NMR
service.
O
Me
O
O
Ar
Footnotes
N
Ar =
* E-mail: h.mcnab@ed.ac.uk
† All new compounds were characterised by their spectra and by elemental
analyses, unless otherwise stated.
Me
Me
Me
O
‡ Crystal data for 9. C15H15NO2, M = 241.28, orthorhombic, space group
P212121, a = 5.557(4), b = 9.645(4), c = 24.272(10) Å, V = 1300.9 Å3
(from setting angles for 10 h0l and 10 0kl data, 2q = 8–24°, l = 0.71073
Å), Z = 4, Dcalc = 1.232 g cm 23, T = 295K, colourless needle, 0.85 3 0.2
9
Scheme 3
3 0.08 mm, m
= = 512. Data collection and
0.077 mm21, F(000)
processing—Stoe¨ STADI-2 2-circle diffractometer, graphite monochro-
mated Mo-Ka X-radiation, T = 295K, w-scans with w-range (2.0 + 0.5
sinm/tanq)°, 1039 unique reflections (2qmax 50°, 0 @ h @ 6, 0 @ k @ 10, 0
@ l @ 26) giving 547 with F ! 4s(F) for use in all calculations. No
significant crystal decay or movement was apparent, and no absorption
correction was made. Structure solution and refinement—automatic direct
methods9 located all non-hydrogen atoms which were then refined
anisotropically; hydrogen atoms were inserted in calculated positions with
fixed atomic displacement parameters of U = 0.05 Å2 (C–H = 1.08 Å).
Only the torsion angles of the methyl groups were refined. At final
convergence, R, Rw = 0.075, 0.026 respectively, S = 1.12 for 173 refined
parameters and the final difference synthesis showed no peak or trough
outside ±0.3 e Å23. An extinction parameter was refined, converging to 4.0
3 1024. No absorption corrections were made. The weighting scheme
C(52)
O(51)
C(51)
C(16)
C(15) C(17)
C(14)
C(6)
C(5)
N(1)
C(41)
C(11)
C(2)
C(13)
C(12)
C(4)
C(3)
O(2)
2
w21 = s (F) gave satisfactory agreement analyses; in the final cycle, the
Fig. 2 Thermal ellipsoid plot of 9 showing the crystallographic numbering
maximum d/s was 0.07. Inlaid10 atomic scattering factors were used,
molecular geometry calculations utilised CALC,11 and Fig. 2 was produced
by SHELXTL.12 Atomic coordinates, bond lengths and angles, and thermal
parameters have been deposited at the Cambridge Crystallographic Data
Centre (CCDC). See Information for Authors, Issue No. 1. Any request to
the CCDC for this material should quote the full literature citation and the
reference number 182/477.
scheme
In contrast to the situation with pyran-2-one and thiopyran-
2-one systems,3 very little is known of the thermal behaviour of
2-pyridones except for some fragmentation reactions under
forcing conditions.8 It is clear from the above result that
2-pyridones are unlikely to ring open under such mild
conditions as their O-analogues, but we were interested to
determine the general feasibility of this process. Accordingly,
we reacted p-toluidine with 2-oxopyran-5-carbaldehyde and
condensed the resulting oxopyridine carbaldehyde (56%) with
15N-labelled p-toluidine to give the imine 10 (96%) (dN 263.76
References
1 C. Wentrup, W. Heilmayer and G. Kollenz, Synthesis, 1994, 1219.
2 C. Wentrup and K.-P. Netsch, Angew. Chem., Int. Ed. Engl., 1984, 23,
802.
3 W. H. Pirkle and W. V. Turner, J. Org. Chem., 1975, 40, 1617.
4 C. L. Hickson and H. McNab, J. Chem. Res. (S), 1989, 176.
5 J. T. Kurek and G. Vogel, J. Heterocycl. Chem., 1968, 5, 275.
6 J. Fried and R. C. Elderfield, J. Org. Chem., 1941, 6, 577.
7 For example, W. H. Pirkle and M. Dines, J. Heterocycl. Chem., 1969, 6,
1.
8 D. A. Brent, J. D. Hribar and D. C. DeJongh, J. Org. Chem., 1970, 35,
135.
9 SHELX86, program for crystal structure solution, G. M. Sheldrick,
University of Go¨ttingen, F.R.G., 1986.
Ar
Ar
*
N
*
O
N
N
Ar
•
*
H
H
H
•
N
O
N
O
NAr
Ar
Ar
10
10 SHELX76, program for crystal structure refinement, G. M. Sheldrick,
University of Cambridge, England, 1976.
11 CALC, program for molecular geometry calculations, R. O. Gould and
P. Taylor, University of Edinburgh, Scotland, 1985.
12 SHELXTL/PC, Version 4.3, Siemens Analytical X-ray Instruments
Inc., Madison, Wisconsin, USA, 1992.
O
Ar
*
N
Ar =
Me
NAr
11
Scheme 4
Received in Cambridge, UK, 8th April 1997; Com. 7/02395B
1294
Chem. Commun., 1997