Methylene meldrums acid derivatives of indoxyl and their cyclization reactions under flash vacuum pyrolysis conditions

Article abstract of DOI:10.1055/s-0029-1218668

Pure indoxyl can be obtained in 75% yield by flash vacuum pyrolysis (FVP) of 2′-azidoacetophenone at 650 °C. Reaction of indoxyl with methoxymethylene Meldrums acid takes place at the 1-position, and FVP of the resulting derivative provides 1-hydroxy-9H-pyrrolo[1,2-a]indol-9-one (54%). FVP of the isomeric 2-methylene compound gives pyrano[3,2-b]indol-2(5H)-one (42%). Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0029-1218668

PAPER
1361
Methylene Meldrum’s Acid Derivatives of Indoxyl and Their Cyclization
Reactions under Flash Vacuum Pyrolysis Conditions
M
ethylene Meldr
u
l
m
’s
A
c
e
id
D
erivati
x
ve
s
of Indox
a
School of Chemistry, The University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, UK
Fax +44(131)6504743; E-mail: H.McNab@ed.ac.uk
Received 27 November 2009
Solution-phase thermolysis of 2¢-azidoacetophenone (4)
is known to give anthranil 5.6 Although it has been report-
ed that further pyrolysis provides indoxyl (3),7a attempts
to replicate this process by spray pyrolysis provided in-
Abstract: Pure indoxyl can be obtained in 75% yield by flash vac-
uum pyrolysis (FVP) of 2¢-azidoacetophenone at 650 °C. Reaction
of indoxyl with methoxymethylene Meldrum’s acid takes place at
the 1-position, and FVP of the resulting derivative provides 1-hy-
droxy-9H-pyrrolo[1,2-a]indol-9-one (54%). FVP of the isomeric 2- doxyl (isolated as indigo) in only 15% yield.7b Under FVP
methylene compound gives pyrano[3,2-b]indol-2(5H)-one (42%).
conditions, decomposition of 4 is essentially complete
around 300 °C and anthranil 5 is indeed the major product
between 300–400 °C (Scheme 1). The temperature profile
(Figure 2) shows that above 400 °C indoxyl 3K begins to
appear, identified by the CH2 resonance (d = 3.89) in its
1H NMR spectrum.4 For preparative purposes, FVP of 4 at
650 °C provided indoxyl (3) in 75% yield, sufficiently
pure for further reactions (Scheme 1).
Key words: gas-phase reactions, cyclisations, heterocycles, pyro-
lysis, ketenes
We report the synthesis and flash vacuum pyrolysis (FVP)
behaviour of the two possible methylene-2,2-dimethyl-
1,3-dioxane-4,6-dione derivatives, 1 and 21 of indoxyl 3
(Figure 1). We have carried out similar reactions in the 3-
hydroxypyrrole series,2 but, in extending the work to the
indole series we were surprised to discover that synthetic
routes to indoxyl (3-hydroxy-1H-indole, 3) are them-
selves poorly defined, perhaps because it is highly prone
to aerial oxidation to indigo.3 Indoxyl (3) is often generat-
ed in situ4 and, indeed, its melting point has been recorded
only five times in the Beilstein database (and two values
are clearly in error). We have, therefore, developed a prac-
tical gas-phase route to indoxyl (3) by FVP of 2¢-azido-
acetophenone (4). A similar strategy was employed for
our recent synthesis of the first heteroindoxyl.5 FVP is an
excellent technique for making such air-sensitive com-
pounds because solvent, workup, and the presence of ox-
ygen during the transformation are all avoided.
O
O
3
O
••
N
••
6
N–
N
+
N
4
N
5
Scheme 1
The formation of indoxyl (3) is likely to involve a CH in-
sertion reaction of 2-acetylphenylnitrene (6), but the an-
thranil 5 may be formed either via the nitrene or by
anchimeric assistance by the acetyl group in the nitrogen
elimination. This issue has been thoroughly explored in
the solution-phase reaction.8
O
OH
O
O
O
O
O
N
O
N
1
H
2
O
O
O
OH
N
N
H
H
3E
3K
Figure 1
Figure 2 Temperature-conversion graph for FVP of 4 (shown in
red) detailing the formation of 5 (kinetic product; shown in black) and
3 (thermodynamic product; shown in blue)
SYNTHESIS 2010, No. 8, pp 1361–1364
Advanced online publication: 05.02.2010
DOI: 10.1055/s-0029-1218668; Art ID: P16209SS
x
x
.
x
x
.2
0
1
0
© Georg Thieme Verlag Stuttgart · New York

1362
A. P. Gaywood, H. McNab
PAPER
Decomposition of the isomeric 4¢-azidoacetophenone (7) eliminated by its 13C NMR spectrum, which shows only
cannot take place via anchimeric assistance so the temper- six CH resonances. The indolone 10b and the pyrrolone
ature dependence of the decomposition of 7 was briefly 10c were distinguished by the absence of the widely
studied. It is clear from Figure 3 that key points of the spaced doublets expected for H2 and H3 of a pyrrolone
temperature profile lie ca. 50 °C higher in the case of the structure such as 10c,12 and the correspondence of the 13C
para-isomer 7. Hence, we favour the anchimeric assis- NMR chemical shift for C9 (dC = 175.13) with that of pyr-
tance mechanism (Scheme 1) for the formation of 3, as rolo[1,2-a]indol-9-one itself (dC = 175.04).13 Therefore,
previously found under other conditions.8
the available evidence is consistent with the hydroxypyr-
role structure 10b.
O
O
OH
8
9
7
OH
O
O
1
6
4N
N
N
5
2
3
10a
10b
10c
Figure 5
In principle 1-hydroxypyrrolo[1,2-a]indol-9-one (10b)
can participate in intramolecular hydrogen bonding,
which is common in analogous fused six-membered
rings.14 The OH signal of 10b occurs at dH = 6.97 showing
that the appropriate bond angles in two fused five-mem-
bered rings are too large for effective hydrogen bonding.
The 2-methylene Meldrum’s derivative of indoxyl 2 is
known,1 but the precursor (dimethylamino)methylene
compound 11 was obtained directly from indoxyl (3) rath-
er than from its N-acetyl derivative15 and gave 2 in 40%
overall yield (Scheme 3). FVP of 2 at 650 °C proceeded in
analogous fashion to the corresponding pyrrolone2 and
provided pyrano[3,2-b]indol-2(5H)-one (12) in 42% yield
by cyclisation of the intermediate ketene onto the carbon-
yl group (Scheme 3), rather than the isomeric 9-hydroxy-
pyrrolo[1,2-a]indol-3-one which would have been
obtained by cyclisation onto the nitrogen atom. The py-
rone ring of 12 was characterized by its vicinal coupling
constant (3J = 9.3 Hz), which is much larger than the cor-
responding coupling in the five-membered ring of the pyr-
rolo[1,2-a]indol-3-one structure. This is the first report of
the parent member of the pyrano[3,2-b]indol-2-one het-
erocyclic system; derivatives are also rare.16
Figure 3 Temperature-conversion graph for FVP of 4 (shown in
red) and 7 (shown in black)
O
O
O
O
O
O
N
N3
H
7
8
9
OMe
Figure 4
Reaction of methoxymethylene Meldrum’s acid 8 with
4,5-dimethyl-1,2-dihydro-3H-pyrrol-3-one (9) in acetoni-
trile solution takes place at the 2-position (Figure 4).2 In
contrast indoxyl (3) behaves as a typical secondary amine
and reaction takes place at the nitrogen atom to provide 1
in 63% yield. FVP of most (dialkylamino)methylene de-
rivatives of Meldrum’s acid provide 1,2-dihydro-3H-pyr-
rol-3-ones by 1,4-hydrogen shift and electro-
cyclisation.9,10 A similar process indeed occurs with 1
(Scheme 2) to give 1-hydroxypyrrolo[1,2-a]indol-9-one
10 (54%). This ring system is not widely known11 and 10
is the first example of a pyrrolo[1,2-a]indol-9-one with an
electron-donating group in the 1-position.
O
O
O
O
O
NMe2
O
O
FVP
2
N
N
H
H
11
12
Scheme 3
Compound 10 can adopt three possible tautomeric forms,
10a, 10b, and 10c (Figure 5). The dione form 10a is easily
In summary, we have developed a robust, gas-phase meth-
od for the preparation of indoxyl (3) which provides an ef-
ficient, controllable alternative to in situ solution-phase
methods. Synthesis and FVP of the two Meldrum’s acid
derivatives 1 and 2 provides access to unusual heterocy-
clic systems 10 and 12, respectively. The former shows
interesting tautomerism and hydrogen-bonding properties
O
O
FVP
–
1
10a
10b
N+
O
N
O
H
Scheme 2
Synthesis 2010, No. 8, 1361–1364 © Thieme Stuttgart · New York

PAPER
Methylene Meldrum’s Acid Derivatives of Indoxyl
1363
3-Methyl-2,1-benzisoxazole (5)
and the latter, formed regiospecifically in the pyrolysis, is
the parent member of an unusual tricyclic system.
1H NMR (CDCl3): d = 2.72 (s, 3 H, CH3), 6.76 (dt, 1 H, Ar-H), 6.84
(dd, 1 H, Ar-H), 7.37 (dt, 1 H, Ar-H), 7.55 (dd, 1 H, Ar-H); consis-
tent with literature data.19
1H and 13C NMR spectra were generally recorded at 250 MHz and
63 MHz, respectively. Chemical shifts are given in ppm relative to
TMS and refer to one CH resonance unless otherwise stated. Mass
spectra were recorded under electron impact conditions.
Temperature Profile of FVP of 4¢-Azidoacetophenone (7)
4¢-Azidoacetophenone20 (7, 50 mg, 0.31 mmol) was pyrolysed at
the temperatures shown in Figure 3 (typical conditions Ti 40 °C; P
2.6 × 10–2 Torr; t 10 min). The pyrolysate was washed out carefully
with DMSO-d6, along with 1,4-dinitrobenzene (15 mg, 0.91 mg) to
act as internal standard. The integral of the 1,4-dinitrobenzene peak
compared with the two peaks for the aromatics from remaining 4¢-
azidoacetophenone (7) was then measured. This value was then nor-
malized against the value obtained for pyrolysis at 200 °C (the tem-
perature at which no pressure increase was noted during reaction,
i.e. all azide remained unreacted). Data are shown in Figure 3.
Flash Vacuum Pyrolysis Experiments; General Procedure
The precursor was volatilized through an empty, electrically heated
silica tube (35 × 2.5 cm) and the products were collected in a U-
tube, cooled with liquid N2, situated at the exit point of the furnace.
CAUTION: for the azide precursors described below, a metal sub-
limation oven was used under rotary pump pressure (ca. 0.02 Torr)
and the scale was limited to a maximum of a few hundred mg. We
experienced no problems with pyrolyses of the azides reported here,
but clearly every precaution must be taken in handing such poten-
tially explosive materials. Upon completion of the pyrolysis, the
trap was allowed to warm to r.t. under an N2 atmosphere. The entire
pyrolysate was dissolved in solvent and removed from the trap. The
precursor, pyrolysis conditions [quantity of precursor, furnace tem-
perature (Tf), inlet temperature (Ti), pressure (P) and pyrolysis time
(t)] and products are quoted.
2,2-Dimethyl-5-(3-oxo-2,3-dihydro-1H-indol-1-ylmethylene)-
1,3-dioxane-4,6-dione (1)
Methoxymethylene Meldrum’s acid (8, 184 mg, 0.99 mmol) and in-
doxyl (3, 159 mg, 1.20 mmol) were dissolved in the minimum
amount of MeCN and stirred for 16 h. The resulting precipitate was
collected and identified as 1 (178 mg, 63%); mp 212–214 °C
(MeOH).
1H NMR (CDCl3): d = 1.76 (s, 6 H, 2 CH3), 4.72 (s, 2 H, CH2), 7.42
(m, 1 H, Ar-H), 7.64 (m, 1 H, Ar-H), 7.80 (m, 2 H, 2 Ar-H), 8.74 (s,
1 H, alkene).
For the Meldrum’s acid derivatives, the pyrolyses proceeded more
smoothly under diffusion pump pressure (ca. 10–5 Torr) to minimize
decomposition in the inlet tube.
13C NMR (CDCl3): d = 26.82 (2 CH3), 59.66 (CH2), 90.98 (Cq),
103.89 (Cq), 112.78, 124.66, 124.87, 126.91 (Cq), 137.21, 147.45,
156.64 (Cq), 159.61 (Cq), 164.66 (Cq), 193.41 (Cq).
MS: m/z (%) = 287 (M+, 17), 229 (65), 185 (66), 157 (52), 129 (64),
43 (100).
Temperature Profile of FVP of 2¢-Azidoacetophenone (4)
Small-scale FVP reactions of 2¢-azidoacetophenone (4)17 were car-
ried out (typically 20 mg, 0.12 mmol; Ti 40 °C; P 2.2 × 10–2 Torr; t
10 min) at the temperatures shown in Figure 2, with the following
peaks in the 1H NMR spectrum of the pyrolysate used to calculate
the product ratio: 2¢-azidoacetophenone (4) [d = 2.55 (s, 3 H, CH3)],
3-methyl-2,1-benzisoxazole (5) [d = 2.72 (s, 3 H, CH3)], and indox-
yl (3) [d = 3.89 (s, 2 H, CH2)]. Data are shown in Figures 2 and 3.
HRMS: m/z [M]+ calcd for C15H13NO5: 287.0794; found: 287.0796.
1-Hydroxy-9H-pyrrolo[1,2-a]indol-9-one (10b)
FVP of 1 (158 mg, 0.55 mmol; Tf 650 °C; Ti 140 °C; P 9 × 10–6 Torr;
t 2 h) gave 10b (55 mg, 54%).
1H NMR (acetone-d6): d = 5.85 (d, J = 2.9 Hz, 1 H, H2), 6.97 (br s,
1 H, OH), 7.14 (m, 1 H, H6), 7.39 (d, J = 2.9 Hz, 1 H, H3), 7.41 (m,
1 H, H4), 7.50 (m, 1 H, H7), 7.54 (m, 1 H, H5).
13C NMR (90 MHz, acetone-d6): d = 104.37 (C2), 109.54 (C4),
115.87 (Cq, C8a), 121.15 (C3), 122.48 (C7), 123.35 (C6), 129.89
(Cq, C7a), 132.48 (C5), 141.91 (Cq, C3b), 149.82 (Cq, C1), 175.13
(Cq, C8); (signals identified by HSQC and HMBC experiments).
Indoxyl (3) by FVP of 2¢-Azidoacetophenone (4)
FVP of 2¢-azidoacetophenone (4, 125 mg, 0.78 mmol; Tf 650 °C; Ti
40 °C; P 2 × 10–2 Torr; t ~50 min) gave indoxyl (3, 82 mg, 75%);
mp 85–87 °C (Lit.18 84 °C),
MS: m/z (%) = 133 (M+, 100), 105 (53), 104 (90), 78 (67), 77 (29),
51 (18).
Keto tautomer 3K
1H NMR (CDCl3): d = 3.89 (s, 2 H, CH2), 6.84 (ddd, J = 0.8, 7.1, 7.8
Hz, 1 H, Ar-H), 6.92 (dd, J = 0.8, 8.3 Hz, 1 H, Ar-H), 7.45 (ddd, J =
1.3, 7.1, 8.3 Hz, 1 H, Ar-H), 7.62 (dd, J = 1.3, 7.8 Hz, 1 H, Ar-H).
MS: m/z (%) = 185 (M+, 48), 86 (87), 84 (100), 51 (74), 49 (100),
47 (53).
13C NMR (CDCl3): d = 54.08 (CH2), 112.92, 118.98, 121.87 (Cq),
124.19, 136.88, 162.57 (Cq), 200.51 (Cq).
HRMS: m/z [M]+ calcd for C11H7NO2: 185.0479; found: 185.0473.
2-[(Dimethylamino)methylene-1,2-dihydro-3H-indol-3-one (11)
Indoxyl (3, 128 mg, 0.96 mmol) was dissolved in toluene (3 mL)
and N,N-dimethylformamide diethyl acetal (512 mg, 3.48 mmol)
was added. The mixture was stirred at r.t. for 1 h, then heated under
reflux for 1 h. The solvent was removed by rotary evaporation, and
EtOAc (2 mL) was added. This was stirred for 30 min, the resulting
precipitate filtered off and washed with i-PrOH followed by Et2O to
give 11 (108 mg, 60%); mp 223 °C (dec) [Lit.15 215 °C (dec)].
1H NMR (DMSO-d6): d = 3.19 [s, 6 H, N(CH3)2], 6.79 (ddd, J = 0.7,
7.2, 7.7 Hz, 1 H, Ar-H), 7.02 (s, 1 H, alkene), 7.09 (dd, J = 0.7, 8.2
Hz, 1 H, Ar-H), 7.33 (ddd J = 0.5, 7.2, 8.2 Hz, 1 H, Ar-H), 7.48 (dd,
J = 0.5, 7.7 Hz, 1 H, Ar-H), 8.99 (s, 1 H, NH).
Enol tautomer 3E
1H NMR (DMSO-d6): d = 6.70 (d, J = 2.1 Hz, 1 H, H2), 6.88 (ddd,
J = 1.0, 6.9, 7.9 Hz, 1 H, Ar-H), 7.00 (ddd, J = 1.3, 6.9, 8.1 Hz, 1 H,
Ar-H), 7.21 (dd, J = 1.0, 8.1 Hz, 1 H, Ar-H), 7.50 (dd, J = 1.3, 7.9
Hz, 1 H, Ar-H), 10.26 (s, 1 H, OH).
13C NMR (DMSO-d6): d = 106.86, 111.05, 116.98, 117.18, 119.67
(Cq), 120.69, 133.47 (Cq), 135.69 (Cq).
3-Methyl-2,1-benzisoxazole (5) by FVP of 2¢-Azidoacetophe-
none (4)
FVP of 2¢-azidoacetophenone (4, 50 mg, 0.31 mmol; Tf 400 °C; Ti
40 °C; P 2 × 10–2 Torr; t 20 min) gave a mixture containing 5 (91%),
4 (7%), and 3 (1%) (by 1H NMR spectroscopy).
13C NMR (DMSO-d6): d = 42.08 (2 CH3), 112.39, 114.12 (Cq),
117.21, 122.31, 131.53, 133.62, 148.86 (Cq), 179.93 (Cq) (1 Cq
overlapping).
Synthesis 2010, No. 8, 1361–1364 © Thieme Stuttgart · New York

1364
A. P. Gaywood, H. McNab
PAPER
(4) For example: Capon, B.; Kwok, F. C. J. Am. Chem. Soc.
MS: m/z (%) = 188 (M+, 100), 173 (48), 158 (23), 147 (35), 146
(65).
1989, 111, 5346.
(5) Gaywood, A. P.; McNab, H. J. Org. Chem. 2009, 74, 4278.
(6) Review: Smalley, R. K. Adv. Heterocycl. Chem. 1981, 29, 1.
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J. Chem. Soc., Perkin Trans. 1 1986, 1107.
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and Azides, Supplement D, Part 1; Patai, S.; Rappoport, Z.,
Eds.; Wiley: Chichester, 1983, 287.
2,2-Dimethyl-5-(3-oxo-2,3-dihydro-1H-indol-2-ylmethylene)-
1,3-dioxane-4,6-dione (2)
Following the literature method,1 a soln of 11 (87 mg, 0.47 mmol)
and Meldrum’s acid (136 mg, 0.95 mmol) in i-PrOH (2 mL) was
stirred at r.t. for 5 h to give 2 (67 mg, 51%); mp 204–206 °C [Lit.1
>210 °C (dec)].
1H NMR (DMSO-d6): d = 1.79 (s, 6 H, 2 CH3), 6.98 (m, 1 H, Ar-H),
7.40 (m, 1 H, Ar-H), 7.55 (m, 1 H, Ar-H), 7.83 (m, 1 H, Ar-H), 8.30
(s, 1 H), 10.75 (br s, 1 H).
13C NMR (DMSO-d6): d = 26.41 (2 CH3), 96.28 (Cq), 103.17 (Cq),
113.79, 116.04 (Cq), 119.18, 121.54, 131.12, 134.00, 141.11 (Cq),
157.30 (Cq), 163.42 (Cq) (2 Cq overlapping).
(9) McNab, H.; Monahan, L. C. J. Chem. Soc., Perkin Trans. 1;
1988, 863.
(10) Reviews: (a) McNab, H.; Monahan, L. C. 3-
Hydroxypyrroles, In The Chemistry of Heterocyclic
Compounds, Vol. 48, Part 2; Jones, R. A., Ed.; Wiley: New
York, 1992, 525. (b) Gaber, A. M.; McNab, H. Synthesis
2001, 2059. (c) McNab, H. Aldrichimica Acta 2004, 37, 19.
(11) (a) Lukes, R.; Zobacova, A. Chimia 1959, 13, 333.
(b) Laschtuvka, E.; Huisgen, R. Chem. Ber. 1960, 93, 81.
(c) Huisgen, R.; Laschtuvka, E. Chem. Ber. 1960, 93, 65.
(d) Merour, J.-Y.; Chichereau, L.; Desarbre, E.; Gadonneix,
P. Synthesis 1996, 519.
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1988, 1459.
(13) Campo, M.; Larock, R. J. Org. Chem. 2002, 67, 5616.
(14) For example: Gaywood, A. P.; Hill, L.; Imam, S. H.; McNab,
H.; Neumajer, G.; O’Neill, W. J.; Mátyus, P. New J. Chem.
2010, in press; DOI: 10.1039/b9nj00474b.
MS: m/z (%) = 262 (M+, 17), 185 (100), 157 (38), 129 (83), 102
(22), 76 (35).
Pyrano[3,2-b]indol-2(5H)-one (12)
After FVP of 2 (55 mg, 0.21 mmol; Tf 650 °C; Ti 120 °C; P 9 × 10–6
Torr; t 2 h) the U-tube trap was first washed through with CHCl3 to
remove soluble impurities, leaving 12 (ca. 20 mg, 42%).
1H NMR (acetone-d6): d = 6.11 (d, J = 9.4 Hz, 1 H, alkene), 7.07 (m,
1 H, Ar-H), 7.23 (m, 1 H, Ar-H), 7.41 (d, J = 9.4 Hz, 1 H, alkene),
7.66 (m, 1 H, Ar-H), 7.88 (m, 1 H, Ar-H).
13C NMR (DMSO-d6): d = 110.76, 112.63, 114.58 (Cq), 117.55,
119.04 (Cq), 119.98, 125.54, 128.26 (Cq), 135.04, 136.27 (Cq),
161.34 (Cq).
(15) Ryabova, S. Y.; Trofimkin, Y. I.; Alekseeva, L. M.;
Bogdanova, G. A.; Sheinker, Y. N.; Granik, V. G. Chem.
Heterocycl. Cpds. (Engl. Transl.) 1990, 1238.
MS: m/z (%) = 185 (M+, 70), 157 (45), 129 (41), 102 (20), 76 (16),
58 (100).
(16) (a) Tominaga, Y.; Natsuki, R.; Matsuda, Y.; Kobayashi, G.
Chem. Pharm. Bull. Jpn. 1973, 21, 1658. (b) Kobayashi,
G.; Matsuda, Y.; Natsuki, R.; Tominaga, Y.; Okamura, T.;
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Y.; Natsuki, R.; Matsuda, Y.; Kobayashi, G. Yakugaku
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(17) Dyall, L. K.; Holmes, A. Aust. J. Chem. 1988, 41, 1677.
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(19) Barth, A.; Corrie, J. E. T.; Gradwell, M. J.; Maeda, Y.;
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1997, 119, 4149.
HRMS: m/z [M]+ calcd for C11H7NO2: 185.0477; found: 185.0473.
Acknowledgment
We are grateful to The Engineering and Physical Science Research
Council (EPSRC, UK) for a research studentship (awarded to
A.P.G.).
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Synthesis 2010, No. 8, 1361–1364 © Thieme Stuttgart · New York