Tandem [4+2] cycloaddition versus electrocyclisation reactions of 1-aryl-2-phenyl-5-alkyl/aryl-1,3-diazapenta-1,3,4-trienes in aza-Wittig reactions of N′-aryl-N-(triphenylphosphoranlidene) benzenecarboximidamides with ketenes

Article abstract of DOI:10.1016/S0040-4039(01)00119-8

1-Aryl-2-phenyl-5-alkyl/aryl-1,3-diazapenta-1,3,4-triene 3 generated in situ aza-Wittig reactions of N′-aryl-N-(triphenylphosphoranylidene)carboximidamides 1 which are shown to undergo selective [4+2] cycloaddition and electrocyclisation reactions leading to the formation of novel pyrimidinone derivatives 5 and quinazoline derivatives 7 with monosubstituted ketenes and diphenylketene, respectively.

Full text of DOI:10.1016/S0040-4039(01)00119-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2235–2237
Tandem [4+2] cycloaddition versus electrocyclisation reactions of
1-aryl-2-phenyl-5-alkyl/aryl-1,3-diazapenta-1,3,4-trienes in
aza-Wittig reactions of N%-aryl-N-(triphenylphosphoranlidene)
benzenecarboximidamides with ketenes†
S. Jayakumar, Vipan Kumar and Mohinder P. Mahajan*
Department of Pharmaceutical Sciences, Guru Nanak Dev University, Amritsar 143005, Punjab, India
Received 5 December 2000; revised 9 January 2001; accepted 17 January 2001
Abstract—1-Aryl-2-phenyl-5-alkyl/aryl-1,3-diazapenta-1,3,4-triene
3 generated in situ aza-Wittig reactions of N%-aryl-N-
(triphenylphosphoranylidene)carboximidamides 1 which are shown to undergo selective [4+2] cycloaddition and electrocyclisation
reactions leading to the formation of novel pyrimidinone derivatives 5 and quinazoline derivatives 7 with monosubstituted ketenes
and diphenylketene, respectively. © 2001 Elsevier Science Ltd. All rights reserved.
The utility of iminophosphoranes in synthesis of azadi-
enes and heterocumulene-associated azadienes has been
well demonstrated.1 The azadienes in turn have proved
to be promising building blocks in hetero Diels–Alder
reactions and in construction of numerous azahetero-
cyclic and substituted open chain systems.2 The com-
pounds containing a heterocumulene attached to a
vinylic group have exhibited rich chemistry and con-
tinue to be the subject of intense investigations.1,3 How-
ever, the compounds containing a heterocumulene
attached to a carbonꢀnitrogen double bond remain
unexplored.4 Rossi et al.5 have observed that the 1,3-
diazabuta-1,3-dienes formed in aza-Wittig reactions
of N%-aryl-N-(triphenylphosphoranylidene)carboximid-
amides 1 with aldehydes undergo a facile electrocyclic
ring closure to yield quinazoline and dihydroquinazo-
line derivatives. It was felt that a similar aza-Wittig
reaction of N-imidoyl iminophosphoranes with hetero-
cumulenes viz. ketenes, isocyanates and isothiocyanates
may lead to an efficient and convenient route to N-imi-
doyl heterocumulenes. As part of our studies on 1,3-
diazabuta-1,3-diene-ketene cycloaddition reactions,6 we
report herein, tandem [4+2] cycloaddition/electrocyclic
ring closure of 1-aryl-2-phenyl-5-alkyl/aryl-1,3-diaza-
penta-1,3,4-trienes 3 formed in aza-Wittig reactions of
N-imidoyl iminophosphoranes 1 with ketenes.
Thus the treatment the of N%-aryl-N-(triphenylphos-
phoranylidene)benzenecarboximidamide 1 with 3 equiva-
lents of chloro-, methyl- and phenylketenes, generated
in situ from the corresponding acid chloride in presence
of triethylamine, resulted in good yields of products
which were characterised as pyrimidinones 5 on the
basis of spectral and analytical data.7 The probable
mechanistic pathway that best explains the formation
of these pyrimidinones 5 is depicted in Scheme 1. In this
mechanism it is assumed that an initial aza-Wittig
reaction of N%-aryl-N-(triphenylphosphoranylidene)
benzenecarboximidamide 1 with ketene leads to 1-aryl-
5-alkyl/aryl-2-phenyl-1,3-diazapenta-1,3,4-triene 3. The
[4+2] cycloaddition reaction of this intermediate with
another molecule of ketene, involving either a stepwise
or the recently proposed concerted Diels–Alder type
mechanism,8 leads to the formation of pyrimidinone 4
which finally isomerises to the stable pyrimidinone 5. It
is surprising to note the absence of an electrocyclisation
route to the quinazoline derivatives in these reactions as
observed by Rossi et al.5 in similar reactions of N-imi-
doyliminophosphoranes 1 with aldehydes. It may be
mentioned that the reactions of 1 with 1.5 equivalents
of ketene did not show the complete disappearance of
starting material (tlc) while the formation of the pyrim-
idinone 5 was indicated.
However, similar reactions of N-imidoyl iminophos-
phoranes 1 with diphenylketene did not form pyrimidi-
none 5 and resulted in good yields of quinazoline
derivatives 7. The quinazolines 7 are presumably the
* Corresponding author. E-mail: mohinderpmahajan@angelfire.com
Dedicated in honour of the 80th anniversary of a great chemist and
valued mentor Professor Rolf Huisgen.
†
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00119-8

2236
S. Jayakumar et al. / Tetrahedron Letters 42 (2001) 2235–2237
R
N
R
R
N
O
C
O
C
Ph
O
Ph
N
C
Ph
R1
R2
+
N
H
R1
R2 =H
N
N
R1
R2
R1
PPh3
1
R1
H
R2
3
2
4
+
2. a. R1=Cl, R2=H
b. R1=Ph, R2=H
c. R1=CH3, R2=H
d. R1=R2=Ph
1. a. R=H
OPPh3
R1=R2=Ph
b. R=CH3
R
Ph
N
Ph
N
C
R
Ph
N
O
N
R
N
H
Ph
N
R1
Ph
Ph
H
Ph
R1
H
6
5
5. a. R=H, R1=Cl (60%)
b. R=CH3, R1=CH3 (65%)
c. R=H, R1=CH3 (62%)
d. R=H, R1=Ph (71%)
e. R=CH3, R1=Ph (68%)
[1,7]H
Ph
N
N
Ph
H
R
Ph
7
7. a. R=H (72%)
b. R=CH3 (66%)
Scheme 1.
result of the electrocyclic ring closure of the interme-
diate 1-aryl-2-phenyl-5-diphenyl-1,3-diazapenta-1,3,4-
triene 3 to yield another intermediate 6 which under-
goes a [1,7]H shift to finally yield the aromatic
quinazolines 7. A number of attempts were made to
intercept the intermediate 3 in a [4+2] cycloaddition
reaction with diphenylketene but such attempts, even
with an excess of diphenylketene, resulted invariably in
the isolation of quinazoline 7. It is likely that the
geminal diphenyls in the intermediate triene 3 lie
orthogonal to the rest of the molecular plane and then
the approach of another diphenylketene molecule to
this intermediate from either face of the molecular
plane is sterically hindered. Therefore, 1,3-diaza-1,3,4-
triene 3 is less likely to undergo [4+2] cycloaddition
reaction with diphenylketene and the alternative elec-
trocyclic ring closure route to quinazoline 7 is favoured.
In conclusion the previously unknown 1,3-diazapenta-
1,3,4-trienes 3 can be easily obtained by the aza-Wittig
reactions of N-imidoyl iminophosphoranes 1 with
monosubstituted/disubstituted ketenes and selectively
utilised in intermolecular [4+2] cycloaddition/electro-
cyclic ring closure reactions for the formation of pyrim-
idinone 5 and quinazoline 7 derivatives.
Acknowledgements
S.J. is grateful to CSIR, New Delhi for a Senior
Research Fellowship and the financial assistance by
CSIR, New Delhi, under grant No: 01(1590/99/EMR-
11) is gratefully acknowledged.

S. Jayakumar et al. / Tetrahedron Letters 42 (2001) 2235–2237
2237
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