Chemistry Reference
In-Depth Information
The construction of tetracyclic 6H-indolo[2,3-b]quinolines 1723 by generating
two annulated rings via biradicals 1721 is achieved using an yne-carbodiimide 1720
[1265]. This is obtained in 71-83% yield by aza-Wittig reaction of phenyl iso-
cyanate 1715 with an iminophosphorane 1719, which is formed in 66-93% yield
by the reaction of an amine 1718 and triphenylphosphine dibromide. An annula-
tion reaction of the yne-carbodiimide 1720 by thermolysis at 138
C creating bi-
radical 1721 and proceeding via 1722 affords the 6H-indolo[2,3-b]quinolines 1723
in yields of 77-91% [1265].
R
R
R
Ph-
N=C=O
Ph
3
PBr
2
1715
N
Et
3
N
benzene
reflux
4 h
PPh
3
benzene
rt, 1 h
N
NH
2
N
71-83 %
1720
1718
66-93 %
1719
terpinene
reflux
14 h
138°C
R
R
R
.
.
H
N
N
N
N
N
76-91 %
1723
1722
1721
R = H, Me, Pr, tBu, Ph, TMS
Typical procedure. N-(2-Ethynylphenyl)-N
0
-phenylcarbodiimide 1720 (R
2-
¼
H) [1265].
(1-Propynyl)-N-(triphenylphosphoranylidene)benzenamine 1719 (R
¼
H): To Ph
3
PBr
2
(4.22 g, 10.0 mmol) were added amine 1718 (R
H) (1.31 g, 10.0 mmol), anhy-
drous triethylamine (2.78 mL), and anhydrous benzene (100 mL) under nitrogen
atmosphere. The reaction mixture was heated under reflux for 4 h. The white tri-
ethylammonium bromide that precipitated was removed by filtration, and the fil-
trate was concentrated. The residue was purified by column chromatography (silica
gel; 40-60% diethyl ether in hexanes) to furnish 2.776 g (7.10 mmol, 71%) of 1719
(R
¼
H) as colorless crystals.
N-(2-Ethynylphenyl)-N
0
-phenylcarbodiimide 1720 (R
¼
H) (0.377
g, 1.00 mmol) under nitrogen atmosphere, a solution of phenyl isocyanate 1715
(0.119 g, 1.00 mmol) in anhydrous benzene (15 mL) was added via a cannula at
room temperature. After 1 h, the reaction mixture was concentrated in vacuo, and
the residue was purified by column chromatography (silica gel; 5% diethyl ether in
hexanes) to furnish 0.181 g (0.83 mmol, 83%) of 1720 (R
¼
H): To 1719 (R
¼
¼
H) as a yellow oil; IR
2258, 2139, 2103 cm
1
.
A method for preparing cyclic bis(carbodiimide)s 1725 in up to 84% yield by re-
acting iminophosphoranes 1724 with Boc
2
O in the presence of DMAP at room
temperature for 12 h has been developed [1266].
(neat): n
max
¼
