3
-Chloro-
4
-mercapto-
5
-ethoxypyridazine (1) was heated under reflux in abs. ethanol for three hours to form concurrently four products with novel rings in the dipyridazo-1,
4
-dithiin system,
4
,
9
-diethoxydipyridazo [
3
,
4
-
b
:
3
',
4
'-
e
]-1,
4
-dithiin (pale yellow needles, mp 211°, 13% in yield) (Ia),
4
, 6-diethoxydipyridazo [
3
,
4
-
b
:
4
',
3
'-
e
]-1,
4
-dithiin (colorless needles, mp 260°, 12% in yield) (IIa), dipyridazo [
3
,
4
-
b
:
3
',
4
'-
e
]-1,
4
-dithiin-
4
,
9
(1H, 6H)-dione (colorless solid, mp>300°, 24% in yield) (Ib), and dipyridazo [
3
,
4
-
b
:
4
',
3
'-
e
]-1,
4
-dithiin-
4
, 6 (1H,
9
H)-dione (colorless solid, mp>300°, 16% in yield) (IIb). Structural assignment of the compounds, Ia, IIa, Ib and IIb, was confirmed by their physico-chemical constants and chemical experimental evidences. Similar concurrent formations of an isomeric pair of
4
,
9
-disubstituted dipyridazo-[
3
,
4
-
b
:
3
',
4
'-
e
]-1,
4
-dithiins (I series isomers) and
4
, 6-disubstituted dipyridazo [
3
,
4
-
b
:
4
',
3
'-
e
]-1,
4
-dithiins (II series ones) were also observed in those cases in which heating
3
,
5
-dichloro-
4
-mercaptopyridazine (
3
),
3
-chloro-
4
-mercapto-
5
-benzylthiopyridazine (
4
) and
3
-chloro-
4
-mercapto-
5
-morpholinopyridazine (
5
) in similar reaction conditions (heating in proper polar solvents for proportionatd periods) formed a pair of the
4
,
9
-dichloro- (colorless needles, mp 275°, only trace in yield) (Ic) and
4
, 6-dichloro derivative (pale yellow needles, mp 307°(decomp.), only trace in yield) (IIc),
4
,
9
-dibenzylthio- (yellow crystals, mp 230°, 29% in yield) (Id) and
4
, 6-dibenzyl derivative (red crystals, mp 252°, 23% in yield) (IId), and
4
,
9
-dimorpholino- (colorless crystals, mp 268°, 38% in yield) (Ie) and
4
, 6-dimorpholino derivative (colorless needles, mp 291°, 25% in yield) (IIe), respectively. A rather complex cyclization furnishing the concurrent formation of two pairs of the dipyridazo-1,
4
-dithiin derivatives was found in the reaction of heating
3
-chloro-
4
,
5
-dimercaptopyridazine (6) in a similar reaction condition, followed by benzylation, in which a pair of the derivatives with another novel ring, dipyridazo [
3
,
4
-
b
:
4
',
5
'-
e
]-1,
4
-dithiin, the
4
,
9
-dibenzylthio- (colorless plates, mp 168°,
5
% in yield) (VI) and
4
, 6-di-benzylthio derivative (yellow needles, mp 210°, 11% in yield) (VII) and another pair of derivatives with a known ring, dipyridazo [
4
,
5
-
b
:
4
',
5
'-
e
]-1,
4
-dithiin, the 1, 6-dibenzylthio-(yellow needles, mp 178°, 19% in yield) (IV) and the 1,
9
-dibenzylthio derivative (colorless needles, mp 170°,
9
% in yield) (V) and none of Id and IId or 1, 6-dichlorodipyridazo-[
4
,
5
-
b
:
4
',
5
'-
e
]-1,
4
-dithiin (If) and 1,
9
-dichloro derivative (IIf) was detected. NMR spectra of some dibenzylthiodipyridazo-1,
4
-dithiin derivatives were compared. A discussion of the cyclization mechanism involved in these reactions is also proposed.
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