2-Benzyl-
4
-chloro-
5
-mercapto-3 (2H)-pyridazinone (
1
) was heated in ethanol under reflux for 10 hours to form concurrently 2,
7
-dibenzyldipyridazo [
4
,
5
-
b
:
4
',
5
'-
e
]-
1
,
4
-dithiin-
1
,
6
(2H,
7
H)-dione (Ia) (57% in yield) and 2,
8
-dibenzyldipyridazo [
4
,
5
-
b
:
4
',
5
'-
e
]-
1
,
4
-dithiin-
1
,
9
(2H,
8
H)-dione (IIa) (14% in yield). Assignment of the two products to the corresponding structures is established by their physico-chemical constants and chemical behaviors. The concurrent formation of the dipyridazo [
4
,
5
-
b
:
4
',
5
'-
e
]-
1
,
4
-dithiin-
1
,
6
(2H,
7
H)-dione and the -
1
,
9
(2H,
8
H)-dione were observed in, not only heating
4
-chloro-
5
-mercapto-3 (2H)-pyridazinones (
1
-3) in polar solvent, but warming
4
-mercapto-
5
-chloro-3 (2H)-pyridazinones (
4
-
6
) in the presence of potassium carbonate in DMF. For the interpretation of the reaction, keto-thioketo carbenes as active intermediate species and a reversible interconversion among them are suggested on the basis of the fact that there was nonexistence of an equilibrium between the two compounds, such as Ia and IIa, under the reaction condition and a trapping keto-thioketo carbene species as 2-phenylimino-
5
-benzylpyridazo [
4
,
5
-d]-
1
, 3-dithiol-
4
(
5
H)-one (IVa) (14% in yield) along with the major products, (Ia) and (IIa), was furnished by warming
1
with phenylisothiocyanate in the presence of triethylamine in dry benzene. On the contrary, participation of a reversible interconversion between Ia and IIa, to a considerable extent, with the concurrent formation of them in the case of benzylation of either Ib or IIb, by warming with benzylchloride in the presence of potassium carbonate in DMF at 80°, might not be neglected, because an attempted approach to an equilibrium between Ia and IIa in a similar reaction condition except the use of benzylchloride realized, whereas non-existence of an equilibrium between Ib and IIb in the similar reaction condition was observed.
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