A novel ring contraction for the conversion of 2, 7-disubstituted 10H-dipyridazino [
4
,
5
-
b
:
4
',
5
'-
e
] [1,
4
] thiazine-1, 6-(2H, 7H)-diones (
4
'a-c) into the corresponding 2, 6-disubstituted
9
H-dipyridazino [
4
,
5
-
b
:
4
',
5
'-d] pyrrole-1,
5
(2H, 6H)-diones (7'a-c), through a base-induced extrusion of sulfur, is described. Heating of
4
'a-c in dimethylformamide in the presence of potassium carbonate smoothly afforded 7'a-c, respectively, in good yields, although neither 2, 8-dimethyl-10H-dipyridazino [
4
,
5
-
b
:
4
',
5
'-
e
] [1,
4
] thiazine-1,
9
(2H, 8H)-dione (
5
'a), nor 3, 7-dimethyl-10H-dipyridazino [
4
,
5
-
b
:
4
',
5
'-
e
] [1,
4
] thiazine-
4
, 6 (3H, 7H)-dione (6'a) was susceptible to similar reaction conditions. The mechanism of the ring contraction, which may involve an intermediate (anion, C
-) containing a thiirane ring, is also discussed.
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