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The shortly performed thermal conversion of 1, 2‐dimethylpyrazolin‐5‐thiones leads to 1‐methyl‐3‐methylthiopyrazoles. The isomerization of these compounds into mixtures of 3‐methylthio and 5‐methylthiopyrazoles is observed after several months at room temperature or more rapidly after prolonged heating. 1‐methyl‐3‐methylthio‐ and 5‐methylthiopyrazoles appear thus respectively as the kinetic and thermodynamic products of the reaction which proceeds at least mainly via an intermolecular mechanism. The thermal intermolecular conversion of 1‐phenyl‐2‐methylpyrazolin‐5‐thiones leads exclusively to 1‐phenyl‐5‐methylthiopyrazoles as described in the literature.

A B S T R A C T . T h e r e a c t i o n s o f pyrazolin-5-thiones a n d b e n z a l d e h y d e ( o r d e r i v a t i v e s ) lead to u n s t a b l e 4 -a r y l i d e n i c c o m p o u n d s , w h i c h u n d e r g o %molecular c y c l i z ations. R e a c t i o n s o f t h e p y r a z o l i n -5 -t h i o n e s w i t h c i n n a m a l d e h y d e a n d a c e t o p h e n on e r e s p e c t i v e l y a f f o r d s u n s t a b l e c i n n a m y l i d e n i c and a r y l i d e n i c products. w h i c h u n d e r g o =molecular c y c l i z a t i o n s . T h e s t r u c t u r e s o f t h e n e w h e t e r o c y c l i c s y s t e m s s o o b t a i n e d a r e e s t a b l i s h e d by IH-a n d I3C N M R t e c h n i q u e s . D a n s d e s p u b l i c a t i o n s p r e c e d e n t e s , nous d e c r i v o n s la s t e r e o i s o m e r i e d e d erives 4 -a r y l i d e n i q u e s o b t e n u s par c o n d e n s a t i o n d'aldehydes o u d e c e t o n e s a r o m at i q u e s s u r des p y r a z o l i n e -, i s o x a z o l i n e -e t o x a z o l i n e -5 -0 n e s . !~ a ')(Figure 1).

I 1 y a u r a i t d o n c l a e g a l e m e n t un p a r a l l e l i s m e d e c o m p o r t e m e n t e n t r e i m i d a z o l in e -5 -0 n e s e t 5 -t h i o n e s . En m i l i e u n e u t r e , c e t a u t o m e r e 3 -N H n'est o b s e r v 6 , e n f a i b l e q u a n t i t e , q u e c h e z l e c o m p o s e t r i m e t h y l 6 7. En m i l i e u polaire,la p a r t i c i p a t i o n a l ' e q u i l i b r e d e c e t i s o m e r e s e t r o u v e a u gm e n t e e en s o r t e q u e la f o r m e 3 -N H a p p a r a f t e g a l e m e n t p o u r l e s c o m p o s e s 8 e t 9 t o u t e n r e s t a n t c e p e n d a n t t r e s m i n o r i t a i r e .