Thieno pyridazines

Thieno[3,4-c]furan dipole moment, 4, 1049 (74JA1817) Thieno[2,3-d]pyridazine... 

Thieno[3,2-c]pyridazine, 3,7-ethano-nomenclature, 1, 24 Thieno[3,2-c]pyridazine, 3,7-ethylene-nomenclature, 1, 24... 

Thieno[3,2-c]pyridazine-3-carboxylic acid synthesis, 4, 819-820 Thienopyridazines, 4, 1015 Thieno[2,3- hjpyridazines nitration, 4, 1016 synthesis, 4, 791, 1015 Thieno[3,2-c]pyridazines synthesis, 4, 1016 Thieno[3,4-d]pyridazines synthesis, 4, 1016 Thieno[2,3-6]pyridine, 4-amino-synthesis, 4, 1005... 

N pyrrolo[3,4-d]pyridazine O furo[3,4-d]pyridazine S thieno[3,4-d]pyridazine... 

The formation of pyrrolo-, furo-, and thieno[2,3- pyridazines has been predicted on the basis of thermodynamic studies <2005JOC998>. 

By far the largest body of the literature of thienopyridazines involves the reactions of thieno[2,3-f]pyridazines. There are two main types of reactions on such systems (1) simple chemistry of existing functional groups and (2) reactions involving cyclization to give tricyclic (and higher) ring systems. 

Other rings have been fused onto thieno[2,3-f]pyridazines. The reaction of 48 with ethyl chloroacetate provides a fused furan derivative 49 (Equation 19) <1999M1117>. 

The only other group of compounds that has exhibited unusual reactions is the thieno[3,4-, pyridazines. The types of reaction involve either ring expansion (or addition) and introduction of a functional group onto the ring. 

A review describing the syntheses of thieno[2,3-f]pyridazines, thieno[3,2-f]pyridazines, thieno[2,3-i/]pyridazines, and thieno[3,4-i/]pyridazines has been published recently <2004PS(179)321>. 

The chemistry of this series parallels that of the nitrogen and oxygen analogs. Hence the synthesis of thieno[2,3-tfl pyridazines can be accomplished from either a pyridazine or thiophene ring precursor. 

Reaction of 348 with the ethyl thioglycolate anion affords ethyl thieno[2,3-,7]pyridazine-2-carboxylate 349 in excellent yield <1994S669> (Equation 126). 

Clearly the most common approach to the synthesis of thieno[3,4-r/]pyridazines, such as 355, proceeds via the reaction of a suitably substituted pyridazine 354 and elemental sulfur (Equation 129) <1995T12745,1995JCM488, 1996PJG589, 1997HAC29, 1997IJB612, 1999JCM648, 2001JHC685>. 

Finally, a Diels-Alder reaction with inverse electron demand, in which the thiophene acts as the 27r-component, has been reported (78AP728). Tetrazinedicarboxylic ester adds to thiophenes to give the adduct (290) loss of nitrogen from this is followed by oxidation, yielding the thieno[2,3-rf]pyridazines (291) in 10-15% yield. With 2,5-dimethyl-thiophene as the substrate, aromatization is blocked, and the product (292) is obtained in 57% yield. 

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