Thieno pyridazines syntheses

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... 

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. 

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>. 

The preparation of selenolopyridazines is accomplished in the usual manner by condensation of diformylselenophenes with hydrazines. Both selenolo[2,3-cf]pyridazine (432) and selenolo[3,"4-tf]pyridazine (433) are obtained in this way (72BSF3453) as crystalline compounds with m.p. 158 and 153 °C, respectively. By strict analogy with the synthesis of boron containing furo- and thieno-pyridines (Sections 3.17.2.3.1, 3.17.3.3.1), boroxazoselenolo-pyridines (e.g. 434) and borazaroselenolopyridines (435,436) could be prepared by condensation of appropriately substituted formylselenophenoboronic acids and hydroxylamine or hydrazine <72IJS(A)(2)257>. 

Miscellaneous Fused Systems.-An elegant synthesis of thieno[2,3-c ]-pyridazine in 40% yield consists of heating thiophen-2-carbaldehyde azine with AlCla-triethylamine at 170-200°C. 2,3-Dimethylthieno[3,4-i>]-pyrazine has been prepared from 2,4-diaminothiophen. A derivative of the unusual thienopterine (303 R = OH) that was isolated from human urine in 1940, namely (303 R = H), has been synthesized from 2-amino-3,5-dicyano-... 

Aza-Wittig reaction of the iminophosphoranes 41, followed by heterocyclization using amines, provides a novel, one pot, synthesis of functionalized pyrimido[4 ,5 4,5]thieno-[2,3-c]pyridazines 42 <97H(45)1319>. 

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