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Phenacyl-malononitrile derivatives 1a and b react with dimethylformamide dimethylacetal (DMFDMA) in refluxing toluene to afford the enaminones 2a and b respectively. Compounds 2a and 2b react with the hydrazine hydrate 3a and phenyl hydrazine 3b in refluxing ethanol to afford the pyridazine derivatives 5a–d, presumably via the intermediates 4. Compounds 5a–d, react with hydrazine hydrate 3a to afford the pyridazino[4,5-d]pyridazines 6a–d respectively. The pyridazines 5a and b and the pyridazino[4,5-d] pyridazines 6a and b could be oxidized into the full aromatic systems 7a and b and 8a and b respectively. Compounds 7a and b react also with hydrazine hydrate 3a to afford 8a and b respectively.
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Shaista S. Khokhar Thomas Wirth 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3-4):1087-1098
Abstract Chiral selenium electrophiles can be employed in cyclization reactions for the stereoselective synthesis of different heterocyclic compounds which are of great interest. 1 The effect of counterions in selenenylations and the interactions of selenium electrophiles with nearby heteroatoms have been reported earlier, but these observations do not yet give a conclusive picture. We recently observed interesting effects of external nucleophiles on the course of selenenylation reactions. To investigate in which way different nucleophiles solvents and counterions affect selenocyclizations, we synthesised alkene 1, which contains two different functional groups, an alcohol group and a carboxylic acid function. Electrophilic 5- exo-cyclization of 1 can lead to either tetrahydrofurans 2 or to lactones 3, depending on the attacking nucleophile (Scheme 1). The relative stereochemistry could not be resolved by NOE due to overlapping signals of 2 and 3, which are both obtained as mixture of cis (2a, 3a) and trans isomers (2b, 3b). An X-ray crystallographic analysis of 3a (E = I) finally allowed an unambiguous assignment. We will discuss the behavior of solvents, interactions of nucleophiles, and counterions with selenium electrophiles, which strongly affect the course of cyclizations.
Control of Selenocyclisations
Published online:
01 February 2007 SCHEME 1 相似文献
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