Krohnke-type pyridines

Cave, Raston et al. used mechanochemistry to prepare several Krohnke-type pyridines, some of them as complex as 38 and 39 (00CC2199, 01CC2159, 01EJO3227, 01JCS(P1)3258). These authors point out that kinetic considerations for the reaction of two solids can only be explained if a eutectic melt is formed during the reaction. 

Double Krohnke reactions have been employed in the preparation of quaterpyridines and sexi-pyridines. Thus, the chalcone dimer (77a,b) reacts with the pyridinium salt (78) to provide (79a,b) (Scheme 13).84 An alternative approach involves a bischalcone species based on bpy that can be elaborated to (76c), as well as several 4, 4""-disubstituted derivatives.85 A dimeric analogue of (78), having a central bpy moiety, undergoes condensation with a Krohnke-type salt to provide the parent sexipyridine (76c).86... 

In the laboratory of P. Kocovsky, novel pyridine-type P,A/-ligands were prepared from various monoterpenes. The key step was the Krohnke pyridine synthesis, and the chirality was introduced by the a,(3-unsaturated ketone component, which was derived from enantiopure monoterpenes. One of these ligands was synthesized from (+)-pinocarvone which was condensed with the acylmethylpyridinium salt under standard conditions to give good yield of the trisubstituted pyridine product. The benzylic position of this compound was deprotonated with butyllithium, and upon addition of methyl iodide the stereoselective methylation was achieved. The subsequent nucleophilic aromatic substitution (Sw/ r) gave rise to the desired ligand. 

Novel, tetrahydroquinoline-based A/,S-type ligands were prepared by the Krohnke pyridine synthesis and their... 

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