Bohlmann-rahtz pyridine synthesis mechanism, bohlmann rahtz pyridine synthesis mechanism
Mechanism of the Bohlmann-Rahtz Pyridine Synthesis
In this efficient procedure, the enamine is generated in situ and then reacts with the alkynone present. As the enamines are not readily available, and to improve the facility of the process, a three-component reaction was developed using ammonium acetate as the amino group source. The direct use of ynones instead of enones obviates the need for an aromatizing oxidation step to get the target pyridines.
A drawback is the limited compatibility with acid-sensitive substrates. For example, acid-catalyzed decomposition of the starting enamines occurs with tert-butylester and cyano as electron withdrawing groups.
Mechanism of the Bohlmann-Rahtz Pyridine Synthesis The reaction is related to the well-known Hantzsch Dihydropyridine Synthesisin which in situ-generated enone and enamine species give dihydropyridines. A mild alternative is the use of Amberlyst ion exchange reagent that allows a simple work up and tolerates tert-butylesters.
Bower, Synlett, Some of the drawbacks have been overcome recently, making the Bohlmann-Rahtz Synthesis more valuable for the generation of pyridines.
In both solvents, spontaneous protodesilylation took place. Instead of using butynone as the substrate, Bagley screened different solvents for the conversion of the less volatile and cheaper 4- trimethylsilyl butynone.
Here, some additional synthetic alternatives are described, such as the use of microwaves, and recent applications such as the synthesis of fused heterocycles and natural products are covered. In a first attempt, AcOH and ZnBr2 were used as additional catalysts with toluene as solvent, but it has been shown more recently that acid-sensitive substrates react under milder acid-free conditions with EtOH as solvent.
Although the Bohlmann-Rahtz Synthesis is more versatile, purification of the intermediate and the high temperatures required for the cyclodehydration are significant drawbacks that have limited its synthetic utility.
Bagley has also shown that acid catalysis allowed the cyclodehydration to proceed at a significantly lower temperature.
Bohlmann-Rahtz Pyridine Synthesis | Chem-Station Int. Ed.
Although Gay matchmaking sydney studies were not undertaken, it is assumed that coordination by the catalyst accelerates the Michael Addition, isomerization and cyclodehydration steps.
A new one-step synthesis of pyridines under microwave-assisted conditions M.
Some methods that allow the synthesis of tri- and tetrasubstituted pyridines in a one-step procedure have recently been developed. DMSO as a polar aprotic solvent. Although no mechanistic studies have been conducted, intermediates can be characterized by 1H-NMR, which clearly shows that the main product of the initial Michael Addition and subsequent proton transfer is a 2Z-4E-heptadienone, which is isolated and purified by column chromatography.
Recent Literature A new mild method for the one-pot synthesis of pyridines X.