One-step synthesis of pyridines and dihydropyridines in a continuous flow microwave reactor

Figure 1: Microwave flow reactor for the Bohlmann–Rahtz synthesis of pyridine 2b.

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The Bohlmann–Rahtz pyridine synthesis and the Hantzsch dihydropyridine synthesis can be carried out in a microwave flow reactor or using a conductive heating flow platform for the continuous processing of material. 
In the Bohlmann–Rahtz reaction, the use of a Brønsted acid catalyst allows Michael addition and cyclodehydration to be carried out in a single step without isolation of intermediates to give the corresponding trisubstituted pyridine as a single regioisomer in good yield. 
Furthermore, 3-substituted propargyl aldehydes undergo Hantzsch dihydropyridine synthesis in preference to Bohlmann–Rahtz reaction in a very high yielding process that is readily transferred to continuous flow processing.

Mark C. Bagley¹, Vincenzo Fusillo², Robert L. Jenkins², M. Caterina Lubinu² andChristopher Mason³

¹Department of Chemistry, School of Life Sciences, University of Sussex, Falmer, Brighton, East Sussex, BN1 9QJ, UK
²School of Chemistry, Main Building, Cardiff University, Park Place, Cardiff, CF10 3AT, UK
³CEM Microwave Technology Ltd, 2 Middle Slade, Buckingham, MK18 1WA, UK

This article is part of the Thematic Series "Chemistry in flow systems III".

Guest Editor: A. Kirschning

Beilstein J. Org. Chem. 2013, 9, 1957–1968.