Microwave Chemistry Highlights
A short summary of articles published within the last month on microwave-assisted organic synthesis (MAOS)
Organocatalyzed Asymmetric Reactions
Sarah Mossé and Alexandre Alexakis from the University of Geneva have reinvestigated asymmetric aldol (a), Mannich (b) and Diels-Alder reactions (c) applying organocatalysis under microwave conditions (Org. Lett. 2006, 8, 3577. DOI: 10.1021/ol0614727). L-proline (a), N-i-Pr-2R,2’R-bipyrrolidine ((R, R)-iPBP, 1, b) and McMillan imidazolidinone (2, c) were used as organocatalysts. In all cases the reaction times were reduced dramatically using microwave heating. Additionally, for the aldol (a) and Mannich reaction (b) catalyst loadings could be decreased maintaining the enantioselectivities achieved by conventional rt experiments.
The amination of various functionalized heterocyclic bromides with aliphatic primary and cyclic amines and pyrazole was disclosed by Vince S. C. Yeh and Paul E. Wiedeman from Abbott Laboratories (Tetrahedron Lett. 2006, 47, 6011. DOI: 10.1016/j.tetlet.2006.06.119). Optimization studies have shown that high conversions to the desired product could be achieved with the catalyst system CuI/proline which proved to be very general in the further process. However, a rather high catalyst (20%) and ligand (40%) loading was necessary to achieve the aminated products in moderate to good yields.
Synthesis of Quinoxalinones
Sukanta Kamila and Edward R. Biehl from the Southern Methodist University, Dallas, have reported on the synthesis of potentially biologically active quinoxalin-2-ones (Heterocycles 2006, 68, 1931. DOI: 10.3987/COM-06-10793). Symmetrical 1,2-diaminobenzenes 1 were reacted with ethyl 2-bromoalkyl/phenyl acetates 2 and DBU as base under solvent-free microwave conditions to give quinoxalinones 3 in good to excellent yields.
Synthesis of 1-Acetyl-2-amino-cyclohexa-1,3-dienes
Oana G. Schramm and Thomas J. J. Müller from the University of Heidelberg have reported on the one-pot reaction of electron-deficient (hetero)aryl bromides 1, (hetero)aryl propargyl alcohols 2 and enamino carbonyl compounds 3 or 4, respectively (Synlett 2006, 1841. DOI: 10.1055/s-2006-947352). The reaction proceeds via a coupling-isomerization-enamine addition-aldol condensation sequence and furnishes dienes 5 when enaminones 3 are used or the hydrolyzed products 6 if β-amino crotonamide (4) is applied. Compared to thermal heating where dihydropyridines are obtained, an aldol-type condensation is favored in the second step under microwave conditions providing cyclohexa-1,3-dienes 5 and cyclohexanones 6, respectively.