Microwave Chemistry Highlights
A short summary of articles published within the last month on microwave-assisted organic synthesis (MAOS)
Scaffold Decoration of C-Glycosides
The group of John A. Porco, Jr. and Scott E. Schaus from Boston University has reported on the synthesis of aryl ether C-glycoside derivatives 3 via Pd(0)-mediated allylic substitution of biscarbonates 1 with phenols 2 (Org. Lett. 2006, 8, 5065. ). This reaction was both regio- and stereoselective with an overall retention of configuration when the enantiomeric (S,S)-Trost ligand was employed. The protected aryl ether C-glycosides 4 can be further diversified by Eu(III)-catalyzed Claisen rearrangement where glycosides 4 undergo a [3,3]-sigmatropic rearrangement and furnish the novel phenol derivatives 5.
Synthesis of 2-Amino-dihydropyrimidin-4-ones
The three-component reaction of Meldrum´s acid (1), aldehydes 2 and guanidine carbonate (3) for the simple synthesis of 2-amino-dihydropyrimidines 4 was developed by Nikolay Yu. Gorobets and co-workers from the National Academy of Scienes of Ukraine (Mol. Diversity 2006, 10, 483. ). The reactions were performed both under conventional (120-130 °C, 20-25 min) and microwave heating (150 °C, 5 min) showing slightly higher yields for most of the derivates applying conventional conditions. Importantly, to avoid high pressure build up in the closed microwave vials due to CO2 formation, the reaction mixture was stirred for 10-15 min at rt before subjecting to microwave irradiation.
Cross-Coupling of Arylboronic Acids with Acid Chlorides
A set of 19 symmetrical and unsymmetrical aryl ketones was prepared via Suzuki coupling of arylboronic acids with acid chlorides in moderate to high yields under very mild reaction conditions (98 °C, 10 min) by the group of Viera Poláčková from Comenius University in Bratislava (Tetrahedron 2006, 62, 11675. ). A catalyst screen for the reaction of phenylboronic acid and 4-nitrobenzoyl chloride revealed that Pd(PPh3)4 and Cs2CO3 as catalyst/base system provided the aryl ketone (R1 = Ph, R2 = 4-NO2-Ph) in highest yield under the given microwave-conditions.
Synthesis of Tricyclic Symmetrical and Unsymmetrical Heterocycles
Mark Lautens and co-workers from the University of Toronto have investigated the synthesis of tricyclic heterocycles (3 and 4) starting from aryl iodide 1 which contains two tethered alkyl bromides and a Heck acceptor 2 or Zn(CN)2, respectively. Products 3 are obtained via a norbornene-mediated Pd-catalyzed intramolecular bis-alkylation/intermolecular alkenylation sequence of 1 and 2 (Synlett 2006, 2969. ) whereas tricyclic benzonitriles 4 are received via tandem intramolecular bis-alkylation/intermolecular cyanation reaction applying the same catalytic system (J. Am. Chem. Soc. 2006, 128, 14436. ).