Microwave Chemistry Highlights
SPOT Synthesis of Pyrimidines on Planar Cellulose Supports
A recent publication by Helen E. Blackwell and co-workers from the University of Wisconsin-Madison (Org. Lett. 2004, 6, 2019. ) reports the multistep synthesis of a spatially addressed pyrimidine library on planar membrane supports (SPOT synthesis). For all of the three steps of the synthesis (see below) microwave irradiation was used to speed up the chemistry on the planar support. Importantly, microwave irradiation did not impact on the integrity of the cellulose support and the chemistry could easily be scaled up employing other (non planar) types of cellulose supports.
Tandem Carbonylation-Lactone Formation Reactions
Mathias Alterman and co-workers from Uppsala University have employed (Tetrahedron Lett. 2004, 45, 4635. ) a tandem carbonylation-lactone formation reaction sequence for the synthesis of phthalides. Optimum conditions involved the use of Mo(CO)6 as a solid source of CO, and Pd(OAc)2/dppf as a catalyst (5 mol%) at 180 °C. The microwave-assisted carbonylation-cyclization method was also applied for the synthesis of other scaffolds, such as dihydroisocoumarins, dihydroisoindones, and phthalimides.
N-Arylation of Sulfoximines
Michael Harmata and coworkers from the University of Missouri-Columbia have disclosed (Tetrahedron Lett. 2004, 45, 5233. ) an efficient protocol for the Pd-catalyzed N-arylation of enantiopure sulfoximines with aryl chlorides. Optimal results were achieved by using Pd(OAc)2 as Pd-source and rac-binap or t-Bu3P as ligands under microwave irradiation conditions. For aryl chlorides bearing ortho-carbonyl substituents the corresponding benzothiazines were obtained.
Heparin Oligosaccharide Synthesis
The synthesis of fully N-differentiated heparin oligosaccharides has been demonstrated by Gregory J. S. Lohman and Peter H. Seeberger from the MIT (J. Org. Chem. 2004, 69, 4081. ). One of the many synthetic steps involves the simultaneous installment of an N-diacetate and O-acetyl functionality in a trisaccharide building block. Microwave irradiation in isopropenyl acetate as solvent in the presence of p-TsOH at 90 °C for 5 hours led to the desired product in 86 % yield. This transformation could not be achieved under a variety of thermal conditions, with only poor yields achieved even after several days.