Cooperative Al(salen)-pyridinium catalysts for the asymmetric synthesis of trans-configured β-lactones by ［2+2］-cyclocondensation of acylbromides and aldehydes: Investigation of pyridinium substituent effects. Molecules, (17)6:7121–7150, June 2012. [PUMA: catalysis,Ion chemie from:alexanderdenzel rauhut catalysis,Ketene,Pyridinium theoretische stuttgart pair theochem 2-Oxetanone,Aluminum,Cooperative] URL