An umpolung strategy for intermolecular [2 + 2 + 1] cycloaddition of aryl aldehydes and nitriles: a facile access to 2,4,5-trisubstituted oxazoles

芳醛和腈分子间[2 + 2 + 1]环加成反应的极性反转策略：一种简便合成2,4,5-三取代噁唑的方法