Iminophosphoranes: Useful Building Blocks for the Preparation of Nitrogen-Containing Heterocycles

The almost centenarian reaction of iminophosphoranes with carbonyl or heterocumulene compounds (aza-Wittig reaction) is presented in this review, but with focus mainly on publications of the last decade. Both inter- and intramolecular versions of the aza-Wittig reaction are discussed, including for the latter mono and tandem cyclizations. The feasibility of preparing several kinds of nitrogen-containing heterocycles by this method has been systematically examined. Several successful examples of the synthesis of natural products, using the iminophosphorane-mediated cyclization as the key step, have demonstrated the maturity of the field and its excellent prospects. 1. Introduction 2. Intermolecular Aza-Wittig Reaction 3. Intramolecular Aza-Wittig Reaction 3.1. Ketones 3.2. Esters 3.3. Acyl Chlorides 3.4. Amides 3.5. Carboxylic Acid Anhydrides 4. Reaction of Iminophosphoranes with Alcohols and Oxiranes 5. Tandem Reactions 5.1.Tandem Aza-Wittig/Electrocyclic Ring Closure 5.1.1.Pyrido Annulation 5.1.1.1. Synthesis of c-Fused Pyridines 5.1.1.2. Synthesis of b-Fused Pyridines 5.1.2. Pyrimido Annulation 5.2. Tandem Aza-Wittig/Heterocumulene-Mediated Annulation 6. Consecutive Processes 7. Cyclization of Phosphazides