Keeping up with the advances in modern heterocyclic chemistry is essential for many of our colleagues in academia and industry. It is the aim of this series on "Stereoselective Heterocyclic Synthesis" to assist the chemical community in this respect by presenting a selection of exciting recent developments. As it was for the first two volumes (1997), the stereoselective synthesis of - or with the aid of - heterocycles is the common motif for all the chapters in this third volume. I am very glad that again leading researchers in this area have contrib- ed highly stimulating accounts with up-to-date coverage. "Stereoselective Heterocyclic Synthesis 111" features chapters on "Stereoselective Intramolecular J,3-Dipolar Cycloadditions" by I. N. N. Nambothiiri and A. Hassner giving an in depth survey of the generation and synthetic application of valuable 1,3-dipoles,"4-Acetoxy- and 4-Cyano-12-dioxanes in Synthesis" by C. J. Sinz and S. D. Rychnovsky presenting a comprehensive summary of the utility of the versatile title compounds in natural products synthesis,"The Synthetic Potential of Three-Membered Ring Aza-Heterocycles" by B. Zwanenburg and l? ten Holte highlighting the fascinating chemistry of aziridine and azirine carboxylic esters, and '(Synthesis of Medium-Sized Ring Lactams" by U. Nubbemeyer discussing a wide range of modern strategies for the stereoselective preparation of these important heterocycles. I wish to express my thanks to all contributors for their dedicated effort and to Mrs. Kollmar-Thoni, Springer-Verlag, for her continuous support. Dresden, March 2001 Peter Metz Contents Stereoselective Intramolecular 1. 3. Dipolar Cycloadditions I. N. N. Namboothiri. A. Hassner.