Cyclizations of Alkoxyallenes: Mechanisms, Intermediates, Products - A Personal Account on Solved and Unsolved Problems with Unique Allene Building Blocks

The additions of lithiated alkoxyallenes to electrophiles, such as carbonyl compounds, thioketones, imines, and nitrones, provide the expected primary addition products. These alkoxyallene intermediates undergo ring-closure reactions under quite different conditions. Whereas allenyl hydroxylamine derivatives spontaneously cyclize to 1,2-oxazine derivatives, the related allenyl amines, thiols, and alcohols require, with distinct exceptions, promotion by acids, base, silver(I), or gold(I). The different mechanisms of these processes are discussed in this account. The serendipitous discovery of a novel three-component reaction of lithiated alkoxyallenes, nitriles, and carboxylic acids followed by a cyclization to pyridine derivatives is also reported and the mechanism involved is illustrated. This account also compiles exemplary examples of natural products and other compounds prepared by subsequent reactions of alkoxyallene-based cyclization products, but the fascinating ring-closing event of the allenyl intermediates is the main focus of this report. 1 Introduction 2 Cyclizations of Allenyl Hydroxylamines to 1,2-Oxazine Derivatives 3 Cyclizations of Allenyl Amines to Dihydropyrrole Derivatives 4 Cyclizations of Allenyl Imines to Pyrroles - Discovery of a New Three-Component Synthesis of Pyridines 5 Cyclizations of Allenyl Thiols to Vinylthiiranes and Dihydrothiophene Derivatives 6 Cyclizations of Allenyl Alcohols to Dihydrofuran Derivatives 7 Conclusions