Through the simple expedient of using a mixture of KF and silica as the stationary phase in column chromatography, levels of organotin impurities from tributyltin hydride mediated reductions have been reduced from stoichiometric levels to approximately 30 ppm.

Homolytic aromatic substitution reactions are used for the efficient regioselective synthesis of helicenes, phenanthrenes, and azahelicenes. The use of halo and alkoxy substituents to control the stereochemical outcome of this approach and X‐ray studies of the resulting helicenes are described (see structure; red O, gray C).

A synthesis of combretastatin A-4 and a small library of analogues led to the discovery of some new cooperative orthoeffects allowing (Z)-stilbenes to be prepared in high yield and diastereomeric ratio.

The thermal rearrangement of vinylcyclobutenones has become established as a reliable method of preparing hydroquinones owing, in large part, to the pioneering work of the research groups of Moore and Liebeskind. [1][2][3] The basic reaction, 1!2, extends to various aryl-and heteroaryl cyclobutenones, [3][4][5] and has been incorporated into a number of more-elaborate domino sequences. [1,6] From a synthetic perspective, the rearrangement has been used primarily to access quinones by the oxidation of 2 into 3 and in that capacity has featured in several natural-products total syntheses. [1,5] Mindful of this, we conceived a simple extension to facilitate the direct conversion of vinylcyclobutenones into quinones. Our plan was to incorporate a leaving group on the vinyl appendage in the hope that thermolysis would induce a domino reaction comprising electrocyclic ring opening to 4, electrocyclization to 5, and elimination of HX to quinone 3 (Scheme 1). Herein we describe our realization of that objective and the discovery of four new thermal rearrangements of cyclobutenones.Our study began with the thermolysis of enol ether 6. Unexpectedly, heating a THF solution of 6 at 120 8C for 30 min by microwave irradiation failed to yield the anticipated quinone and gave instead a complex product mixture from which two diastereoisomers of spirocycle 7 and dione 8 were isolated in yields ranging from 20 to 27 %. Further experimentation showed dione 8 to be an artifact derived by aerial oxidation of 7, and that it could be produced in 73 % yield by oxidation of the crude product mixture with the Dess-Martin periodinane reagent (Scheme 2). [7] Our discovery of a new vinylcyclobutenone rearrangement raised questions as to the mechanistic course of the reaction and the factors responsible for promoting this pathway over the classical sequence depicted in Scheme 1. Scheme 1. The Moore rearrangement and our planned approach to quinones.Scheme 2. Thermal rearrangements of (alkoxyvinyl)cyclobutenones that lead to spirocycles. DCM = dichloromethane.

Homolytische aromatische Substitutionen werden für die effiziente regioselektive Synthese von Helicenen, Phenanthrenen und Azahelicenen genutzt. Der Einsatz von Halogen‐ und Alkoxysubstituenten, um den stereochemischen Verlauf der Reaktionen zu steuern, und röntgenographische Untersuchungen der erhaltenen Helicene wie des im Bild gezeigten (rot O) werden beschrieben.