A New Type of Claisen Rearrangement Involving 1,3‐Dipolar Intermediates. Preliminary communication

Abstract

During the reaction of allyl ethers, allyl sulfides and allyl selenides with in situ prepared dichloroketene (1), 2 competing pathways are observed. Besides [2+2]‐cycloaddition, an unprecedented [3,3]‐sigmatropic (Claisen) rearrangement via a 1,3‐dipolar intermediate takes place. It leads to O‐, S‐ or Se‐esters of α,α‐dichloro‐γ, δ‐unsaturated acids containing an inverted allylic group. Starting from cyclic n‐membered α‐vinyl‐substituted ethers, lactones with n+4‐membered rings are formed. A very facile synthesis of the naturally occurring macrolides (±)‐phoracantholide I (10) and (±)‐phoracantholide J (11) illustrates the synthetic utility of this new ‘ketene’ Claisen rearrangement.

1,2‐Asymmetrische Induktion bei der Keten‐Claisen‐Umlagerung von Allylsulfiden

“…Keten-Claisen~Umlagerung der Allylsulfide: In 15 mL Ether werden unter Argon und kriftigcm Ruhren 1.5 mmol Allylsulfid und etwa 15 mmol Zn/Cu-Legierung [4] …”

1,2‐Asymmetric Induction in the Ketene Claisen Rearrangement of Allyl Sulfides

“…After removal of the solvent, the thioester was chromatographed or purified by distillation. Ketene Claisen rearrangement of allyl thioether: Ally1 thioether (1.5 mmol) and about 15 mmol of Zn/Cu alloy [4] were placed in 15 mL of vigorously stirred ether under argon and the mixture was heated to reflux. A solution of 3 mmol of freshly distilled trichloroacetyl chloride in 5 mL ofether was added dropwise to the solution over 4 h by means of a syringe pump (7 h infusion for the generation of chloromethylketene from 2,2-dichloropropionyI chloride).…”

“…The most outstanding feature of the structure is its general resemblance to that of 7, particularly with respect to the asymmetric disposition of the alkenylidene, C(3) being at- [4] ing, there is a large change in the "twist" angle of the terminal methylene group relative to the metallacyclopropane core when going from 7 (69") to 11 (160"). It is curious that, while both experience an unusually large distortion (typically only about I O")r39 91 from the "ideally" expected perpendicular and coplanar configurations, respectively, the net "rotation" is exactly that predicted: 90".…”

Die Belluš‐Claisen‐Umlagerung

“…[4] Besonderes Augenmerk liegt dabei auf den synthetischen Anwendungen chemo-, regio-, stereo-und enantioselektiver Belluš-Claisen-Umlagerungen für C-C-Verknüpfungen. …”

“…[4,14] Zunächst wurde für die cyclische Variante dieser Reaktion eine enttäuschende Ausbeute von 8 % erhalten. [14] Vedejs et al optimierten dieses Verfahren zur Ringerweiterung, indem sie Dichlorketen (11) durch langsame Zugabe eines Überschusses an CCl 3 COCl zu einer Mischung von Zn/CuLegierung und dem Substrat in siedendem Ether generierten.…”