Tandem Processes in C-Aryl Ketenes and Ketenimines Triggered by [1,5]-Hydride-Like Migration of an Acetalic Hydrogen Atom

• Published in: European journal of organic chemistry Vol. 2014; no. 4; pp. 878 - 886
• Main Authors: Vidal, Angel, Marin-Luna, Marta, Alajarin, Mateo
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 01-02-2014; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: Cycli­zation; Electrocyclic reactions; Ketenes; Nitrogen heterocycles; Sigmatropic rearrangement
• ISSN: 1434-193X; 1099-0690
• DOI: 10.1002/ejoc.201301501

Heating a range of suitably substituted diazoacetoacetates produced a family of 2‐(1,3‐dioxolan‐2‐yl)phenyl ketenes that, under thermal conditions, smoothly underwent a [1,5]‐H shift/6π‐electrocyclic ring‐closure sequence to give 1H‐2‐benzopyrans. The application of such processes to ketenes, produced by replacing the phenyl scaffolding with a thiophene ring, afforded thienopyrans. The aza‐Wittig reaction of these 2‐(1,3‐dioxolan‐2‐yl)phenyl and thienyl ketenes with N‐aryliminophosphoranes provided analogous ketenimines that transform into the respective 1(2H)isoquinolinones and thienopyridinones under similar thermal conditions, following the same type of cascade sequence.
Under mild thermal conditions, 2‐(1,3‐dioxolan‐2‐yl)phenyl ketenes and ketenimines convert into the respective spirocyclic 1H‐2‐benzopyrans and 1(2H)‐isoquinolines by a [1,5]‐H shift/6π‐electrocyclization cascade sequence.