The Synthesis of Annulated Azepin-3-one Derivatives from 1,3,4-Pentatrienyl Nitrones by a Heterocyclization−Rearrangement Sequence

Abstract

Treatment of various o-propargylaryl nitrones of type 6 with potassium hydroxide or sodium methoxide in methanol at room temperature provides 1,2-dihydro[c]benzazepin-3-ones 9. The high product yields and the ease of the reactions under surprisingly mild conditions are particularly intriguing in view of the complex mechanistic pathway involved in the overall transformation. A mechanism based on a multistep rearrangement is proposed, involving conjugated allene nitrones of type 13 as precursors of a 1,7-dipolar cyclization process that is followed by further bond reorganizations, with cyclopropanones 16 as key intermediates. In agreement with the allene formation is the fact that the same transformation can be achieved with the triple bond isomers 12 and 37, which contain terminal alkyl groups. The intermediacy of cyclopropanones 16 is supported by the competing formation of the isoindoles 20 as minor products. On treatment of dihydronaphtho-annulated nitrones 30 with base, formation of the azepinones 31 as the main products is also accompanied by that of the isomeric isoindoles 32. Some selective C=O and C=C hydrogenation reactions, together with conversions into the thioketone 42 and the vinyl bromide 9p, have been demonstrated with representative examples of 9.