An efficient synthesis of oxindoles is achieved from a thermal and rhodium(H)-catalyzed Wolff rearrangement of diazoquinolinediones in moderate yields, 0 1999 Elsevier Science Ltd. All rights reserved.
Oxindoles and their derivatives are very important compounds as synthetic intermediates of natural products and pharmaceutical reagents. t Numerous methods for the synthesis of oxindoles have been carried out by Ftiedel-Crafts reaction2 Gassman reaction,3 metal-catalyzed reactions,4 photoinduced cyclization,5 and radical cyclizations. 6 Although a number of methods for the synthesizing of oxindoles have been reported, simple and efficient approaches still remain scarce.
The thermal, photochemical, and metal-mediated Wolff rearrangement is a well-known and useful reaction in organic synthesis. 7 We have been interested in the synthesis of oxindoles. Our strategy for the synthesis of oxindoles begins with a ring contraction of diazodicarbonyl compounds 1 and 3 utilizing Wolff rearrangement. The Wolff rearrangement of diazocarbonyl compounds has been widely investigated by many groups.* However, no study has been made of a thermal and rhodium(R)-catalyzed Wolff rearrangement of diazoquinolinediones. We describe here a novel route to the synthesis of biologically interesting oxindoles and their derivatives by using thermal and rhodium(II)-catalyzed Wolff rearrangements.
Diazoquinolinediones 1 and 3 were prepared by the diazotransfer reaction of the corresponding 4hydroxy-Zquinolones with mesyl azide according to Taber's method.9 In order to check the reactivity of N-methyl diazoquinolinedione 1 for a Wolff rearrangement, thermal reactions in severa solvents were first investigated (Scheme 1). When acetonitrile, fluorobenzene, or toluene as undried solvents (containing a little amount of water) was heated at reflux for 24 h in the absence of a metal catalyst, only starting material was recovered. However, when xylene was used at reflux for 6 h, N-methyloxindole 2 was obtained in a 55% yield. Support for the structural assignment of 2 comes from their spectroscopic * Corresponding author.