Synthesis of heterocyclic systems by transition-metal-catalyzed cyclization migration reactions -: A diversity-oriented strategy for the construction of spirocyclic 3(2H)-furanones and 3-pyrrolones

Two platinum(II) -catalyzed heterocyclization-migration reactions that provide five-membered heterocycle products are described. With 5 mol-% of PtCl2 as a catalyst, 2-alkynyl-2-hydroxy carbonyl compounds 1 are converted into 3(2H)-furanones 2 at 80 C in moderate to excellent yields under very mild reaction conditions. The reaction is proposed to proceed through an oxonium ion intermediate B, which triggers a stereospecific 1,2-shift analogous to an a-ketol rearrangement. When exploited in a different manner, the 2-alkynyl-2-hydroxy carbonyl compounds 1 afford 3-pyrrolones 3 in 33-81% yield. For this purpose, the starting compounds 1 are treated with primary amines in the presence of 5 mol-% of PtCl2 at 100 degrees C in a convenient one-pot process. These cyclization-migration reactions give novel access to 3(2H)-furanones and 3-pyrrolones in which the 2-position bears aryl or alkyl substituents. Synthetically challenging spirocyclic compounds can be prepared in this fashion by ring contraction. Studies that define the scope and limitations of the cyclization-migration synthesis of heterocyclic systems are also described. ((c) Wiley-VCH Verlag GmbH & Co.