Heterocycles from Ylides. Reactivity of 2-Acetyl-5-bromothiophene and -5-Methylfuran with Stabilised and Non-stabilised Ylides

The title substrates 3 and 10 react with phosphonium salts 1a,b under phase-transfer catalysis conditions to afford phosphoranes 9a,b or pyrans 13, respectively, along with the corresponding 2,3-fused products with iso-furan rings 7a,b or 11a,b instead of the expected Wittig products. The reaction of 3 with stabilised phosphorus ylides 15a–c affords in the case of 15a,b, the stereoisomers (Z) 17 and (E) 18 in addition to the corresponding phosphonium salts. Compounds 10 and 15a,b yield, on the other hand, the iso-furans 23 and the dimeric products 22a,b as well, according to a sequence involving an initial Wittig reaction between 10 and 15a,b.