Catalytic and selective conversion of (Z)-2-en-4-ynoic acids to either 2H-pyran-2-ones in the presence of ZnBr2 or (Z)-5-alkylidenefuran-2(5H)-ones in the presence of Ag2CO3

Article abstract of DOI:10.1016/S0040-4039(02)01128-0

Treatment of (Z)-5-alkyl-2-en-4-ynoic acids (1), prepared by the Pd-catalyzed alkynylzinc-β-haloacrylic acid coupling, with 5-10 mol% of ZnBr₂ can produce 6-alkyl-2H-pyran-2-ones (2) along with minor amounts of (Z)-5-alkylidenefuran-2(5H)-ones (3) in >90% combined yields, with often very high (≈95/5) pyranone/furanone ratios. On the other hand, lactonization of 1 catalyzed by Ag₂CO₃ provides a selective synthesis of (Z)-5-alkylidenefuran-2(5H)-ones (3) in >90% yields along with minor amounts (≤5%) of 2.