An efficient three-step synthesis of cyclopenta[b]pyrans via 2-donor-substituted Fischer ethenylcarbenechromium complexes

A wide range of cyclopenta[b]pyrans 4 has been synthesized in a one-pot reaction by treatment of different 2-donorsubstituted ethenylcarbene-chromium complexes 2 with alkynes in THF in moderate to excellent yields (41-90% for 14 out of 25 examples). The starting materials 2 are readily available in good to excellent yields (76-99% for 25 out of 36 examples) by Michael addition of amines, alcohols and thiols, respectively, to the corresponding alkynylcarbenechromium complexes 1. Due to their 10 pi-electrons in a cross-conjugated bicyclic system, cyclopenta[b]pyrans have been termed pseudoazulenes, as they indeed have similar UV/Vis-spectroscopic properties.