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 ethenylcarbenechromium 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 π‐electrons in a cross‐conjugated bicyclic system, cyclopenta[b]pyrans have been termed pseudoazulenes, as they indeed have similar UV/Vis‐spectroscopic properties. Pseudoazulenes, that is highly substituted cyclopenta[b]pyrans, are obtained in moderate to very good yields in the reaction of 2‐donor‐substituted Fischer chromiumcarbenes with alkynes, which proceeds with twofold insertion of the latter and subsequent intramolecular [4+2] cycloaddition (see scheme).