Selectivity in the Formal [2 + 2 + 2] Cycloaromatization of Enyne–Allenes Generated by the Alder-ene Reaction from Triynes

Abstract

1,3-Diynyl propiolates undergo the Alder-ene reaction to generate enyne–allenes, which participate in the Diels–Alder reaction to provide products of a formal [2 + 2 + 2] cycloaromatization of three alkynes. Without an external alkyne, enyne–allene reacts with one of the alkyne moieties of 1,3-diynyl propiolate, whereas external alkynes can be used to trap enyne–allene to provide various arene products. The substituents on the dienophilic alkynes have a profound impact on their reactivity. In this Diels–Alder reaction, 1,3-diynes display higher reactivity than monoynes; thus, an excess amount (4–5 equiv) of external monoynes needs to be employed to get good product selectivity.