Cascade Cyclization of <i>o</i>‑(2-Acyl-1-ethynyl)benzaldehydes with Amino Acid Derivatives: Synthesis of Indeno[2,1‑<i>c</i>]pyran-3-ones and 1‑Oxazolonylisobenzofurans via the Erlenmeyer–Plöchl Azlactone Reaction

A highly regioselective divergent approach is reported for the synthesis of both indeno[2,1-c]pyran-3-one and 1-oxazolonylisobenzofuran derivatives using the Erlenmeyer–Plöchl azlactone (EPA) reaction. This approach involves the synthesis of o-(2-acyl-1-ethynyl)benzaldehydes, which reacted with various amino acids. Reaction with N-acylglycines resulted in the formation of indeno[2,1-c]pyran-3-ones, involving the sequential formation of two C–C bonds and two C–O bonds. Conversely, when the same conditions were applied to free amino acids, 1-oxazolonylisobenzofurans were obtained. This reaction involved the formation of a C–C bond between oxazolone and o-(2-acyl-1-ethynyl)benzaldehyde, followed by the formation of a C–O bond through a selective 5-exo-dig cyclization.