Hetero-Diels–Alder reactions of α-carbonylated styrylphosphonates with enol ethers. High-pressure influence on reactivity and diastereoselectivity

Abstract

Variously substituted α-carbonylated styrylphosphonates 5 were easily prepared by Knoevenagel-type syntheses, and used as oxadienes in hetero-Diels–Alder [4 + 2] cycloadditions with enol ethers, to give new phosphonylated 3,4-dihydro-2H-pyrans 6. It was confirmed that the reactivity , as well as the trans-diastereoselectivity of the reaction, was significantly enhanced by the use of high-pressure conditions, particularly in the presence of Bu^tOH as a co-solvent. Moreover, a one-pot synthesis of 6 via a tandem-sequence Knoevenagel and hetero-Diels–Alder reactions was achieved.