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

Variously substituted alpha-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 (BuOH)-O-t as a co-solvent. Moreover, a one-pot synthesis of 6 via a tandem-sequence Knoevenagel and hetero-Diels-Alder reactions was achieved.