H. Al-badri et al., Hetero-Diels-Alder reactions of alpha-carbonylated styrylphosphonates withenol ethers. High-pressure influence on reactivity and diastereoselectivity, J CHEM S P1, (16), 1999, pp. 2255-2266
Variously substituted alpha-carbonylated styrylphosphonates 5 were easily p
repared by Knoevenagel-type syntheses, and used as oxadienes in hetero-Diel
s-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 (B
uOH)-O-t as a co-solvent. Moreover, a one-pot synthesis of 6 via a tandem-s
equence Knoevenagel and hetero-Diels-Alder reactions was achieved.