Synthesis and absolute configuration of the four possible stereoisomers of prandiol

Prandiol and its three stereoisomers were synthesized in five steps starting either from (R)-(+)-2,3-dihydro-2-(1′-methylethenyl)-6-methoxybenzofuran 1a or from its enantiomer (S)-(−)-1b. The synthesis involved condensation of the corresponding phenol 4 with ethyl propiolate to afford furocoumarins 6. The absolute configuration of the newly formed stereogenic center at C(1′) was established by circular dichroism spectroscopy in combination with X-ray crystallographic analysis.