Syntheses totales et etudes de lignanes biologiquement actifs—6: Syntheses totales de la (±)-iso-β-peltatine et de ses analogues

2-Benzyloxypiperonal 15, a key intermediate in our synthesis of (±)- iso - β - peltatin 7, was obtained by bromination of 4 - hydroxy - 1,3 - benzodioxole 12, followed by treatment with (i) excess n-BuLi,o li]N-methylformanilide, li]HCl/H₂O, and li]benzyl chloride/K₂CO₃. The aromatic aldehydes 15 and 6 were subsequently transformed into the corresponding β - (2 - alkoxy 3,4 - methylenedioxybenzyl) -γ- butyrolactones 19 and 29, respectively. α-Hydroxyalkylation of 19 with 3,4,5-trimethoxybenzaldehyde 20, followed by cyclisation and hydrogenolysis afforded (±)- iso -β- peltatin 7 in good yield. Similarly, α-hydroxyalkylation of 29 with 20 and syringaldehyde 21, followed by cyclisation, afforded good yields of (±)- iso -β- peltatin O-methyl ether 5 and (±)-iso-α- peltatin O-methyl ether 8, respectively.