Synthesis of α,β-unsaturated dioxanes, dioxolanes and dioxepanes by trans-acetalisation of dimethylacetals with meso or C2-symmetrical 1,2-, 1,3- and 1,4-diols
摘要:
Several o-dibenzylic diols were prepared reacting organometallics with o-phthalaldehyde at room temperature in ether. The identity of the meso and C-2-symmetrical (D,L) isomers as well as their ratio were determined by chiral gas chromatography. The meso and C-2 (racemic) stereoisomeric diols were easily separated by flash chromatography on silica gel. A set of 18 alpha,beta-unsaturated acetals were then prepared reacting those, as well as commercially available 1,2, 1,3 and 1,4 diols, with the corresponding methylacetals in acidic medium. A trans-acetalisation procedure adapted to the cases of fragile allylic alcohols or unfavorable 1,6 diols-derived dioxonanes based on a Dean-Stark trapping of methanol was also employed. (C) 2003 Elsevier Ltd. All rights reserved.