Bridging the final gap in stereocontrolled wittig reactions: Methoxymethoxy-armed allylic phosphorus ylides affording conjugated dienes with high cis selectivity

After treatment with an appropriate base (butyllithium or sodium amide), 2-alkenyltris(2-methoxymethoxyphenyl)phosphonium salts carrying an allyl, crotyl, or prenyl (3-methyl-2-butenyl) side chain condense with saturated or unsaturated aldehydes to give conjugated dienes with ZIE ratios ranging from 90:10 to > 99:1 and averaging 96:4. Owing to steric congestion, yields are only moderate (on average 41 %; extremes 10-79 %). The nonvolatile tris(2-methoxymethoxyphenyl)phos-phine oxide by-product can be readily isolated and reduced to recover the phosphane starting material, or it may be hydrolyzed to the water-soluble tris(2-hydroxyphenyl)phosphine oxide.