Stereoselective alkenylation of aldehydes with phosphorus carbanions : preparation of E- and Z-2-alkoxy- and 2-aryloxy-2-alkenoates

Eighteen phosphorus-based alkerrylation reagents 1a-6c were synthesized and condensed with n-butyraldehyde 7a and benzaldehyde 7f. They were condensed with the aldehydes 7b-e,g-n to produce 2-methoxy-2-alkenoates 8a-n and 2-phenoxy-2-alkenoates 9a-n. A discussion on different selectivities for the alkerrylation reagents under different experimental conditions (base, soh em, temperature), the influence of the aldehydes on the geometrical outcome (electron rich or poor substituents steric hindrance) and the selection of the most E-and Z-selective reaction conditions for each class of aldehydes are reported Namely, triphenyl phosphonium salt la at RT in THF with DBU was better for Z-selectivity while trifluoroethylphosphonates 5b, 6b and 6c at -78 degrees in THF with KN(SiMe3)(2) were better for E-selectivity.