STEREOCONTROLLED SYNTHESIS OF CONJUGATED POLYENE ISOPRENOIDS USING PHOSPHINE OXIDE ANION INTERMEDIATES

Article abstract of DOI:10.1016/S0040-4020(01)93264-8

The potential for the Horner-Wittig reaction in the stereocontrolled synthesis of conjugated polyene isoprenoids is investigated.It is shown that metallations of the Z- and E-phosphine oxides (4 and 5), followed by reaction with Z- and E-citrals lead, in single steps, to the corresponding isomeric pentaenes (6) showing complete preservation of the Z- and E-geometries in the starting materials.Although the single step reactions leading to 6 proceed in low overall yields (10-20percent), the olefinations occur with >98percent E-stereoselectivity.Isolation of the intermediate β-hydroxyphosphine oxides (e.g. 14), followed by subsequent treatment with s odium hydride in dimethylformamide, demonstrate that these features of the reaction are associated largely with the remarkably slow rates of elimination from the erythro-intermediates in comparison with the threo-intermediates.Condensation between 4 and the ester-aldehyde (20) leads to the Z-6 ester 21 (34percent).After conversion of 21 into the tetraenal (25), reaction with the phosphine oxide (26), followed by separation of the erythro-β-hydroxy-phosphine oxide intermediate 27 and treatment with NaH in DMF, leads to the di-Z-octaene (28).The octaene 28 shows closely similar spectral data to those found in natural di-Z-phytofluene (2) isolated from fruit of the Tangerine tomato, Lycopersicon escalentum var.In a similar manner, reaction between the Z-phosphine oxide (4) and the dialdehyde (29) produces the di-Z-nonaene (33) which has the same chromophore as that found in di-Z-ζ-carotene (3) from Tangerine tomato fruits.