Stereoselective Functionalization at C-9 of Retinal: Synthesis of 9-trans-19-Nor-9-haloretinal Analogs

Stereoselective functionalization at C-9 of retinal based on the use of the C 4 +X-unit is described. A sequential linkage of C 4 +X-units (X=Cl, Br, and I) with ylide 8 (C 10 ) and phosphonate 12 (C 5 )selectively gave 9-trans-19-nor-9-Cl, Br, and I-retinal derivatives.