Popis: |
Epoxy alcohol anti-10, derived from a desymmetrizing Sharpless epoxidation (up to 97 % ee) of divinylcarbinol 9, provided the unsaturated 1,3-diol syn-11 upon treatment with RedAl®; syn-11 was converted into the α,β-unsaturated esters (E)- or (Z)-7b in three steps. Cu-promoted 1,4-addition of vinylmagnesium halides to the (E)-ester proceeded with diastereoselectivities of up to 91 % and Cu-catalyzed 1,4-additions with diastereoselectivities of up to 82 %. The potassium enolate of the major vinylation product syn-22b was hydroxylated by the Davis oxaziridine with perfect but unprecedented diastereoselectivity. The resulting hydroxy ester, α,βsyn,β,γsyn-32, furnished the “eastern moiety” building block 6 of the title compounds in three steps. |