ASYMMETRIC-SYNTHESIS OF A SERIES OF CHIRAL AB(2) MONOMERS FOR DENDRIMER CONSTRUCTION

Efficient preparation of a series of four chiral, nonracemic AB(2) mon omers suitable for the construction of dendrimers is presented. Monome rs 1-4 possess the common structural features of a diphenolic moiety a nd a benzylic or aliphatic hydroxyl which render these molecules suita ble for convergent dendrimer synthesis. The same basic, high-yielding, five-step sequence is employed for 1-4. Stilbene derivatives 13 and 1 4 are prepared by a Horner-Wadsworth-Emmons modified Wittig reaction b etween 3,5- or 3,4-bis(benzyloxy)benzaldehyde (8 and 10) and an ester- substituted benzylphosphonate (11 or 12). Cinnamate derivatives 21 and 22 are prepared similarly from 8 and 10 and triethyl phosphonoacetate . Chirality is introduced in the form of a 1,2-diol unit by Sharpless asymmetric dihydroxylation (AD) (>97% ee in all cases), Protection of the 1,2-diols as their acetonide derivatives provides dioxolane interm ediates 17, 18, 25, and 26. Reduction of the ester groups followed by hydrogenolysis of the benzyl ethers yields AB(2) monomers 1-4 in 57-67 % overall yield from 8 and 10.