Asymmetric synthesis of a series of chiral AB(2) monomers for dendrimer construction

Efficient preparation of a series of four chiral, nonracemic AB(2) monomers suitable for the construction of dendrimers is presented. Monomers 1-4 possess the common structural features of a diphenolic moiety and a benzylic or aliphatic hydroxyl which render these molecules suitable for convergent dendrimer synthesis. The same basic, high-yielding, five-step sequence is employed for 1-4. Stilbene derivatives 13 and 14 are prepared by a Horner-Wadsworth-Emmons modified Wittig reaction between 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 intermediates 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.