Jr. Mcelhanon et al., ASYMMETRIC-SYNTHESIS OF A SERIES OF CHIRAL AB(2) MONOMERS FOR DENDRIMER CONSTRUCTION, Journal of organic chemistry, 62(4), 1997, pp. 908-915
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.