Syntheses of new 1 → (2+1) C-branched monomers for the construction of multifunctional dendrimers

For the purpose of providing a route to multifunctionalized dendrimers, new types of 1 --> (2 + 1) C-branched monomers, possessing either ester and protected hydroxy groups or mixed esters, were designed and synthesized. Thus, di-tert-butyl 4-(3-[X]oxypropyl)-4-aminoheptanedioate (X = MeCO, 4; X = CH2CH2CN, 6; X = CH2C6H5, 7) was prepared in high yields via the protection of the nitro precursor 2 (X = H), which was readily accessible from the Michael addition of 4-nitrobutanol with tert-butyl acrylate, followed by catalytic reduction. These monomers were readily attached to an appropriate four-directional core to produce the first-generation dendrimers (e.g., 9-11). The related second-generation dendrimer (15), possessing two different functional groups on both the surface and interior, was convergently synthesized from monomer 3. Alternatively, the mixed ester 17 was prepared starting with benzyl 4-nitrobutanoate (16); selective deprotection offered access to the representative 1 --> (2 + 1) C-branched monomer 20, which was converted to dendrimers 29 and 30 with a single novel terminus per dendron. These 1 --> (2 + 1) C-branched monomers offer synthetic versatility to place different functionality at different levels within or on the surface of the dendritic construct.