Syntheses of 1 → 3 branched isocyanate monomers for dendritic construction

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Abstract
The synthesis of a novel series of 1 → 3 C-branched, isocyanate-based building blocks (monomers) useful for the rapid construction of dendritic architectures is reported. More specifically, the preparation of di-t-butyl 4-isocyanato-4-[(t-butoxycarbonyl)ethyl]-1,7-heptanedioate (3), 4-isocyanato-4-(3-acetoxypropyl)-1,7-diacetoxyheptane (7a), 4-isocyanato-4-[3-(benzyloxy)propyl]-1,7-di(benzyloxy)-heptane (7b), 4-isocyanato-4-[3-(t-butyldimethylsiloxy)propyl]-1,7-bis(t-butyldimethylsiloxy)heptane (7c), 4-isocyanato-1,7-bis[N-(t-butoxycarbonyl)amino]-4-[3-(N-(t-butoxycarbonyl)amino)propyl]heptane (7d), 5-isocyanato-1,9-dicyano-5-(4-cyano-2-oxabutyl)-3,7-dioxanonane (9), and dimethyl 6-isocyanato-6-(4-carbomethoxy-2-oxabutyl)-4,8-dioxaundecanedioate (11), via treatment of the corresponding amines with triphosgene, is described. Characteristic features of each monomer include a triad of protected functionality connected to a single, air-stable isocyanate moiety. Isocyanate stability is postulated to arise from the close proximity of the tertiary alkyl branching center. This series of dendritic building blocks facilitates the introduction of a variety of functional groups, such as alcohol, amines, carboxylic acids, esters, nitriles, and ethers, onto the surface of dendrimers as well as other suitable materials. Isocyanate utility is demonstrated by reaction of a DSM-based, 32-amine-terminated dendrimer with the triscarbamate isocyanate monomer (7d) to afford a poly(t-butyl carbamate)-'coated' dendrimer (12).