Synthesis of AB4-type carbohydrate scaffolds as branching units in the glycosciences

• Mannosides with four functional groups B and one orthogonal A are prepared.
• (NHBoc)4 functionalisation was achieved by tetra-cyanoethylation and subsequent reduction.
• A 2-benzyloxyethyl aglyon was converted into an azidoethyl moiety.
• To reach an N3/(NHBoc)4 carbohydrate scaffold the N3 function had to come late.

Carbohydrate scaffolds, functionalised according to an AB4-type, were prepared on the basis of α-d-mannopyranosides with various ethyl aglycone moieties, functionalised with ‘A’. Four functional groups ‘B’ were installed at positions 2, 3, 4, and 6 of the sugar ring. In particular, we were interested in preparing N3(NH2)4-functionalised mannosides as multifunctional branching units for further orthogonal derivatisation or immobilisation on surfaces. A detailed synthetic study was performed which revealed that an azido function ‘A’ had to be installed at an advanced stage of the synthesis for successful preparation of the desired AB4-type carbohydrate scaffolds. The most successful synthetic sequence involved tetra-cyanoethylation of a 2-benzyloxyethyl mannopyranoside and subsequent reduction with in situ Boc protection to achieve (NHBoc)4 functionalisation. Finally, the benzyloxyethyl aglycon was converted into the corresponding azidoethyl moiety to gain access to the desired N3(NHBoc)4-functionalised carbohydrate scaffold. Its utilisation was exemplified by straightforward synthesis of a photosensitive glycoconjugate and a tetravalent glycocluster. Such compounds may be immobilised on functional surfaces to serve as tools in cell adhesion studies.

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