The mono-Boc-protected diamine (II) was obtained by reaction of 1,4-diaminobutane (I) with Boc2O. Reductive alkylation of (II) to the benzylic amine (V) was carried out in a two-step procedure consisting in condensation of amine (II) with the formyl boronic ester (III) in the presence of molecular sieves, followed by reduction of the resultant imine (IV) with NaBH4. The N-Boc protecting group of (V) was then removed by acidic treatment, yielding diamine (VI). The title boronic acid was finally obtained by boronate ester exchange in (VI) with phenylboronic acid in a two-phase system.