Neomycin (Bought as a 85:15 mixture of neomycin B and neomycin C)
Boc anhydride
Triethylamine
Water
Methanol

The amine of interest is dissolved in a 10/10/7 solution of water, methanol and triethylamine.  As the solution is stirred Boc₂O_(s) (1.6eq) is slowly added, with a small amount of methanol used to rinse the weigh boat/paper.  The solution is then heated to 55°C for 16hr (overnight), during which time the production of t-butanol will result in a thick slurry.

Following the reaction the solution is allowed to cool to room temperature and the stir bar is removed (if possible).  The slurry is evaporated to dryness on a combined high vacuum/rotary evaporator setup, using dry ice as a coolant.  The resulting white solid is then left under high vacuum for a minimum of 72hr, during which time residual Boc₂O will sublimate into the dry ice trap.  The product may then be used without further purification, extracted if the aminoglycoside was provided as a salt, or columned to separate out isomers (ie. neomycin B from neomycin C).

This reaction has been successfully applied to the protection of Neomycin B, Neamine, Nebramine, Tobramycin and Amikacin.  It failed during an attempted protection of Streptomycin, likely due to the guanidine.

In this reaction, the starting material is a 85:15 mixture of neomycin B and neomycin C, which seems to be a fairly constant ratio among chemical suppliers. At the end of the reaction it is possible to isolate Boc-protected neomycin B via column chromatography (3:2 DCM:acetone).  Yields for NBoc-neomycin B were ~60%, due to poor separation between the closely moving spots, but yields with tobramycin approached 97%.

1H NMR (300 MHz, MeOD) δ 5.38 – 5.12 (m, 2H), 4.92 (s, 1H), 4.21 (s, 2H), 4.10 – 3.06 (m, 20H), 2.05 – 1.88 (m, 1H), 1.71 – 1.19 (m, 55H).

Quader S, Boyd SE, Jenkins ID, & Houston TA (2007). Multisite modification of neomycin B: combined Mitsunobu and click chemistry approach. The Journal of organic chemistry, 72 (6), 1962-79. PMID: 17298096

1H NMR spectra attached.