Ammonia → Ninhydrin

Participants

Reaction Steps

An aminomethyl polystyrene resin support (6.0 g, 0.95 mmol/g, 5.7 mmol) was derivatized with Nα -Fmoc, Nδ -Boc-ornithine by standard procedures. The resin was swollen and washed with dichloromethane, neutralized with 5% DIEA in Dichloromethane (1×5 min+3×10 min), and washed with dichloromethane. Coupling of the Orn derivative was mediated by N,N'-diisopropylcarbodiimide (DIPCDI)/HOBt in DMF method (3 equiv. each, 15 min preactivation, 40 mL reaction volume), 3 hours. Washing followed with DMF and dichloromethane. A ninhydrin test on a sample was nearly negative, but unreacted sites were blocked by capping with acetic anhydride (4.5 mL, 5 equiv.) and triethylamine (7.0 mL, 5 equiv.) in DMF (20 mL), 30 minutes. The protected Orn-PS resin (0.78 mmol/g, matches theoretical loading as adjusted for expected weight gain) was washed with DMF, dicholoromethane, and treated with piperidine-DMF (1:4, v/v) (2+10 min) to remove selectively the Nα -Fmoc group. Further washings with DMF, dichloromethane, 2 -propanol, and DMF-dichloromethane (3:7, v/v) prepared the resing for coupling of the MPEG-acid derivative, produced as described in Example 3. A solution of the MPEG-acid (37 g, 18 mmol) in DMF-dichloromethane (3:7 v/v, 75 mL) was combined with HOBt (2.75 g, 18 mmol) and DIPCDI (2.83 mL, 18 mmol) for 15 minutes, and was then added to the H-Orn(Boc) -PS resin. Coupling proceeded for 5 hours, giving a slightly positive ninhydrin test. A second coupling with smaller amounts of MPEG-acid derivative and activating agents (9 mmol scale), and acetylation by already-described procedures followed. The final MPEG-Orn(Boc)-PS resin was washed with dichloromethane, DMF, dichloromethane and methanol, and dried in vacuo over P2O5 for 48 hours. There was obtained 16.9 g of resin (excellent agreement with theoretical weight gain) with a loading of 0.31 mmol/g, comprising PEG:PS:Orn-0.57:0.36:0.07 by weight (based on elemental analysis and amino acid analysis). Similar results were obtained when BOP/HOBt/NMM in DMF protocols were used for activation and coupling.

Provenance & Evidence