The article describes the synthesis of cyclen based chelators which can accommodate and stabilize di-and trivalent metal ions for biomedical applications. The chelators were synthesized first by monoalkylating cyclen (1,4,7,10 tetraazacyclododecane) with 2-bromoglutamic -1-tert-butyle-5-benzylester followed by alkylating the remaining nitrogens with three equivalents of bromoacetic acid-tert-butyl ester. The carboxylic acid functions from the alkylating agents and the nitrogens from the cyclen are supposed to stabilize the metal ions (e.g.Gadolinium, Gd3+) placed inside the cyclen ring.
Review of the article
.We came across the article while working to synthesize enzyme activated rotaxane based magnetic resonance imaging (MRI) contrast agents. Rotaxanes are mechanically interlocked molecules which are composed of a wheel and a dumbbell shaped axle. The ends of the "dumbbell" serve as blocking groups so that the wheel cannot fall off. One of the blocking groups of the rotaxane was designed to be a cyclen complexed with Gd3+ and the other component is a tripeptide (Ala-Phe-Lys) which is a substrate for the enzyme Cathepsin B. This enzyme is synthesized by the ovarian cancer cells and cleaves the tripeptide allowing for the wheel to fall of the rotaxane structure and the Gd3+ stabilized by the cyclen will serve as a MRI contrast agent.
To make the cyclen based blocking group, we followed the procedures given in the article. First 2-bromoglutaric -1-tert-butyle-5-benzylester from glutamic acid-5-benzyl ester was synthesized as one of the alkylating agents. As described in the article, the amine of glutamic acid-5-benzyl ester was replaced by bromine using sodium bromide, hydrobromic acid, sodium nitrite and sulfuric acid. The carboxylic acid was then tert-butylated using tert-butyl-trichloroacetimidate in the presence of boron trifluoride ethyl etherate as a catalyst and cyclohexane and dimethylacetamide as solvents to give 2-bromoglutaric -1-tert-butyle-5-benzylester. Our previous attempt to make 2-bromoglutaric acid from glutamic acid using different reagents and conditions did not work. Getting this article after several failures to make 2-bromoglutaric acid was a huge relief. To alkylate the nitrogens of the cyclen, we applied a slightly different approach but used the same conditions and reagents as described in the article.
We started first by alkylating the three nitrogens of cyclen with three equivalents of bromoacetic acid-tert-butyl ester. We decided to use this approach because we found it easier to purify trialkylated cyclen than monoalkylated cyclen. We used 100% ethylacetate to run the trialkylated cyclen through a column chromatography. In doing so we avoided the need to use ammonia (which smells bad) and alcohols. The fourth nitrogen was then alkylated with 2-bromoglutaric -1-tert-butyl-5-benzylester using chloroform as a solvent and potassium carbonate as a weak base.
The authors of the article were interested in converting the benzyl ester to carboxylic acid in order to attach the chelator to amine terminated peptide. However we are interested in converting the benzyl ester into amine in order to form amide bond with the carboxylic acid terminated part of the axle. To obtain amine terminated chelator, benzyl ester was converted into an amide using ammonia in methanol solution. Attempts were made to reduce the amide into amine using borane in tetrahydrofuran solution but we only got partial conversion of amide into amine and to our disappointment the amine formed from the reduction reaction attacks the tert-butyl esters forming six membered ring. This was verified with mass spectroscopy and NMR. We are now working to overcome this problem by changing our synthetic strategy. One way of overcoming the problem is to have a carboxylic acid terminated chelator so as to couple it with amine terminated axle.
In conclusion, the authors of the article really did good job in identifying and synthesizing the right alkylating agents. All the procedures described in the paper are reproducible and the article has been extremely helpful in the project that I am undertaking at this time.