A synthesis of the bifunctional chelator 2-(p-thiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid [2-(p-NCS-Bz)-NOTA] is described which illustrates the especial utility of the (4-methoxy-2,3,6-trimethylphenyl)sulfonyl (Mtr) protecting group as an alternative to the p-tolylsulfonyl (Ts) moiety commonly used for Richman-Atkins type cyclizations. Reaction of N,N'-bis(p-tolylsulfonyl)-1-(p-benzamidobenzyl)ethylenediami ne with N,N-bis[2-[(p-tolylsulfonyl)oxy]ethyl]-p-toluenesulfonamide gave 2-(p-benzamidobenzyl)-1,4,7-tris(p-tolylsulfonyl)-1,4,7-tria zacyclononane in 55% yield, whereas the analogous reaction using the Mtr-protected starting materials gave the corresponding Mtr-protected macrocycle in 34% yield. However, deprotection of the Ts- and Mtr-protected macrocycles (H2SO4, 90 degrees C) afforded 2-(p-benzamidobenzyl)-1,4,7-triazacyclononane in 23% and 60% yield, respectively, illustrating the relatively facile cleavage of the Mtr moiety. A modest improvement in overall percent conversion of (p-nitrobenzyl)ethylenediamine into substituted macrocyclic polyamine was observed when comparing the Mtr vs Ts protection (12.6 vs 10.6%). The macrocyclic triamine was converted to 2-(p-NCS-Bz)-NOTA by alkylation with bromoacetic acid (pH 9, 73%) followed by hydrolysis of the benzamide protecting group (6 M HCl, 70 degrees C, 87%) and reaction with thiophosgene (90%). The serum stability of the 67Cu complexes of 1,4,7-triazacyclononane (I), 2-(p-nitrobenzyl)-1,4,7,10-tetraazacy-clododecane (II), 2-(p-nitrobenzyl)-1,4,8,11-tetraazacyclotetradecane (III), 2-(p-PhCONH-Bz)-NOTA (IV), 2-(p-nitrobenzyl)-1,4,7,10-tetraazadodecane-1,4,7,10-tetraacetic acid (V), 2-(p-nitrobenzyl)-1,4,8,11-tetraazatetradecane-1,4,8,11-tetraaceti c acid (VI), and the acyclic ligand 1-(p-nitrobenzyl)-4-methyldiethylenetriamine-N,N,N',N", N"-pentaacetic acid (VII) was measured at 37 degrees C (5% CO2) and showed the following order of relative stability: I < VII < VI < IV < V approximately equal to II approximately equal to III.
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