DOTA Tris(phenylmethyl) Ester - American Chemical Society

Synthesis of DOTA Monoamides Containing Acid-Labile Bonds ... The synthesis of DOTA tris(phenylmethyl) ester 2, a new monoreactive derivative of DOTA,...
0 downloads 0 Views 57KB Size
Download PDF
Bioconjugate Chem. 2001, 12, 1081−1084

1081

TECHNICAL NOTES DOTA Tris(phenylmethyl) Ester: A New Useful Synthon for the Synthesis of DOTA Monoamides Containing Acid-Labile Bonds Pier Lucio Anelli, Luciano Lattuada,* Milena Gabellini, and Paola Recanati Bracco Imaging spa, Milano Research Centre, via E. Folli, 50; 20134 Milano, Italy. Received April 8, 2001

The synthesis of DOTA tris(phenylmethyl) ester 2, a new monoreactive derivative of DOTA, is described. This versatile synthon can be easily coupled to compounds bearing an amino group and then deprotected to DOTA monoamide under mild and neutral conditions by catalytic hydrogenolysis. Accordingly, compound 2 has been used in the synthesis of a DOTA monoamide gadolinium complex containing two palmitic esters, which is a component of mixed micelles as MRI contrast agents.

INTRODUCTION

The 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) is a well-studied ligand which forms extremely stable complexes with a variety of metals (1-5). Such metal complexes have found a widespread use in therapy and diagnostic imaging (6-9). For example, the gadolinium complex of DOTA is currently used as extravascular contrast agent for magnetic resonance imaging (MRI) (10, 11). The search for more and more specific therapeutic and diagnostic entities incorporating a metal complex has led to the conjugation of the DOTA moiety to a variety of biomolecules [e.g., peptides (12, 13), bile acids (14, 15), proteins (16)] which should act as carriers, delivering the product to a selected district of the body. Since the conversion of a carboxyl moiety of DOTA into a carboxamide does not dramatically affect the stability of the final metal complex (13, 17), the easiest way to conjugate DOTA to a carrier is by formation of an amide bond. In this respect the activation with isobutyl chloroformate (13, 18) or by formation of a sulfo-NHS active ester (16) have been reported. The statistical polyactivation of the four carboxylic groups of DOTA is the main drawback of this strategy, and to overcome this problem a triprotected derivative should be employed. Indeed, the synthesis of DOTA tris(tert-butyl) ester 1 has been recently reported (12, 19), and conjugation of 1 to a somatostatin derivative gives rise, after acid deprotection of the tert-butyl esters, to a DOTA monoamide analogue of OctreoScan (12). In the past we took advantage of 1 for the coupling of DOTA to bile acid derivatives (14, 15) in order to obtain gadolinium complexes for the magnetic resonance imaging (MRI) of the liver (Figure 1). Recently, we focused our attention on the synthesis of lipophilic gadolinium complexes which were found to be very useful agents for magnetic resonance angiography (MRA) when formulated as mixed micelles (20, 21). Mixed micelles are multicomponent micelles containing a phos* Corresponding author: Phone: ++39.0221772621. Fax: ++39.0221772770. E-mail: [email protected].

Figure 1. Structures of DOTA tris(tert-butyl) ester 1 and DOTA tris(phenylmethyl) ester 2.

pholipid, a biocompatible nonionic surfactant (e.g., Synperonic F-108), and a lipophilic gadolinium complex, such as 8 or 9 (Figure 2). In particular, compound 8 was found very interesting. The mixed micelles containing 8 show high relaxivities and long blood permanence in rats, two essential features for a MRA agent. The only drawback of 8 is its elimination. In fact, 7 days after injection in rats of the formulation only 50% of injected dose is eliminated by the liver while the remaining is still present into the animal body. To increase the elimination, the synthesis of 9 was taken into account. Compound 9 contains two palmitic esters which should be hydrolyzed in vivo, giving rise to a small, hydrophilic gadolinium complex more easily eliminable. We now report here the synthesis of DOTA tris(phenylmethyl) ester 2, which is a valid alternative to compound 1 expecially in all those cases in which the conjugated counterpart cannot survive acid deprotection conditions, as assessed for example in the synthesis of 9. EXPERIMENTAL PROCEDURES

General. Organic and inorganic reagents were purchased from Merck KGaA (Darmstadt, Germany) and Fluka A.G. (Buchs, Switzerland) except for 1-propanephosphonic acid cyclic anhydride 12 (Lancaster, Mu¨hlheim am Main, Germany). 1,4,7,10-Tetraazacyclododecane 3 was synthesized following our patented procedure (22). Resin Amberlite XAD 16.00 was purchased from Rohm and Haas Italia Srl (Gessate, Italy). Thinlayer chromatography (TLC) was carried out on silica gel plates (Merck KGaA, Silica 60 F254, 0.2 mm), and spots were visualized with 1% KMnO4 in 1 N NaOH. Flash chromatography was performed on Merck KGaA silica

10.1021/bc010046x CCC: $20.00 © 2001 American Chemical Society Published on Web 10/13/2001

1082 Bioconjugate Chem., Vol. 12, No. 6, 2001

Figure 2. Structures of the lipophilic gadolinium complexes 8 and 9.

gel 60 (230-400 mesh). IR spectra were recorded on KBr cells on a Perkin-Elmer Infrared 882 spectrometer. 1H and 13C NMR spectra (in CDCl3, if not differently specified) were recorded on a Bruker AC 200 spectrometer. Electrospray ionization was performed on a Finnigan TSQ 700 triple quadrupole mass spectrometer fitted with a Finnigan ESI interface. Melting points were determined in open capillaries with a Bu¨chi B-540 apparatus and are uncorrected. Elemental analysis were performed by Redox Laboratories (Monza, Italy). 1,4,7,10-Tetraazacyclododecane-1-acetic acid 1,1Dimethylethyl Ester (4). A solution of tert-butyl bromoacetate (25.3 g, 130 mmol) in CHCl3 (500 mL) was added dropwise in 7 h to a solution of 1,4,7,10-tetraazacyclododecane 3 (112.3 g, 650 mmol) in CHCl3 (2 L) maintained under nitrogen at room temperature. After 14 h, the solution was concentrated to 800 mL, washed with H2O (9 × 200 mL) to eliminate the excess of 3, dried over Na2SO4, and evaporated to give 4 (39 g, 99%) as a pale yellow oil: IR 1728 cm-1; 1H NMR δ 1.12 (s, 9H, tBu), 2.26 (m, 8H, CH2N), 2.43 (m, 8H, CH2N), 2.93 (s, 2H, CH2COO); 13C NMR δ 27.77 (CH3), 44.9, 45.7, 46.6, 51.4 (CH2N), 56.7 (CH2COO), 80.4 (C), 170.4 (CO); MS m/z 287 [M + H]+. Anal. Calcd for C14H30N4O2: C, 56.20; H, 10.06; N, 18.56. Found: C, 56.36; H, 10.34; N, 17.84. 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic acid 1,1-dimethylethyl tris(phenylmethyl) ester adduct with NaCl (5). A solution of 4 (36 g, 126 mmol) in DMF (200 mL) was added dropwise in 7 h to a stirred suspension of benzyl bromoacetate (94.96 g, 414 mmol) and K2CO3 (86.8 g, 628 mmol) in DMF (250 mL) maintained under nitrogen at room temperature. After 14 h the suspension was filtered and the orange solution evaporated to dryness. The residue was dissolved in EtOAc (500 mL), washed with H2O (3 × 400 mL) then with brine (2 × 300 mL). The organic phase was separated, dried over Na2SO4 and evaporated. The resi-

due was purified by flash chromatography (CH2Cl2/MeOH 15:1) to give 5 (51 g, 51%) as a sticky pale orange solid: IR 1730 cm-1; 1H NMR δ 1.29 (s, 9H, tBu), 2.19-3.26 (m, 24H, CH2N), 4.94-4.98 (m, 6H, CH2Ph), 7.13-7.16 (m, 15H, Ph); 13C NMR δ 27.8 (CH3), 48-53 (broad signal, CH2N), 55.0, 55.6 (CH2COO), 66.8 (CH2Ph), 82.1 (C), 128.2, 128.4, 128.4, 134.9, 135.0 (Ph), 172.9, 173.4 (CO); MS m/z 754 [M + Na]+. Anal. Calcd for C41H54ClN4NaO8: C, 62.38; H, 6.91; N, 7.10; Na, 2.91; Cl, 4.49. Found: C, 61.77; H, 6.74; N, 6.90; Na, 2.90; Cl, 4.95. 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic Acid Tris(phenylmethyl) Ester (2). Compound 5 (47.11 g, 60 mmol) was dissolved in dioxane (500 mL), and then 12 N HCl (500 mL) was added under nitrogen at room temperature, obtaining a white precipitate. After stirring for 16 h, the pale yellow suspension was evaporated and the residue dissolved in H2O (700 mL) by ultrasound sonication. The solution (pH 2) was loaded onto an Amberlite XAD-16.00 resin column (900 mL) and eluted with a CH3CN/H2O gradient. The product elutes with 40% CH3CN/H2O. The fractions containing the product were concentrated to remove CH3CN and then extracted with EtOAc. The organic phase was dried over Na2SO4 and evaporated. The pale yellow residue was triturated with EtOAc (150 mL) to give 2 (21 g, 52%) as a white solid: mp 108-110 °C; IR 1735 cm-1; 1H NMR δ 2.64 (m, 8H, CH2N), 2.90 (m, 4H, CH2N), 3.34-3.58 (m, 12H, CH2N), 5.01 (m, 6H, CH2Ph), 7.24 (m, 15H, Ph); 13C NMR δ 48.1, 50.2, 53.1, 53.2, 55.4, 55.6, 56.1 (CH2N), 66.1, 66.3 (CH2Ph), 128.1, 128.2, 128.3, 128.4, 128.5, 135.2 (Ph), 166.5, 170.4, 171.1 (CO); MS m/z 697 [M + Na]+. Anal. Calcd for C37H46N4O8: C, 65.86; H, 6.87; N, 8.30. Found: C, 66.00; H, 7.03; N, 8.33. 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic Acid Phenylmethyl Ester (6). This product elutes with 10% CH3CN/H2O from the column used for purification of 2. Evaporation of the eluate gave 6 (3 g, 10%) as a pale yellow solid: mp 87-92 °C (dec); IR 1737 cm-1; 1H NMR δ (CD3OD) 3.14-3.85 (m, 24 H, CH2N), 5.13 (s, 2H, CH2Ph), 7.36 (m, 5H, Ph); 13C NMR δ (CD3OD) 49.6, 50.1, 52.6, 53.0, 54.3, 54.9, 58.2 (CH2N), 67.8 (CH2Ph), 129.7, 129.9, 137.4 (Ph), 170.4, 172.4, 175.0 (CO); MS m/z 517 [M + Na]+. Anal. Calcd for C23H34N4O8: C, 55.85; H, 6.93; N, 11.33. Found: C, 55.84; H, 7.09; N, 11.08. 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic Acid 1,4-bis(Phenylmethyl) Ester (7). This product elutes with 20% CH3CN/H2O from the column used for purification of 2. Evaporation of the eluate gave 7 (3 g, 8.5%) as a pale yellow solid: mp 78-82 °C (dec); IR 1736 cm-1; 1H NMR δ (DMSO-d6) 2.71-3.63 (m, 24H, CH2N), 5.10 (s, 4H, CH2Ph), 7.35 (m, 10H, Ph); 13C NMR δ (DMSO-d6) 49.3, 49.9, 50.8, 51.1, 54.2, 55.6 (CH2N), 65.3 (CH2Ph), 127.9, 128.4, 136.0 (Ph), 169.2, 170.9 (CO); MS m/z 607 [M + Na]+. Anal. Calcd for C30H44N4O8: C, 61.20; H, 7.54; N, 9.52. Found: C, 61.52; H, 6.97; N, 9.43. 10-[2-[Bis[2-[(1-oxohexadecyl)oxy]ethyl]amino]-2oxoethyl]-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic Acid Tris(Phenylmethyl) Ester Adduct with NaCl (13). A 50% EtOAc solution of 1-propanephosphonic acid cyclic anhydride 12 (23) (16.9 g, 26.6 mmol) was added to a solution of 2 (13 g, 19.3 mmol), hexadecanoic acid iminodi-2,1-ethanediyl ester hydrochloride 11 (24) (12 g, 19.4 mmol) and Et3N (12.6 mL, 90.5 mmol) in CH2Cl2 (600 mL). The reaction mixture was stirred under nitrogen at room temperature for 24 h, and then more 50% EtOAc solution of 12 (6.1 g, 9.6 mmol) was added. After another 24 h, the mixture was washed with brine (2 × 300 mL), dried over Na2SO4, and evaporated.

Bioconjugate Chem., Vol. 12, No. 6, 2001 1083

The residue was purified by flash chromatography (CH2Cl2/MeOH 9:1) to give 13 (12 g, 48%) as a yellow oil: IR 1735 cm-1; 1H NMR δ 0.81 (t, 6H, J ) 6.7 Hz, CH3), 1.19 (m, 48H, aliphatic chain), 1.49 (m, 4H, CH2CH2COO), 2.20 (m, 4H, CH2COO), 2.26-3.56 (m, 28H, CH2N), 4.14 (m, 4H, CH2OOC), 5.07 (m, 6H, CH2Ph), 7.27 (m, 15H, Ph); 13C NMR δ 13.9 (CH3), 22.5, 24.3, 24.6, 29.0, 29.1, 29.2, 29.3, 29.5, 31.7, 33.9 (aliphatic chain CH2), 46.4, 46.8 (CONCH2), 48-53 (broad signals, tetraazacyclo CH2), 55.0 (NCH2CO), 61.8 (CH2OOC), 66.7, 66.8 (CH2Ph), 126.9, 128.0, 128.3, 128.4, 135.1, 135.2 (Ph), 172.0, 173.2, 173.4 (CO); MS m/z 1261 [M + Na]+. Anal. Calcd for C73H115ClN5NaO11: C, 67.59; H, 8.94; N, 5.40; Cl, 2.73; Na, 1.77. Found: C, 66.91; H, 9.09; N, 5.34; Cl, 3.10; Na, 1.92. 10-[2-[Bis[2-[(1-oxohexadecyl)oxy]ethyl]amino]-2oxoethyl]-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic Acid (14). Pd (10%) on carbon (1.43 g) was added to a solution of 13 (10 g, 7.7 mmol) in EtOH (150 mL), and the suspension was stirred over 12 h under a hydrogen atmosphere at room temperature. The mixture was filtered over Millipore FT 0.45 µm and the solution evaporated under reduced pressure. The residue was suspended in H2O (200 mL), stirred for 1 h, and then centrifuged (10 °C, 8000 rpm, 30 min). The solution was decanted away to eliminate the dissolved NaCl, and the remaining solid was dried (1.3 kPa, P2O5) to give 14 (7.4 g, 98%) as a white solid: mp 82 °C (dec); IR 1736 cm-1; 1H NMR δ 0.84 (t, 6H, J ) 7 Hz, CH ), 1.22 (m, 48H, 3 aliphatic chain), 1.55 (m, 4H, CH2CH2COO), 2.25 (m, 4H, CH2COO), 2.5-3.9 (broad m, 28H, CH2N), 4.15 (m, 4H, CH2OOC); 13C NMR δ 13.9 (CH3), 22.5, 24.4, 24.7, 29.1, 29.2, 29.6, 31.8, 34.0, 34.8 (aliphatic chain CH2), 46.958.0 (broad signals, CH2N), 61.7 (CH2OOC), 173.3, 173.4 (CO); MS m/z 991 [M + Na]+. Anal. Calcd for C52H97N5O11: C, 64.50; H, 10.10; N, 7.23. Found: C, 64.07; H, 9.78; N, 7.22.

Figure 3. Structures of the byproducts recovered during the purification of 2.

Peralkylation of 4 with benzyl bromoacetate in DMF and K2CO3 gave tetraester 5 as NaCl adduct, after workup with brine. The ability of DOTA tetraesters to coordinate sodium ions has been already assessed (25, 26). The presence of NaCl made the purification of 5 easier by flash chromatography on silica gel, avoiding the use of large amounts of ammonia in the eluent. Deprotection of the tert-butyl ester of 5 with 12 N HCl in dioxane gave compound 2, which was purified and desalted by elution through an Amberlite XAD 16.00 resin column with CH3CN/H2O. Minor amounts of DOTA benzyl ester 6 and 1,4dibenzyl ester 7 were also recovered during the purification of 2. The structure of 7 was assigned on the basis of symmetry and the 13C NMR spectrum (Figure 3). In fact, the 13C NMR spectrum of 7 shows six different methylene groups on nitrogen, while the corresponding 1,7-dibenzyl ester should have only four. The usefulness of intermediate 2 was proved in the synthesis of ligand 14 (Scheme 2). DOTA tribenzyl ester Scheme 2. Synthesis of Ligand 14

RESULTS AND DISCUSSION

The use of compound 1 for the synthesis of 9 is hampered by the fact that the acid deprotection of tertbutyl esters leads to a significant hydrolysis of the palmitic esters too. For this reason we planned the synthesis of synthon 2 (Scheme 1), since the benzyl esters Scheme 1. Synthesis of DOTA Tris(phenylmethyl) Ester 2

can be deprotected by catalytic hydrogenolysis without affecting the esters with the palmitic residues. Reaction of an excess of 1,4,7,10-tetraazacyclododecane 3 (5 mol equiv) with tert-butyl bromoacetate gave the monoalkylated derivative 4 in nearly quantitative yield.

2 was coupled to amine hydrochloride 11 (24) using 1-propanephosphonic acid cyclic anhydride 12 as condensing agent (23) to obtain 13 as NaCl adduct. Cleavage of the benzyl ester protections of 13 by hydrogenolysis yielded the triacid 14. Complexation of 14 with (CH3COO)3Gd in CHCl3/MeOH/H2O at 50 °C gave the complex 9 in quantitative yield.

1084 Bioconjugate Chem., Vol. 12, No. 6, 2001

Mixed micelles containing complex 9 show a better elimination profile than those containing complex 8. In fact, 7 days after injection in rats, 67% of the injected dose is eliminated (for complex 8 was only 50%). Moreover, 27% of injected dose is found in urine, and this is an indirect evidence of a partial hydrolysis in vivo of complex 9 (21). In conclusion, compound 2 can be easily coupled to a variety of carriers bearing an amino group and then deprotected to DOTA monoamide under mild and neutral conditions by catalytic hydrogenolysis. This is a considerable advantage over compound 1 which requires acidic conditions for the deprotection. Indeed, this was proved with the synthesis of gadolinium complex 9, which is a promising component of mixed micelles for MRI coronarography. LITERATURE CITED (1) Clarke, E. T., and Martell, A. E. (1991) Stabilities of the alkaline earth and divalent transition metal complexes of the tetraazamacrocyclic tetraacetic acid ligands. Inorg. Chim. Acta 190, 27-36. (2) Clarke, E. T., and Martell, A. E. (1991) Stabilities of trivalent metal ion complexes of the tetraacetate derivatives of 12-, 13- and 14-membered tetraazamacrocycles. Inorg. Chim. Acta 190, 37-46. (3) Kumar, K., Magersta¨dt, M., and Gansow, O. A. (1989) Lead(II) and bismuth(III) complexes of the polyazacycloalkane-N-acetic acids nota, dota, and teta. J. Chem. Soc., Chem. Commun. 145-146. (4) Cacheris, W. P., Nickle, S. K., and Sherry, A. D. (1987) Thermodynamic study of lanthanide complexes of 1,4,7triazacyclononane-N,N′,N′′-triacetic acid and 1,4,7,10-tetraazacyclododecane-N,N′,N′′,N′′′-tetraacetic acid. Inorg. Chem. 26, 958-960. (5) Loncin, M. F., Desreux, J. F., and Merciny, E. (1986) Coordination of lanthanides by two polyamino polycarboxylic macrocycles: formation of highly stable lanthanide complexes. Inorg. Chem. 25, 2646-2648. (6) Volkert, W. A., and Hoffman, T. J. (1999) Therapeutic radiopharmaceuticals. Chem. Rev. 99, 2269-2292. (7) Guo, Z., and Sadler, P. J. (1999) Metals in medicine. Angew. Chem., Int. Ed. 38, 1512-1531. (8) Reichert, D. E., Lewis, J. S., and Anderson, C. J. (1999) Metal complexes as diagnostic tools. Coord. Chem. Rev. 184, 3-66. (9) Thunus, L., and Lejeune, R. (1999) Overview of transition metal and lanthanide complexes as diagnostic tools. Coord. Chem. Rev. 184, 125-155. (10) Caravan, P., Ellison, J. J., McMurry, T. J., and Lauffer, R. B. (1999) Gadolinium(III) chelates as MRI contrast agents: structure, dynamics, and applications. Chem. Rev. 99, 2293-2352. (11) Aime, S., Botta, M., Fasano, M., and Terreno, E. (1998) Lanthanide(III) chelates for NMR biomedical applications. Chem. Soc. Rev. 27, 19-29. (12) Heppeler, A., Froidevaux, S., Ma¨cke, H. R., Jermann, E., Powell, P., and Henning, M. (1999) Radiometal-labeled macrocyclic chelator-derivatised somatostatin analogue with superb tumor-targeting properties and potential for receptormediated internal radiotherapy. Chem. Eur. J. 5, 1974-1981.

(13) Li, M., and Meares, C. F. (1993) Synthesis, metal chelate stability studies, and enzyme digestion of a peptide-linked DOTA derivative and its corresponding radiolabeled immunoconjugates. Bioconjugate Chem. 4, 275-283. (14) Anelli, P. L., Calabi, L., de Hae¨n, C., Lattuada, L., Maiocchi, A., Morosini, P., and Uggeri, F. (1997) Hepatocytedirected MR contrast agents. Can we take advantage of bile acids? Acta Radiologica, 38, Supplement 412, 125-133. (15) Anelli, P. L., de Hae¨n, C., Lattuada, L., Morosini, P., and Uggeri, F. (1995) Bile acid conjugates, derivatives thereof with metal complexes and related uses PCT Int. Appl. WO 95/ 32741, Chem. Abstr. 124, 192411. (16) Lewis, M. R., Raubitschek, A., and Shively, J. E. (1994) A facile, water-soluble method for modification of proteins with DOTA. Use of elevated temperature and optimized pH to achieve high specific activity and high chelate stability in radiolabeled immunoconjugates. Bioconjugate Chem. 5, 565576. (17) Aime, S., Anelli, P. L., Botta, M., Fedeli, F., Grandi, M., Paoli, P., and Uggeri, F. (1992) Synthesis, characterization, and 1/T1 NMRD profiles of gadolinium(III) complexes of monoamide derivatives of DOTA-like ligands. X-ray structure of the 10-[2-[[2-hydroxy-1-(hydroxymethyl)ethyl]amino]-1[(phenylmethoxy)methyl]-2-oxo-ethyl]-1, 4, 7, 10-tetraazacyclododecane-1,4,7-triacetic acid-gadolinium(III) complex. Inorg. Chem. 31, 2422-2428. (18) Sieving, P. F., Watson, A. D., and Rocklage, S. M. (1990) Preparation and characterization of paramagnetic polychelates and their protein conjugates. Bioconjugate Chem. 1, 65-71. (19) Rajopadhye, M., Edwards, D. S., Harris, T. D., Heminway, S. J., Liu, S., and Singh, P. R. (1999) Pharmaceuticals for the imaging of angiogenic disorders. PCT Int. Appl. WO 99/ 58162, Chem. Abstr. 131, 351678. (20) Anelli, P. L., Lattuada, L., Uggeri, F., Lux, G., Serleti, M., Gabellini, M., and Tournier, H. (2000) Amphipatic polycarboxylic chelates and complexes with paramagnetic metals as MRI contrast agents. PCT Int. Appl. WO 00/30688, Chem. Abstr. 133, 26120. (21) Anelli, P. L., Lattuada, L., Lorusso, V., Schneider, M., Tournier, H., and Uggeri, F. (2001) Mixed micelles containing lipophilic gadolinium complexes as MRA contrast agents. MAGMA 12, 114-120. (22) Argese, M., Ripa, G., Scala, A., and Valle, V. (1997) A process for the preparation of tetraazamacrocycles. PCT Int. Appl. WO 97/49691, Chem. Abstr. 128, 114973. (23) Wissman, H., and Kleiner, H.-J. (1980) New peptide synthesis. Angew. Chem., Int. Ed. Engl. 19, 133-134. (24) Tundo, P., Kippenberger, D. J., Politi, M. J., Klahn, P., and Fendler, J. H. (1982) Redox active funtionally polymerized surfactant vesicles. Syntheses and characterization. J. Am. Chem. Soc. 104, 5352-5358. (25) Tsukube, H., Mizutani, Y., Shinoda, S., Okazaki, T., Tadokoro, M., and Hori, K. (1999) Sidearm effects on cyclenalkali metal cation complexation: highly selective and threedimensional encapsulation of Na+ ion. Inorg. Chem. 38, 3506-3512. (26) Kumar, K., Chang, A. C., Francesconi, L. C., Dischino, D. D., Malley, M. F., Gougoutas, J. Z., and Tweedle, M. F. (1994) Synthesis, stability, and structure of gadolinium(III) and Yttrium(III) macrocyclic poly(aminocarboxylates). Inorg. Chem. 33, 3567-3575.

BC010046X