The cyclic quadridentate 1-methyl-1,4,7,10-tetraazacyclododecane (Mecyclen) has been prepared in three simple steps from p-tosylaziridine, methylammonium acetate, and tris(p-tosyl)diethanolamine. Coordination to Co(III) gives [Co(Mecyclen)Cl-2]Cl in high yield, which on treatment with S-alanine (S-AlaOH), or its methyl ester (50 degrees C, pH 7.5-8.0), gives a mixture of six [Co(Mecyclen)(S-AlaO)](2+) isomers. Five of these have been isolated by a combination of cation ion-exchange chromatography and selective crystallization. Each has been structurally assigned using a combination of nOe and COSY H-1 NMR spectroscopies (d(6)-DMSO solvent) as, in order of decreasing yield from S-AlaOMe, the following : anti(Me), syn(N), anti(O) (isomer 4); anti(Me), anti(N), syn(O) (isomer 5); syn(Me), syn(N), anti(O), (isomer 1); syn(Me), anti(N), syn(O) (isomer 2); syn(Me), syn(N), syn(O) (isomer 3). The structures of 1, 2, and 4 have been confirmed by single-crystal X-ray analysis : [Co(Mecyclen)(S/R-AlaO)](ClO4)(2) . H2O (50:50 mixture of isomer 1 containing S-AlaO and isomer 4 containing R-AlaO), monoclinic, P2(1)/n, a = 14.131(3) Angstrom, b = 8.777(2) Angstrom, c = 17.745(4) Angstrom, Z = 4, R = 0.0734; [Co(Mecyclen)(S-AlaO)]ZnCl4 . 2H(2)O (isomer 2), orthorhombic P2(1)2(1)2(1), a = 8.842(2) Angstrom, b = 14.794(7) Angstrom, c = 17.157(8) Angstrom, Z = 4, R = 0.0415; [Co(Mecyclen)(S-AlaO)](ClO4)(2) . H2O (isomer 4); orthorhombic P2(1)2(1)2(1), a = 8.757(2) Angstrom, b = 9.684(5) Angstrom, c = 25.291(6) Angstrom, Z = 4, R = 0.0380. Structural assignment by the H-1 NMR method depends critically on the presence of separate N-H resonances for the different sites; this can usually be achieved by addition of DCl to a d(6)-DMSO solution of the complex.