acid chain was carried out in about 90% yield by catalytic hydrogenolysis of the 2-iodoethyl derivative l c which was prepared from hexamethyl dicyanocobyrinate [2] via the primary alcohol l a and the corresponding mesylate l b (cf. Exper. Part). When oxidized with O2 in the presence of ascorbic acid, 2 yielded a brownish yellow corrinoid whose unexpected structure 5 was elucidated by spectroscopic methods (Scheme 2).
The UVjVIS spectrum of 5 shows a distinct similarity to those of the other yellow corrinoids characterized so far. The hypsochromic shift of the maximum at 476 nm indicates, however, that the chromophore must be slightly different. The presence of a carbonyl group at C( 5) is revealed both by an IR-absorption band at 1700 cm-' and by a low-field peak (194.21 ppm) in the I3C-NMR spectrum.
Particularly, the configuration at C(6) of 5 could be established, after the signals of all CH, groups in the molecule had been assigned by 'H-NMR NOE difference spectroscopy and selective I3C,'H decoupling experiments (cf. Exper. Part), on the ground of the NOE observed between the two CH, groups at C(6) and C(7), which must be, therefore, cis to each other (see Table ).
The formation of 5 as the main product of the reaction of 2 with Udenfriend's reagent, surprising at the first glance, can be explained in a straightforward manner by the Table. Assignments of 'H-NMR Resonance Signals in 5 by 'H,'H-NOE-Difference Experimentsa) Irradiated Resonance Iuuml Enhanced Signals h m l ~~ 2.15 CH,-C(IS) 2.85 (H-C(13)); 1.22 (CH3-C(17)) 1.66 CH,-C(l) 1.33 (CH,-C(2)); 2.90 (H-C(18)) 1.33 CH,-C(2) 1 66 (CH3-C(1)) 131 CH,-C(7) 1.55 (CH,-C(6)); 2.05, 1.67 (CHZ(8')); 1.65, 1.36 (CHZ(7')) 1.29 a-CH3-C(12) 1.14 (8-CH,-C(12)); 5.13 (H-C(10)) 1.22 CH3-C(17) 3.90 (H-C(19)); 2.15 (CH,-C(lS)) 1.14 p-CH3-C(12) 2.85 (H-C(13)); 5.13 (H-C(I0)) 0.83 CH,-C(7') 2.63 (H-C(8)); 1.65, 1.36 (CHZ(7'))
") In CDCI, solution at 400 13 MHz.
7,
Obviously, step 9 to 10 can be also formulated as a one-electron process. In this case, the Co(II1) ion would change to the Co(I1) oxidation state.