Dithionite reductive activation

Dithionite-mediated reductive activation of mitomycin C has been employed in the study of its DNA alkylation chemistry.6,63 However, dithionite activated mitomycin C possesses different DNA alkylation properties than that activated by catalytic hydrogenation and enzymatic reduction. We postulated that a new alkylating species is produced by dithionite reductive activation resulting in different reactivity than the iminium methide species. To investigate dithionite-mediated reductive activation further, we treated 13 C-labeled analogues of WV-15 with dithionite and carried out spectral and product studies. 

Scheme 7.11 shows the product structures resulting from the dithionite reduction of a simplified version of WV-15. The symmetric sulfite diester was extracted from the reaction mixture with methylene chloride. The isolation and characterization of the sulfite diester confirmed that this species can form in dithionite reductive activation reactions and provided the chemical shift for the 10a-13C center of a mitosene sulfite ester (49.37 ppm). The aqueous fraction of the reaction contained the mitosene sulfonate and trace amounts of Bunte salt, based on their 13C chemical shifts. 

The requirement for reduction prior to DNA alkylation and crosslinking was first demonstrated by Iyer and Szybalski in 1964 [29], and can be induced both by chemical reducing agents such as sodium dithionite and thiols in vitro and by various reductive enzymes such as DT-diaphorase (NAD(P)H-quinone oxidoreduc-tase) in vitro and in vivo [47]. Much work to characterize the mechanism of reductive activation and alkylation has been carried out, principally by the Tomasz and Kohn groups, and Figure 11.1 illustrates a generally accepted pathway for mitomycin C [16, 48-50] based on these experiments, which is very similar to the mechanism originally proposed by Iyer and Szybalski [29]. 

Schiltz, P. Kohn, H. Sodium dithionite-mediate mitomycin C reductive activation processes. Tetrahedron Lett. 1992, 33, 4709 1712. 

A redox system (50/51) to affect brain delivery of y-aminobutyric acid (GABA) derivatives and analogues was also developed. Zincke reaction of 41 with acetal 49 followed by dithionite reduction afforded the 1,4-dihydropyridine prodrug 50, which was hydrolyzed and oxidized in vivo to the active GABA analogue 51. The neutral and lipophilic 1,4-dihydropyridine 50 can penetrate the blood-brain barrier (BBB), whereas the oxidized pyridinium salt 51 is retained in the brain for an extended period and then eliminated. 

Figure 3.11 Mo(v) X-band EPR spectra of Rs NapAB recorded at 15 K. (a) Inactive high- resting Mo(v)-oxidized [4Fe-4S] state detected in the air-purified enzyme, (b) Inactive high-g resting Mo(v)-reduced [4Fe-4S] state obtained after dithionite-reduction of the enzyme for a short incubation time, (c) High-g nitrate Mo(v)-reduced [4Fe-4S] " state obtained after reductive activation of the enzyme and incubation with nitrate. Experimental parameters microwave frequency 9.475 GHz temperature 15 K modulation amplitude 0.1 mT at 100 kHz microwave power 1 pW (a), 3 pW (b), 10 pW (c).

The rate of reduction of cytochrome a was examined by addition of various chemical reductants such as p-phenylenediamine, hydro-quinone, and ascorbic acid. These were then tested as substrates for the cytochrome oxidase, the activity of which was measured manometrically. Onlyp-phenylenediamine reduced cytochrome a, while the other reagents tested, except sodium dithionite, had weak reductive activity. In Fig. 11, it is shown that cytochrome a is oxygenated under aerobic conditions and is reduced under anaerobic conditions by addition of 10 Af/>-phenyl-enediamine and the trace of borohydride, used to decrease the concentration of oxygen in the solution in the cuvette. When a trace of oxidized cytochrome c is added to either system, both forms of cytochrome a are instantaneusly oxidized. As shown in Table IX (see Section III.C), the cytochrome a acquires an oxidase activity in cooperation with cytochrome c when the oxygen consumption is measured manometrically in the presence and absence of cytochrome c. 

The Zincke reaction has also been adapted for the solid phase. Dupas et al. prepared NADH-model precursors 58, immobilized on silica, by reaction of bound amino functions 57 with Zincke salt 8 (Scheme 8.4.19) for subsequent reduction to the 1,4-dihydropyridines with sodium dithionite. Earlier, Ise and co-workers utilized the Zincke reaction to prepare catalytic polyelectrolytes, starting from poly(4-vinylpyridine). Formation of Zincke salts at pyridine positions within the polymer was achieved by reaction with 2,4-dinitrochlorobenzene, and these sites were then functionalized with various amines. The resulting polymers showed catalytic activity in ester hydrolysis. ... 

Semi-continuous and continuous reductive processes are best carried out with activated hydroxymethanesulphinates, since dithionites (even when stabilised) are unsuitable due to being oxidised too quickly by air [273]. 

An alternative route to sulphones utilizes the reaction of the appropriate activated halide with sodium dithionite or sodium hydroxymethanesulphinite [6], This procedure is limited to the preparation of symmetrical dialkyl sulphones and, although as a one-step reaction from the alkyl halide it is superior to the two-step oxidative route from the dialkyl sulphides, the overall yields tend to be moderately low (the best yield of 62% for dibenzyl sulphoxide using sodium dithionite is obtained after 20 hours at 120°C). The mechanism proposed for the reaction of sodium hydroxymethanesulphinite is shown in Scheme 4.20. The reaction is promoted by the addition of base and the best yield is obtained using Aliquat in the presence of potassium carbonate. It is noteworthy, however, that a comparable yield can be obtained in the absence of the catalyst. The reaction of phenacyl halides with sodium hydroxy-methane sulphinite leads to reductive dehalogenation [7]. 

---