Scheme 81

In the case of pyridoxine (299), both hydroxymethyl groups are sequentially oxidised to carboxy acids under oxygen to finally give (300), while under nitrogen dimeric pyridinium-3-olate zwitterion (301) is obtained (Scheme 82) 280

Scheme 82

With pyridoxal-5-phosphate (302) the reaction involves the aldehyde function, giving the acyloin (303) under anaerobic conditions and various oxidised products, such as the carboxylic acid (304) and the dipyridyl a-diketone (305) in the presence of oxygen (Scheme 83).281 Vitamin B12 (306) is liable to photo-oxygenation. The related dicyanocobirinate (307) undergoes two consecutive oxidative cleavages (via singlet oxygen, compare Scheme 1, f) to give the corrin macrocycle (308) (Scheme 84).282



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