O Oh

This work cannot be detailed here; a typical example is the photorearrangement of calciferol (vitamin D2) (273) which undergoes both sigmatropic hydrogen shift to give (274) and electrocyclic ring-closure to give (275) (Scheme 75).254> 255 Obviously vitamins D must be protected from light.256

Scheme 75

a-Tocopherol (276) (vitamin E) is well known as an antioxidant. The sensitised photo-oxygenation involves singlet oxygen addition to the phenol moiety giving hydroperoxycyclohexadienone (277). This in turn slowly decomposes to give benzoquinone derivatives (278) and (279) (Scheme 76).257

Irradiation of menadione (280) (vitamin K3 or vitamin K2(0)) leads to epoxidation in aerated ethanol (product 281) and to hydroxylation and disproportionation (products 282 and 283) in micellar solution (Scheme 77).258> 259 This molecule also undergoes photodimerisation: the syn cyclobutane dimers (284) and (285) were obtained by irradiation in the solid state (Scheme 77),260 and the anti dimers by irradiation in acetone solution.261 Other vitamins K bearing an allylic chain undergo E-Z isomerisation262 as well as ene addition with singlet oxygen to give allyl hydroperoxides or related derivatives263"267 [see as an example the peroxides (286) and (287) formed by singlet oxygen addition to menaquinone-1, vitamin K2(i), (288) in Scheme 78],268 Menadiol derivatives are also photoreactive, as it has been shown for solid menadiol acetate (vitamin K4).260

Scheme 77

Me oh hv h20 me

Scheme 79

2.7.2 Hydrosoluble Vitamins. Thiamine (289) undergoes cleavage of the thiazolyium ring when irradiated, giving an aminopyrimidylmethylamine (290)269 and sulfurated (and odoriferous) aliphatic fragments (291) (Scheme 79).270> 271 Riboflavin (292) is known for its high sensitivity to solar light. Irradiation under anaerobic conditions causes reversible reduction,272 while in the presence of oxygen the A/0-ribityl side chain is oxidised; the main product is lumichrome (293), with elimination of the chain (see Scheme 80), but various products of intermediate oxidation have been characterised.273"278

Both niacinamide (294) (vitamin PP) and the related vitamin factor nicotinic acid (295) are hydroxylated at position 6 upon irradiation under acidic conditions to give (296) and

(297) respectively; compound (295) is decarboxylated and gives the dipyridyl derivative

(298) in neutral solution (Scheme 81).279

+ Further Products

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