7.8- Dihydro-6- pteridin-2,4-dione

The molecular features of covalent hydration are also present in the dihydroxy series, i.e., in pteridine-2,6-dione (30) and in pteridine-4,6-dione. The latter compound is hydrated only at the C(7)—N(8) double bond, whereas (30) forms two hydrated species, 7-hydroxy-7,8-dihydro- (29) and 4-hydroxy-3,4-dihydro-pteridin-2,6-dione (31) (equation 8). Structure (29) is thermodynamically the more stable substance (31) is formed more rapidly in solution but disappears slowly with time (63JCS5151). Insertion of a 4-methyl group greatly reduces the extent of 3,4- in favour of 7,8-hydration by a blocking effect . 

As a final example of regioselective control (Scheme 25), sulfonium ylides usually give nitrones when reacted with nitroso groups via oxaziridine intermediates. However, 7-aryl-2-dimethylamino-3,4,5,6-tetrahydropteridine-4,6-diones 120 were directly formed when nitrosopyrimidine 121 reacted with dimethylphenacylsulfonium bromides 122 instead of the isomeric 5-oxides <1996H(43)437>. It was also reported that the resulting pteridines were reduced to give 7,8-dihydro derivatives 123 with sodium dithionite (Table 7). 

Oxo-groups located at other positions on the pteridine ring were shown by Albert and Matsuura152 to affect the structure of the dihydropteridine formed. Thus, pteridin-4- and 6-ones and 2,6- and 4,6-diones were converted to the corresponding 7,8-dihydro-derivatives (153), whereas pteridin-7-one and 4,7-dione formed the corresponding 5,6-dihydro-derivatives (154) on reaction with potassium borohydride. 

Dihydro-6-(D-arahino-tetritol-l-yl)pteridine-2,4-dione 668 was unequivocally prepared by condensation of l-amino-l-deoxy-o-fructose (42) with 6-chloro-5-nitrouracil followed by concomitant reduction and cycliza-tion of 667 (62JCS44) (Scheme 172). 

The chemical names of riboflavin (RF) are 3,10-dihydro-7,8-dimethyl-10-[(2S,3S,4R) 2,3,4,5-tetrahydroxypentyl]benzo[g]pteridine-2,4-dione or 7,8-dimethyl-10-(T-D-ribityl) isoalloxazine. RF or lactoflavin is known as vitamin B2. The chemical formula is Ci7H2oN40 and its CAS number is 83-88-5. 

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