Bromo-l,3-dimethyluracil (la) reacted with equimolar sodium cyanide in dimethylformamide (DMF) to give 6-cyano-l,3-dimethyluracil (2a), which was also obtained from 6-chloro-l,3-dimethyluracil (4) by the same treatment as above. Compound 2a was readily converted to 5-cyano-l,3-dimethyluracil (3) when heated with a catalytic amount of sodium cyanide in DMF. The mechanism for these cine substitutions was shown by deuterium-exchange experiments to involve an addition-elimination process. Furthermore, a variety of N-substituted 6-cyanouracils (2b,c and 14a,c) and N-substituted 6-carbamoyluracils (12a-c) were prepared by the reaction of 5-bromouracils (la-c and 13a,c) with sodium cyanide in DMF or 50% aqueous ethanol.

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Photolysis of 5-substituted 6-azidouracil derivatives in the presence of nucleophiles has been studied.Irradiation of 5-alkyl-6-azidouracils (3) in the presence of amines caused a ring expansion to give 1,3,5triazepine derivatives (7). Photolysis of 6-azido-1,3,5-trimethyIuracil (3a) in alcohols gave the corresponding 6,6-dialkoxy-5-amino-5,6-dihydrouracils (8). When compound (3a) was irradiated in water, a ring contraction occurred to afford 3,5-dimethylhydantoin (9). On the other hand, 6-azido-5cyano-l,3-dimethyluracil (4) was converted into 1,3,5-triazepines (13) only in the presence of alcohols.Several studies on the chemistry of azido compounds have been carried out because of their interesting reactivity and utility in organic synthesis as reagents or intermediate^.^ Aryl azides display a variety of thermochemical and photochemical reactivitie~.~ For example, it has long been known that photolysis of phenyl azides in the presence of amines produces a ring expansion to give azepine~.~ Much effort has recently been expended in elucidating the reaction mechanism6 of this conversion and in applying it to other aromatic azides.' On the other hand, although the isomerization between the azido group and the tetrazole ring for aromatic heterocyclic azides has been extensively investigated,8 their ring expansion to sevenmembered heterocycles has received less attention apart from a few example^.^^'^ We recently reported that the photolysis of 6-azido-1,3dimethyluracil (1) with amines and acyl chlorides led to the formation of 6-alkylamino-5-amino-1,3-dimethyluracils (2; X = NRR'. Y = H) l o and 5-acylamino-6-chloro-1,3-dimethyluracils (2; X = C1, Y = COR)," respectively, oiu an aziridine intermediate (A) (Scheme 1). In connection with these studies, it was clearly of interest to examine the photochemical reaction of 5-substituted 6-azidouracils with nucleophiles because of the predicted difficulty of converting the intermediate, corresponding to (A), into the 5-aminouracil(2).In this investigation 5-alkyl-6-azido-1,3-dimethyluracils (3a and b), 6-azido-5-cyano-1,3-dimethyluracil (4), and 6-azido-5benzyl-3-methyluracil (5) were used as the 5-substituted derivatives. It was found that the photolysis of these compounds takes a quite different path from that of 5unsubstituted uracils l o and causes a ring transformation to give a 1,3,5-triazepine or hydantoin ring system. Results and Discussion5-Substituted 6-azidouracils (3)-(5), employed here as starting materials, were prepared by treating the corresponding 6-chlorouracils 12,' with sodium azide. Me h e (3a) ( 3 b ) I Me I HFirst, photolysis of 6-azido-1,3,5-trimethyluracil (3a) was carried out in the absence of any nucleophile. When a solution of (3a) in tetrahydrofuran (THF) was irradiated, a reduction product, 6-amino-1,3,5-trimethyluracil (6),12 was formed.On the other hand, irradiation of the azidouracil (3a) in the presence of methylamine in THF gave a ring-expansion product, 1,3,6-trimethyl-4-methylamino-2,3,6,7-tetrahydro-lH-1,3,5-triazepine-2,7-dione (7a), i...