摘要:
1162 CHEMICAL COMMLNICATIONS, 1968 Photodecarboxylation and Photohydration of Substituted Pyrimidines By SHIH YI WANG,* JOHN C. NNADI, and DANIEL GREENFELD (Department of Biochemistry, School of Hygiene and Public Health, The Johns Hopkins University, 615 Novth Wove Street, Baltimore, Maryland 21205) ULTRAVIOLET irradiation of thymine- 1-acetic acid (I) in aqueous solution resulted in quantitative decarboxylation, forming 1-methylthymine (11) , while uracil- 1-acetic acid (IIIa) gave 5,6-dihydro- 6-hydroxy- 1 -methyluracil (IVa) through de- carboxylation and hydration. It is known that thymine derivatives form photohydration products but they are too unstable for iso1ation.l Thus, it is of interest to investigate whether processes of hydration and decarboxylation are inter-related. When (I) and (IIIa) were irradiated as dry, solid films,2 no detectable change was observed and the original compounds were recovered.These results suggest that the absorption of photic energy alone was not responsible for decarboxylation. Further- more, the lactone formation, which may occur in the solid state, is unlikely in these photoreactions. The latter aspect was further shown by quantitative isolation of the corresponding photohydration products (V) from ethyl uracil-1-acetate (IIIb) and uracil- 1-acetamide (IIIc) irradiated in aqueous solutions. In no instance was the formation of lactone or azalactone detected. Irradiation of (I) and (IIIa) in frozen aqueous solutions3 resulted in the formation of cyclobutyl dimer [(VII) and (VIII)] as the major product with no decarboxyla- tion.Thus, it is unlikely that photochemical saturation of the 5,6-double bond or the absorptionCHEMICAL COMMUNICATIONS, 1968 1163 of photic energy after such a process caused decarboxylation. Ground-state decarboxylation after saturation of the 5,6-double bond by the formation of the bromo- hydrin (IX)4 is unlikely since, on heating, (IX) gave 5-bromouracil-l-acetic acid (X) without decar- boxylation. Hydrogenolysis of (IX)5 resulted in the formation of an unstable water addition- product ( I n ) which was isolated as (IIIa). Therefore, decarboxylation does not depend on the presence of the 6-OH group in the saturated molecules. In addition, thermal decarboxylation of (I) and (IIIa) occurs only at temperatures >280°.When (IIIa) was irradiated in aqueous acetonitrile, the decarboxylation, or the amount of (IVa) formed, was found to be directly related to the percentage of reaction proceeding via hydration. The above indicate that decarboxylation and hydration processes are interdependent. They must occur either simultaneously or decarboxyla- tion must occur immediately after hydration. If decarboxylation should precede the hydration step, l-methyluracil should be the intermediate for (IIIa) and the rates of hydration of 1-methyluracil and (IIIa) should be the same. However, the hydration rate of (IIIa) was shown to be faster than that of l-methyluracilJ6 suggesting that decarboxy- lation did not occur before hydration. ,4ny mechanism that assumes that a certain electronic configuration favours the stabilization of a carbanion or a free-radical intermediate for decarboxylation would require the decarboxylation to precede the hydration process, which is in- consistent with the observations.Likewise, several conceivable “concerted” processes (XII) would suggest that thymine-l-propionic acid (XIa) and uracil-l-propionic acid (XIb) should decarboxylate in a manner similar to (I) and (IIIa). However, (XIa) was isolated without change after irradiation, and (XIb) was converted into 5,6-dihydro- 6-hydroxyuracil-l-propionic acid (XIII) . It is in- teresting to note that with an additional CH, group in (XI) decarboxylation no longer occurred. For both the decarboxylation and hydration processes to occur, the intermediate species must possess energy of ca.40 kcal/mole.’ The possibility of reaction occurring from upper vibrational levels8 may be favoured by the large “energy drop” between this and the lowest lying singlet state,S the energy of which is about 90 kcal./mole.1° Acknowledgements are made to the same bodies as mentioned in the preceding communication. (Receiued, May 13tlz, 1968; Corn. 610.) S. Y . Wang, Nature, 1959, 184, 59. S. Y. Wang, Nature, 1963, 200, 879. S. Y. Wang, Nature, 1961, 190, 690; Fed. PYOC., 1965, 24, S-71. S. Y . Wang, J . Org. Chenz., 1959, 24, 11. S. Y. Wang, J . Amer. Chew.., SOC., 1959, 80, 6196. S. Y. Wang and J. C . Nnadi, preceding Communication. The energy of activation for dehydration of these hydrates was found to be ca. 20 kcal./mole [J. C. Nnadi, Ph.D. J. C . Nnadi and S. Y. Wang, submitted for publication. N. J. Turro, “Molecular Photochemistry,” W. A. Benjamin, New York: 1965, p. 186. Thesis (1968)], and that for decarboxylation is also in this range (€5. R. Brown, Quart. Rev., 1951, 5, 131). lo From luminescence data, M. Grierson, J. Eisinger, and R. G. Shulman, J. Chem. Phys., 1967, 47,4077, 90 kca1.l These results also show that mole may be obtained as energy of the lowest lying S, state of derivatives of uracil. internal conversion processes are important for both S, and T , states of pyrimidines.
ISSN:0009-241X
DOI:10.1039/C19680001162
出版商:RSC
年代:1968
数据来源: RSC