Conversion of formycin into the fluorescent isoguanosine analogue 7-amino-3-(β-D-ribofuranosyl)-1H-pyrazolo[4,3-d]pyrimidin-5(4H)-one

Abstract
On u.v. irradiation, in aqueous solution, formycin N(6)-oxide (3) undergoes photorearrangement to give 5-cyano-3-(β-D-ribofuranosyl)-4-ureido-1H-pyrazole (5) as the major product; small amounts of 7-amino-3-(β-D-ribofuranosyl)-1H-pyrazolo[4,3-d]pyrimidin-5(4H)-one (4) and formycin (1) are also formed. When treated with aqueous ammonia, compound (5) cyclizes to the isoguanosine analogue (4). The latter compound is strongly fluorescent and its c.d. spectrum resembles that of formycin. Its u.v. absorption and fluorescence characteristics are compared with those of the corresponding guanosine analogue 5-amino-3-(β-D-ribofuranosyl)-1H-pyrazolo[4,3-d]pyrimidin-7(6H)-one (2).

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