Rhodamine 123 (I) is reduced with NaBH4 and iodinated with N-chlorosuccinimide/NaI in CCl4 to yield iodinated derivatives of dihydrorhodamine 123 (II). Reduction of (I) gave a 90% yield of (II) after flash column chromatography. The structure was confirmed bv NMR. This compound was sensitive to oxidation unless stored at -20 C in the dark. The goal of this work was to synthesize no-carrier-added 125l derivatives; therefore, the mildest possible conditions were sought since iodination and oxidation would be competing reactions. The use of N-chlorosuccinimide (NClS)/NaI in CCl4 proved to be the most satisfactory iodination method: if the reaction was carried out with 1 Eq of NaI, the main product was the 2'-iodo derivative (III), isolated in 44% yield, with 5% of the 4'-iodo compound (IV) and 9% of the symmetrical diiodo derivative (V). When the reaction was performed without NaI, only the starting material was observed after workup on TLC, indicating that no chlorinated byproducts were formed (the intense red color of rhodamine 123 derivatives which developed as the TLC plates were exposed to air enabled the visualization of very small amounts of material). Using 4 Eq of NaI gave 19% of (III), 11% of (IV), 17% of (V) and 12% of (VI). With the reverse addition of dihydorhodamine to NClS/NaI the yields were 25% (V), 23% (VI) and 7% (VII). These yields were not realized unless an NaBH4 reduction step was included; this was possible since no reductive deiodination of the purified iodo derivatives occurred following treatment with NaBH4. All iodinated derivatives were identified by NMR. None of the dihydrorhodamine derivatives was fluorescent. Radioiodination was carried out with carrier free Na125 (I) / NClS / (II) in a mole ratio of 0.002:1:1. After flash chromatography was performed twice, the total radiochemical yield was 60%. In a typical run 30% of the recovered activity was (III), and the rest was distributed among (IV), (IV), (VI) and (VII). The radioiodinated compounds were identified by autoradiography of thin layer chromatograms with radioactive samples spotted alone and with overspotted authentic nonradioactive samples.

Rhodamine 123 (I) is reduced with NaBH4 and iodinated with N-chlorosuccinimide/NaI in CCl4 to yield iodinated derivatives of dihydrorhodamine 123 (II). Reduction of (I) gave a 90% yield of (II) after flash column chromatography. The structure was confirmed bv NMR. This compound was sensitive to oxidation unless stored at -20 C in the dark. The goal of this work was to synthesize no-carrier-added 125l derivatives; therefore, the mildest possible conditions were sought since iodination and oxidation would be competing reactions. The use of N-chlorosuccinimide (NClS)/NaI in CCl4 proved to be the most satisfactory iodination method: if the reaction was carried out with 1 Eq of NaI, the main product was the 2'-iodo derivative (III), isolated in 44% yield, with 5% of the 4'-iodo compound (IV) and 9% of the symmetrical diiodo derivative (V). When the reaction was performed without NaI, only the starting material was observed after workup on TLC, indicating that no chlorinated byproducts were formed (the intense red color of rhodamine 123 derivatives which developed as the TLC plates were exposed to air enabled the visualization of very small amounts of material). Using 4 Eq of NaI gave 19% of (III), 11% of (IV), 17% of (V) and 12% of (VI). With the reverse addition of dihydorhodamine to NClS/NaI the yields were 25% (V), 23% (VI) and 7% (VII). These yields were not realized unless an NaBH4 reduction step was included; this was possible since no reductive deiodination of the purified iodo derivatives occurred following treatment with NaBH4. All iodinated derivatives were identified by NMR. None of the dihydrorhodamine derivatives was fluorescent. Radioiodination was carried out with carrier free Na125 (I) / NClS / (II) in a mole ratio of 0.002:1:1. After flash chromatography was performed twice, the total radiochemical yield was 60%. In a typical run 30% of the recovered activity was (III), and the rest was distributed among (IV), (IV), (VI) and (VII). The radioiodinated compounds were identified by autoradiography of thin layer chromatograms with radioactive samples spotted alone and with overspotted authentic nonradioactive samples.