Radical reactions are potentially useful as tritiation techniques because they are less subject to steric hindrance or rearrangement than ionic reactions. We have demonstrated the 4-step synthesis of selectively tritiated (2'R) 2'-deoxynucleosides a) Selective protection of 3'- and 5'-hydroxyl functions of the adenosine with 1,3-dichloro-1,1,3,3-tetraisopropyldisiloxane in dry pyridine afforded 3', 5'-O-TPDS-adenosine in 95% chemical yield. b) The 2'-hydroxy group was reacted with 4-phenyl chlorothionoformate in dry acetonitrile to yield 2'-O-phenoxythiocarbonyl-3', 5'-O-TPDS-adenosine. c) For deoxygenation of the 2'-position, three methods were studied i) TBTH / Triethylborane (TEB) at room temperature, ii) diphenylsilane / TEB / O2, and iii) TBTH / AIBN / 75oC in toluene. After optimization, method iii) gave 60% yield of the deoxygenated product. The reduction of 3', 5'-O-TPDS-adenosine with TBTT (100%T) using method iii) yielded [2'-3H]-3', 5'-O-TPDS-adenosine with a specific activity of 20.2 Ci/mmole. d) Deprotection of the 3',5' positions using CsF and methanol at 65oC gave [2'-3H]-2'-deoxyadenosine. The tritium NMR spectra of [2'-3H]-2'-deoxyadenosine showed a singlet tritium peak at 2.42 ppm corresponding to the 2' position on the ribose moiety (2'R) as well as a small singlet (9% of the major peak) at 2.80 ppm corresponding to the 2"""""""" position (2'S). The chemical yield varied from 38-60%, partly due to the formation of a radioactive organotin impurity which was removed by hexane extraction. Whereas the TBTH reaction resulted in complete reduction in 3 hrs, 10% of the precursor remained when using TBTD and 30% when using TBTT. This apparent isotope effect manifested itself in the corresponding increase of adenosine production (from the deprotection of the starting material). Radical deoxygenation of steroids has also been demonstrated. Two precursors were prepared a) cholesteryl-4-fluorophenoxythiocarbonate, and b) epicholesteryl-4-fluorophenoxythiocarbonate. Reaction of cholesteryl-4-fluorophenoxythiocarbonate with TBTD in toluene at 110oC using di-t-butylperoxide as the radical initiator yielded [3-2H]cholest-5-ene in 77% yield with 80% deuterium incorporation. The same reaction conditions were applied to the second precursor and yielded the same product in 86% yield with 83% deuterium incorporation. The analogous tritium reactions with high specific activity tributyltin tritide were reproducible and gave the tritiated products in 77% and 81% chemical yields and specific activities of 17.90 and 18.75 Ci/mmole respectively. A feature of both the deuterium and tritium results is that the ratio of tritium (deuterium) in the a vs. b position of the product was independent of the isomeric precursor, a feature of radical reactions. A manuscript is in preparation.
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