Synthesis of enzymatically noncleavable carbocyclic nucleosides for DNA- N-Glycosylase studies

Carbocyclic nucleosides have been of great interest as antiviral agents and in studies in the area of antisense technology. The recent finding that the replacement of a single 2'-deoxynucleoside in DNA by a carba analogue does not alter the Watson-Crick base pairing, yet at the same time provides a chemically and enzymatically stable 'glycosidic' linkage, led us to examine this class of compound as enzyme inhibitors of the DNA-repair enzymes involved in oxidative damage. We now report the synthesis and incorporation into oligomeric DNA via suitable derivatives, the carbanucleosides 8-oxo- 7,8-dihydro-2'-deoxycarbainosine, 8-oxo-7,8- dihydro-2'-deoxycarbaguanosine, and 2'-deoxyaristeromycin. Aristeromycin (1) was deoxy genated at the 2'- position as follows. Treatment of 1 with TPDSCl₂ gave the 3',5'-protected derivative 3 (76%) which on phenylthiocarbonylation at the 2'-position gave 4 in 51% yield. The latter compound on reduction with Bu₃SnH led to the 2'- deoxy derivative 5 (90%). Benzoylation followed by deprotection with TBAF in THF then gave the desired intermediate (6) in 65% yield. N²-Isobutyryl-8- oxo-7,8-dihydro-2'-deoxycarbaguanosine (16) was synthesized from 3-chloro- 2'-deoxycarbainosine (9). Treatment of 9, either with hydrazine followed by catalytic reduction of the 2-hydrazino derivative or with 1-(2- nitrophenyl)ethylamine followed by photolysis of the resulting 2-substituted derivative, in both instances gave the desired 2'- deoxycarbaguanosine (12) in ~50% overall yield in each case. Bromination of 12 gave 13 (90%) which, when treated with BnONa in DMSO at 65 °C, led to the 8-benzyloxy derivative 14 (46%). Isobutyrylation of 14 followed by catalytic reduction then afforded 16. 8-Oxo-7,8- dihydro-2'-deoxycarbainosine (23) was prepared in four steps. Bromination of 2'-deoxyaristero-mycin (19) at the 8-position gave 20 (> 95%) which was converted to the 8-benzyloxy derivative 21 (61%) using BnONa/DMSO at 80 °C. Reductive debenzylation of 21 then led to 8-oxo-7,8-dihydro-2'- deoxyaristeromycin (~100%) which, when treated with adenosine deaminase, provided the desired carbainosine derivative 23 in quantitative yield. Compounds 6, 16, and 23 were converted to their respective 5'-O-DMT, 3-O- [(2-cyanoethoxy)-(N,N-diisopropylamino)-phosphine] derivatives (8, 18, and 25) in excellent overall yields. The latter were then used to synthesize a series of DNA oligomers by automated procedures.