Palladium-catalyzed synthesis of nucleoside adducts from bay- and fjord-region diol epoxides.

Palladium-catalyzed C-N bond formation has been utilized to synthesize covalent 2'-deoxyadenosine (dA) and 2'-deoxyguanosine (dG) adducts of benzo[a]pyrene (BaP) series 1 (syn) and benzo[c]phenanthrene (BcPh) series 2 (anti) diol epoxides. For this, (+/-)-10 alpha-amino-7 beta,8 alpha,9 beta-trisbenzoyloxy-7,8,9,10-tetrahydro BaP and (+/-)-1 beta-amino-2 alpha,3 alpha,4 beta-trisbenzoyloxy-1,2,3,4-tetrahydro BcPh were coupled with 6-halo-9-[3,5-bis-O-(tert-butyldimethylsilyl)-beta-D-erythro-pentofuranosyl]purine and O6-benzyl-3',5'-bis-O-(tert-butyldimethylsilyl)-2-bromo-2'-deoxyinosine, using a (+/-)-BINAP-Pd complex and Cs2CO3. For the synthesis of the dA adducts, both the 6-chloro- as well as the 6-bromopurine nucleoside derivatives were analyzed for the C-N coupling reaction with the hydrocarbon amino tribenzoates. With the BaP amino tribenzoate, the 6-chloronucleoside provided satisfactory results, whereas the 6-bromo analogue proved to be superior with the BcPh amino tribenzoate. Overall, lower yields of the dA adducts were obtained with the more hindered fjord-region BcPh amino tribenzoate as compared to the bay-region BaP amino tribenzoate. In contrast to reactions leading to the dA adducts, the C-N reactions of both BaP and BcPh amino tribenzoates with the 2-bromo-2'-deoxyinosine derivative proceeded in comparable yields. This seems to indicate that such Pd-catalyzed adduct forming reactions at the C-6 position may be influenced by steric constraints of the amine component, whereas those at the C-2 position are less sensitive. Diastereomeric adduct pairs were separated and characterized by spectral methods and by comparisons to adducts produced by direct displacement reactions as well as those formed from DNA alkylation by diol epoxides.