C-13-H-1 AND C-13-C-13 SPIN COUPLINGS IN [2'-C-13]2'-DEOXYRIBONUCLEOSIDES - CORRELATIONS WITH MOLECULAR-STRUCTURE

2'-Deoxyribonucleosides (2'-deoxyadenosine (1), 2'-deoxycytidine (2), thymidine (3)) singly enriched with C-13 at C2' have been prepared and used to obtain one-, two-, and three-bond C-13-H-1 and C-13-C-13 spin -coupling constants involving C2'. Coupling data are interpreted with assistance from complementary (3)J(HH) data (PSEUROT analysis), furano se structural parameters obtained from molecular orbital calculations, structure-coupling correlations found for J(CH) and J(CC) in carbohyd rates, and calculated J values. Spin couplings in 1-3 involving C1' an d C2' are also compared to corresponding values in ribonucleosides in order to assess the effects of nucleoside structure and conformation o n J values within the furanose ring. (1)J(C2',H2'R) and (1)J(C2',H2'S) in 1-3 and (1)J(C2',H2') in ribonucleosides depend on C-H bond orient ation; (1)J(C1',H1') in 1-3 and in ribonucleosides exhibits a similar dependence. The latter couplings appear to be essentially unaffected b y N-glycoside torsion. (1)J(CC) values depend on the number and distri bution of electronegative substituents on the C-C fragment. A modified projection curve is proposed to aid in the interpretation of (2)J(C2' ,H1') values; the presence of N substitution at C1' causes a shift to more negative couplings relative to the O-substituted analog. In contr ast, (2)J(C1',H2') is essentially unaffected by the same change in the electronegative substituent at C1'. (2)J(CC) values within the furano se ring are determined by two coupling pathways; in one case (i.e., (2 )J(C1',C3')), the observed coupling is shown to be the algebraic sum o f the two couplings arising from each pathway. (3)J(CH) and (3)J(CC) v alues depend in general on appropriate molecular dihedral angles as ex pected (Karplus relationships); however, (3)J(C2',H4') values exhibit unexpected behavior, thus suggesting potential limitations in its use as a structural probe.