Stereoselective synthesis of pyrrolo[2,3‐ d ]pyrimidine α‐ and β‐ D ‐ribonucleosides from anomerically pure D ‐ribofuranosyl chlorides: Solid‐Liquid Phase‐Transfer Glycosylation and 15 N‐NMR Spectra

Solid‐liquid phase‐transfer glycosylation (KOH, tris[2‐(2‐methoxyethoxy)ethye]amine ( = TDA‐1), MeCN) of pyrrolo[2,3‐ d ]pyrimidines such as 3a and 3b with an equimolar amount of 5‐ O ‐[(1,1 ‐dimethylethyl)dimethylsilyl]‐2,3‐ O ‐(1‐methylethylidene)‐α‐ D ‐ribofuranosyl chloride (1) [6] gave the protected β‐ D ‐nucleosides 4a and 4b , respectively, stereoselectively (Scheme). The β‐ D ‐anomer 2 [6] yielded the corresponding α‐ D ‐nucleosides 5a and 5b with traces of the β‐ D ‐compounds. The 6‐substituted 7‐deazapurine nucleosides 6a , 7a , and 8 were converted into tubercidin (10) or its α‐ D ‐anomer (11) . Spin‐lattice relaxation measurements of anomeric ribonucleosides revealed that T 1 values of HC(8) in the α‐ D ‐series are significantly increased compared to HC(8) in the β‐ D ‐series while the opposite is true for T 1 of HC(1′). 15 N‐NMR data of 6‐substituted 7‐deazapurine D ‐ribofuranosides were assigned and compared with those of 2′‐deoxy compounds. Furthermore, it was shown that 7‐deaza‐2′deoxyadenosine ( = 2′‐deoxytubercidin; 12 ) is protonated at N(1), whereas the protonation site of 7‐deaza‐2′‐deoxyguanosine ( 20 ) is N(3).