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 15N‐NMR Spectra

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Abstract
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 T1 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 T1 of HC(1′). 15N‐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).

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