Deoxygenative [1,2]-hydride shift rearrangements in nucleoside and sugar chemistry: analogy with the [1,2]-electron shift in the deoxygenation of ribonucleotides by ribonucleotide reductases

A variant of the semipinacol rearrangement that was observed in our laboratory has been applied to the synthesis of several furanose and pyranose derivatives. The process consists of an "orchestrated" [1,2]-hydride shift with departure of a leaving group from the opposite face. Transient formation of a C=O group is followed by rapid transfer of a hydride-equivalent from the same face from which the leaving group departed, which results in double inversion of stereochemistry at the two vicinal carbon atoms. Treatment of 2'-O- and 3'-O-tosyladenosine with lithium triethylborohydride in DMSO/THF gave the respective 2'- and 3'-deoxynucleoside analogues with beta-D-threo configurations. Identical treatment of 5'-O-TPS-2'-O-tosyladenosine gave 9-(5-O-TPS-2-deoxy-beta-D-threo-pentofuranosyl)adenine. The same [1,2]-hydride shift and stereochemistry with the 5'-OH and 5'-O-TPS compounds demonstrated the absence of remote hydroxyl-group participation. Application of this process to other nucleoside 2'-O-tosyl derivatives gave the 2'-deoxy-threo compounds in good yields. The reaction-rate order was OTs approximately Br >> Cl for 2'-O-tosyladenosine, 2'-bromo-2'-deoxyadenosine, and 2'-chloro-2'-deoxyadenosine (all with beta-d-ribo configurations). Analogous results were obtained with mannopyranoside derivatives with either 4,6-O-benzylidene protection or a free OH group at C4. Deuterium labeling clearly defined the stereochemical course as a cis-vicinal [1,2]-hydride shift on the face opposite to the original cis OH and OTs groups followed by hydride transfer from the face opposite to the [1,2]-hydride shift. Synthetic and mechanistic considerations are discussed.

Synthesis of the ribosomal P-site substrate CCA-pcb

[reaction: see text] CCA-pcb (cytidylyl-(3'5')-cytidylyl-(3'5')-3'(2')-O-(N-(6-D-(+)-biotinoylaminohexanoyl)-L-phenylalanyl)adenosine), a ribosomal P-site substrate, was synthesized by phosphoramidite chemistry in 26 steps with an overall yield of 18%, starting from biotin. The synthesis relies on the judicious selection of orthogonal silyl protecting groups for the 5'-hydroxyls and acid-labile protecting groups (DMTr, AcE, and MeE) at other reactive sites to ensure the intactness of the labile ester. Both 3'-esterification and nucleotide coupling were accomplished by in situ activation with imidazolium ions.