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Synthesis of the Cyclopentyl Nucleoside (−)‐Neplanocin A from D‐Glucose via Zirconocene‐Mediated Ring Contraction
Authors:Leo&#x;A Paquette  Zhenjiao Tian  Christopher&#x;K Seekamp  Tony Wang
Institution:Leo A. Paquette,Zhenjiao Tian,Christopher K. Seekamp,Tony Wang
Abstract:Two approaches for the conversion of d‐ glucose to (?) ‐neplanocin A ( 2 ), both based on the zirconocene‐promoted ring contraction of a vinyl‐substituted pyranoside, are herein evaluated (Scheme 1). In the first pathway (Scheme 2), the substrate possesses the α‐d‐ allo configuration (see 6 ) such that ultimate introduction of the nucleobase would require only an inversion of configuration. However, this precursor proved unresponsive to Cp2Zr (=ZrCl2(Cp)2]), an end result believed to be a consequence of substantive nonbonded steric effects operating in a key intermediate (Scheme 5). In contrast, the C(2) epimer (see 7 ) experienced the desired metal‐promoted conversion to an enantiomerically pure polyfunctional cyclopentane (see 5 in Scheme 3). The substituents in this product are arrayed in a manner such that conversion to the target nucleoside can be conveniently achieved by a double‐inversion sequence (Scheme 4). Recourse to palladium(0)‐catalyzed allylic alkylation did not provide an alternate means of generating 2 .
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