合成路线1

The addition of HCl to cyclopentadiene (I) gives 3-chlorocyclopentene (II), which is converted to racemic 2-cyclopentene-1-carboxylic acid (III). Optical resolution of (II) by crystallization of its (-)-1-phenylethylamine yields the desired enantiomer (IV), which is reduced with LiAlH4 in ethyl ether affording the chiral carbinol (V) (ee 98%). The reaction of (V) successively with BuLi, CO2 and I2 provides the iodinated cyclic carbonate (VI), which is treated with DBU in hot toluene to give the unsaturated cyclic carbonate (VII). The condensation of (VII) with 2-amino-6-chloropurine (VIII) by means of a Pd catalyst yields the carbocyclic purine (IX), which is finally hydrolyzed with aqueous NaOH to afford the target carbocyclic guanine. Alternatively, carbinol (V) can also be obtained by the vitamin B12/Zn/NH4Cl-catalyzed isomerization of 1,2-epoxycyclopentane (X) to the chiral cyclopentenol (XI), which by a [2,3]-sigmatropic Wittig rearrangement with KH, ICH2-SnBu4 and BuLi yields the target carbinol (V). However, the enantiomeric excess obtained is only ee 54%.