Synthesis of cyclobutane nucleoside analogues 3: Preparation of carbocyclic derivatives of oxetanocin

A synthesis of cyclobutene nucleoside analogs in which the nucleobase is tethered by a methylene group is described. The coupling of 6-chloropurine with 3-hydroxymethyl-cyclobutanone proceeds via its triflate to give both N-7 and N-9 regioisomers with relative yields corresponding to the calculated charge distribution of the 6-chloropurinyl anion. The stereoselective reduction of the N-alkylated ketones yielded quantitatively one stereoisomer in each case. The structural assignments were based on spectroscopic data and single crystal X-ray diffraction. Attempts to photoexcite the N-7 and N-9 ketones in order to promote ring-expansion did not ensue. Preliminary evidence suggests a photodecarbonylation to cyclopropanes took place.