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The protected 5′‐oxo‐6,5′‐cyclouridine 13 reacts with diazomethane to afford mostly the spiro‐epoxide 18 (79%), but it also undergoes ring‐expansion to give the corresponding 5′‐oxo‐6,6′‐cyclonucleoside 16. Under the conditions of the reaction, ketone 16 reacts further with diazomethane to give the enol ether 20 (12% overall), the isomeric 4‐methoxy nucleoside 15 (2%), and the spiro‐epoxide 19 (4.4%). Acid hydrolysis of the enol ether 20, followed by reduction of the resulting ketone with sodium borohydride, affords a separable mixture of the 5′S (L‐talo) and 5′R (D‐allo) methylene‐bridged cyclonucleosides 7 and 8, respectively. From proton nmr measurements, it appears that the 6′‐methylene groups of these cyclonucleosides project towards the 2′,3′‐edge of the furanose ring.