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We present a diastereoselective synthesis of disubstituted cyclopentane 8 having a nitrogen-containing quaternary carbon center, which is found in axinellamine A ( 5) and related compounds. During this work, we found that the 1,3-dipolar cycloaddition product 24 immediately underwent intramolecular redox reaction at the newly formed morpholin-2-one moiety, thus affording disubstituted cyclopentane containing a tertiary amine ( 9) stereoselectively in good yield. The amine 9 was successfully converted into guanidine 31, which corresponds to 8, through iminium cation-enamine isomerization. [ABSTRACT FROM AUTHOR]