An unexpected oxidation in the generation of cyclopenta[c]piperidines by ring-closing metathesis.

Ring-closing metathesis reaction of dienes A (n = 1, R = CO(2)R') leads to fused cyclopentenones C instead of the expected cyclopentene derivatives B. RCM reaction of the other dienes A takes place satisfactorily, affording the expected fused cycloalkene derivatives B. Cyclohexene B (n = 2, R = CO(2)R') also undergoes oxidation to the corresponding cyclohexenone C.