2-cyclopentenone, followed by acidic quench, under kinetic control, leads to different ratios of cis and trans 2,3-disubstituted cyclanones according to ring size. From 2-methyl and 2-phenyl 2-cyclohexenone, the cis isomer is highly predominant (85 to 98%). From 2-methyl 2-cyclopentenone a cis/trans mixture is obtained: the cis isomer only predominates when a bulky reagent (1c) is used (80%); in the other cases