AbstractThe structure of the cyclo‐addition products of ketenes (2) and methylcyclopentadiene could not be predicted, since this ketenophile exists as a rapidly equilibrating 45:54:1 mixture of the 1‐ (4), 2‐ (5) and 5‐methyl (6) isomers. We studied this reaction using dimethylketene (2a) with a view to monoterpenoid synthesis. Only two (A and B) of the many possible cyclo‐adducts were formed in good yield. Using three methods to generate dimethylketene, the ratios of A:B were 90:6, 77:16 and 55:31 respectively.The structures of A and B were proved without making use of the known ketene cyclo‐addition rules: Of the many possibilities, all but 7, 8, 9 and 10 were excluded by the CO and CCCH3 IR.‐bands and NMR. signals. Structure 7 (filifolone) was rejected by the difference of its NMR. spectrum from those of both A and B, leaving only 8, 9 and 10.For A, structures 9 and 10 were eliminated by two chemical approaches: (1) Peracetic acid oxidation of a mixture rich in A resulted in a combination of Baeyer‐Villiger reaction, epoxidation, and epoxide‐ketone rearrangement affording a C10‐keto‐lactone 14, which was cleaved by alkali to acetone and a C7‐keto‐acid 18. (2) Another peracetic acid treatment produced a keto‐epoxide 11, which was converted to a diol 22 and then to a diketo‐aldehyde 23 with an NMR. doublet for the aldehyde proton.Of the two structures (9 and 10) left for B, the latter was excluded by the NMR. spectrum of keto‐epoxide 15 isolated from the peracid treatment of a mixture rich in B: A decoupling experiment with 15 showed that the ‐CH2‐ was placed next to the α‐carbon of the carbonyl function.Thus the major cyclo‐adduct of dimethylketene and methylcyclopentadiene is 8, and the minor product is 9. This result demonstrates further ketene cyclo‐addition specificities on top of the ones expressed by the three known rules. The different ratios of 8 : 9 are considered to be due to varyingly effective competition between the preliminary interconversion of the methylcyclopentadienes and their cyclo‐addition of dimethylkctene.A number of other oxidation and reduction products of 8 are described. One of them proved that C7 had migrated in the Baeyer‐Villiger reaction.A partial separation of the enantiomers of the major cycloadduct 8 was achieved. By the use of 0.5 equivalent of three chiral primary amines (31, 32, and 33) to form a Schiff base, the excess ketone 8 was in each case distilled off in a chirally enriched form with positive optical rotation. The derived 2.3‐dihydroketone 36 was shown to have a positive Cotton effect and, by an application of the octant rule, to have the (1R,5S) chirality orientation.