Allenyl alkenyl ketones

As mentioned in the previous section, the small steric interaction at the P-carbon along with the release of allenic strain favors the conversion of allenyl alkenyl ketones into the desired cyclopentenones. In the example depicted in Scheme 3.8, the axial chirality of the allene enforces the formation of one product, owing to the steric interactions between the newly forming bond and the f-butyl group at the allene moiety. 

Ma et al. reported the hydrohalogenation of 1,2-allenyl ketones leading to /1-halo-/8,y-unsaturated enones [87, 207]. The reaction of 3-substituted-l,2-allenyl ketone with MX in HOAc affords ( )-2-halo-2-alkenyl ketones as the major product. In HOAc, the reaction of l-substituted-l,2-allenyl ketones affords a mixture of/3-halo-a,/3- and -/3,y-unsaturated enones whereas in CF3C02H-H0Ac (1 1) the same reaction affords /3,y-unsaturated enones. 

Palladium(0)-catalyzed asymmetric arylation of tert-cyclobutanol 33 with bromobenzene, involving enantioselective C-C bond cleavage, furnished the chiral y-arylated ketone 34 (Scheme 3.13) [24]. Good enantioselectivity was observed when a chiral ferrocenyl P,N-ligand was used. y-Alkenylated and y-allenylated ketones were also obtained by asymmetric ring opening. 

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