The oxyallyl cation intermediate from the Lewis acid mediated Nazarov reaction of an allenyl vinyl ketone was intercepted by acyclic, 2-silyloxy-substituted butadienes by highly regioselective (4 + 3) cycloadditions. Stereoselectivity was often modest, but in some instances steric interactions were responsible for high selectivity. The results are consistent with concerted (4 + 3) cycloadditions. In many instances, the (4 + 3) products were susceptible to fragmentation or rearrangement in the presence of the Lewis acid.

Capture of the cyclic oxyallyl cation intermediates from the BF3‐mediated Nazarov reactions of three allenyl vinyl ketones with various dienes was accomplished by (3+2)‐ and (4+3)‐cycloaddition. The relative amounts of these types of products were dependent on the substitution on the diene, and this could be linked to steric hindrance. Treatment of the (3+2)‐cycloaddition products with BF3·Et2O led mainly to decomposition but also to ring‐opened molecules and ring‐enlarged structures. The computed Gibbs energies of the (3+2)‐cycloaddition products, the products of the acid treatment and of some transition states leading to rearranged products were compared.

Abstract:The cationic intermediate of the Lewis acid-initiated Nazarov cyclization of an allenyl vinyl ketone (AVK) was trapped by pyrroles and indoles. The yields ranged from modest to high (up to 93%), and in both cases, only two of the three possible products were produced. Cyclopent-2-enones substituted at the 5-position were predominantly produced, however with increasing alkyl substitution or placement of an electron-withdrawing group on the nitrogen, an alternative regioisomer could also be formed. The results of this study suggest that a position a to the oxygen of the oxyallyl cation is the electronically preferred trapping site, whilst an exocyclic position is preferred for more sterically encumbered reacting partners.

The cyclization of a series of nonheterocyclic allenyl aryl ketones was examined using boron trifluoride etherate and indium triflate to mediate the reaction. Yields with BF3 were low in most instances due mainly to competitive destruction of the substrates. With In­(OTf)3, there was less decomposition, and the yields of the cyclized product were much higher, but only for substrates with electron-donating substituents. Cyclization did not occur without those substituents. A computational study using the ωB97X-D/6-311+G­(2d,p)//ωB97X-D/6-31+G­(d,p) method confirmed better stability of the σ-complexed substrate by indium­(III) and that meta-substituents on the phenyl ring of the substrate significantly influenced the activation barrier of the cyclization, whereas the effect of para-substituents was almost negligible. The computational results supported the idea that the cyclization is a 4π-electrocyclization and not a 5-endo-dig ring closure as had been proposed in the literature.

Nazarov reactions Intercepted by (4 + 3) Cycloadditions with Oxygen-SubstitutedDienes. -An oxyallyl cation, generated by Nazarov reaction of an allenyl vinyl ketone, is trapped by regioselective cycloaddition with 2-siloxy-1,3-butadienes to give bicyclo[4.2.1]nonane derivatives. -(LEFORT, F. M.; MISHRA, V.; DEXTER, G. D.; MORGAN, T. D. R.; BURNELL*, D. J.; J. Org. Chem. 80 (2015) 11, 5877-5886, http://dx.doi.org/10.1021/acs.joc.5b00914 ; Dep. Chem., Dalhousie Univ., Halifax, Nova Scotia B3H 4R2, Can.; Eng.) -T. Stabingis 43-077