PtCl2- and PtCl4-catalyzed cycloisomerization of polyunsaturated precursors

The PtCl2- and PtCl4-catalyzed cycloisomerization of polyunsaturated precursors with different O-protecting groups at the propargylic position is reported. The free hydroxy, O(bromomethyl)dimethylsilyl, O-methyl and O-propenyl derivatives 1a-f have afforded mixtures of skeletal rearrangement products and/or polycyclic molecules incorporating a cyclopropyl ring system in variable chemical yields. Other parameters such as the variation of the alkenyl chains and the triple-bond substitution have been examined and gave new mechanistic elements. Very interestingly, for analogous compounds containing an ester moiety, such as the O-4-nitrobenzoyl or acetate group (1g-j and 22) bicyclo[4.1.0]hept-2-enyl derivatives were obtained in good yields. These products are formally the result of a 1,2-migration of an O-acyl group, coupled to a cyclopropanation step. Related propargylic acetates with a pendant aromatic group (25) or substituted at the terminal acetylenic carbon with a methoxycarbonyl group (24) did not afford this class of rearranged compounds. This type of reaction seems restricted to propargylic derivatives, because the suitably functionalized homopropargylic acetate 24 only afforded 1,3-dienes. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).