PtCl2‐ and PtCl4‐Catalyzed Cycloisomerization of Polyunsaturated Precursors

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Abstract
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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)