Mechanistic and sterochemical insights on the Pt-catalyzed rearrangement of oxiranylpropargylic esters to cyclopentenones.

A mechanism for the rearrangement of oxiranylpropargylic esters to cyclopentenones catalyzed by PtCl(2) is proposed based on DFT calculations (M06/6-31++G(d,p)). Although the basic steps are coincidental with those proposed by Sarpong et al., who characterized a 2H-pyran as intermediate, calculations have revealed other intricate details of this complex rearrangement. The 2H-pyran is proposed to result from the ring-opening of a bicyclic oxonium ion that follows the nucleophilic capture by the epoxide of a platinum carbene generated by an initial Pt-mediated 1,2-propargylic rearrangement. The key steps in the evolution of this system are the electrocyclic ring-opening of the 2H-pyran to a α-methoxycarbonyl dienone and an iso-Nazarov ring closure. Prior to those, changes in hapticity and in the conformation of the dienone are required in order to produce the helical conformation needed to generate a single diastereomer of the cyclopentenone product obtained experimentally. The metal is needed well beyond the first step of the mechanism, and both electrocyclic reactions are favored by coordination to the metal when compared to their uncomplexed counterparts. Moreover, we have experimentally demonstrated that the rearrangement is stereoconvergent, a feature that is traced back to the initial configuration of the epoxide, which determines the somewhat counterthermodynamic placement of the metal syn to the methyl group of the stereogenic center in the 2H-pyran intermediate. Finally, starting from enantiopure oxiranylpropargylic ester 13, a racemate of cyclopentenone (R*,S*)-16 was obtained. Thus, the sequence does not proceed with memory of chirality, and the absolute stereochemical information is already lost at the stage of the 2H-pyran 14.