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 PtCl2 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.