THF-enabled PtBr2-catalyzed desymmetric hydrogenative [3+2] cycloaddition of 2-alkynylbenzaldehyde-tethered cyclohexadienones

A THF-enabled PtBr2-catalyzed desymmetric hydrogenative [3 + 2] cycloaddition of 2-alkynylbenzaldehyde-tethered cyclohexadienones has been developed. The protocol provides a highly functionalized 6-7-6 polycyclic skeleton with four contiguous stereocenters in good to excellent yields of up to 97% yield and with exclusive exo-selectivity. The suggested cascade reaction pathway delineates a rare instance in platinum catalysis of the involvement of an in situ formed Pt-bound benzopyrylium species that participates in a Huisgen-type [3 + 2] cycloaddition with an electron-deficient alkene tether in the highly reactive organometallic intermediate. It also offers the first example of a metallocarbene species generated in this manner which undergoes hydrogenation with THF as the hydride transfer source. The synthetic utility of the catalytic method was exemplified by the late-stage stereoselective modification of the steroids estrone and estradiol under mild reaction conditions.

Automated summary

A THF-enabled PtBr2-catalyzed desymmetric hydrogenative [3 + 2] cycloaddition of 2-alkynylbenzaldehyde-tethered cyclohexadienones has been developed. The protocol provides a highly functionalized 6-7-6 polycyclic skeleton with four contiguous stereocenters in good to excellent yields. The involvement of an in situ formed Pt-bound benzopyrylium species in the cascade reaction pathway and the generation of a metallocarbene species that undergoes hydrogenation with THF as the hydride transfer source are novel findings. The catalytic method also demonstrates its synthetic utility in the late-stage stereoselective modification of steroids.