Synthesis of the Spirastrellolide A, B/C Spiroketal: Enabling Solutions for Problematic Au(I)-Catalyzed Spiroketalizations

A synthesis of the spirastrellolide A, B/C-ring monounsaturated spiroketal is reported. The key step relies on a Au-catalyzed spiroketalization of a propargyl triol employing an acetonide as a regioselectivity regulator. Through observation and analysis, a set of conditions has been developed that facilitates the use of a mixture of diastereomeric substrates, obviating the need to control the stereochemistry of the propargyl stereocenter and enabling a convenient retrosynthetic disconnection. The key reaction proceeds in 80% yield in 1 min at ambient temperature with the Me3PAuCl/AgOTf catalyst system. These conditions should be widely applicable for new synthetic endeavors as they appear to overcome all issues with the Au-catalyzed spiroketalization.