An Approach towards the Synthesis of the Spiroimine Fragment of 13-Desmethylspirolide C and Gymnodimine A.

A general access to the spiroimine skeleton of gymnodimine and spirolides is described, relying on the construction of the cyclohexene fragment using an enantiocontrolled Diels-Alder reaction, the installation of the all-carbon quaternary stereocenter through a stereocontrolled alkylation or aldolisation and the elaboration of the lateral chains at C7 and C22 using Wittig-Horner olefinations. The spiroimine core of gymnodimine is made available through a 16-step linear sequence in a 21 % overall yield.