Enantiocontrolled synthesis of a tetracyclic aminal corresponding to the core subunit of diazonamide A.

A chiral benzylic ether serves as an auxiliary for oxindole carboxylation (dr 5.2:1.0) that sets C10 configuration in a potential diazonamide precursor. The chiral substituent allows diastereomer separation and departs during a subsequent acid-catalyzed ring closure to form a tetracyclic aminal. With suitable N-protection, crystallization affords the aminal with 98-99% ee.