A hidden catalysis: metal-, and organocatalyst-free one-pot assembly of chiral aza-tricyclic molecules

Development of a rapid synthesis of complex molecules from simple building blocks under a metal-and organocatalyst-free condition is both conceptually and chemically challenging. Here, we developed a hidden catalysis that allow the straightforward assembly of enantiopure aza-tricyclic molecules containing six contiguous stereocenters from aminophenols, α,β-unsaturated aldehydes and α-amino acids. Without using a metal or an organocatalyst, our approach relies on a temporary formation of a spiroimidazolidinone intermediate and its participation in a sequential aza-Michael/Michael reaction as both a substrate and a catalyst under an iminium/enamine catalysis. The formation of the putative iminium intermediate was supported by spectroscopic data and its interruptive reduction derivative was isolated and fully characterized. Whereas a conventional catalyst is always present and does not undergo a permanent chemical change in a classic catalysis, the spiroimidazolidinone intermediate is conceptualized as a sub-catalyst as it is only temporary produced from precursors and catalyzes its own consumption. This unique substrate-catalyst (sub-catalyst) dual role of the spiroimidazolidinone induces a substantial steric discrimination in the transition state and an excellent overall diastereoselectivity (>20:1 dr). It allows the use of an amino acid precursor as the sole chirality genesis and avoids the use of transition metals or organocatalysts. An enantiomer of an aza-tricyclic imidazolidinone can be prepared from a commercially available amino acid precursor. The aqueous-based reaction is practical and scalable for multi-gram synthesis. The success of implementing this sub-catalysis concept in the synthesis will pave the way for many efficient chiral catalyst-free preparations of chiral complex molecules.