Total synthesis of (+)-isatisine a: application of a silicon-directed mukaiyama-type [3 + 2]-annulation

Complete details of an asymmetric synthesis of (+)-isatisine A (1) are described. The synthesis highlights the use of a highly diastereoselective Mukaiyama-type [3 + 2]-annulation of allylsilane 5 with the unsaturated aldehyde 9a to assemble the functionalized tetrahydrofuran core of isatisine A. A convergent route to the framework of the natural product was established that employed a substrate-controlled indole coupling that was followed by a late-stage intramolecular copper(I)-mediated amidation to complete the assembly of the tetracyclic framework of (+)-isatisine A. In addition, the scope of the [3 + 2]-annulation was evaluated and enhanced utilizing diastereomeric allylsilanes anti-5 and syn-5 to establish an efficient route to stereochemically well-defined tetrahydrofurans.