Stereoselective synthesis of fused-, spiro- and bridged heterocycles via cyclization of isoquinolinium salts: a recent update

Isoquinoline and its derivatives are ubiquitous in natural alkaloids, synthetic materials and pharmaceuticals with a broad spectrum of biological activities. In particular, isoquinolinium salts are important in organic synthesis because they can serve as building blocks for the rapid construction of various condensed heterocyclic systems involving cyclization processes. This review highlights novel stereoselective strategies that emerged in the last few years (2014-2021) for the synthesis of various fused-, spiro- and bridged heterocycles exploiting bench-stable or in situ generated isoquinolinium salts. Most of the reactions described here provide quick access to the desired products with high stereoselectivity starting from readily available starting materials under mild conditions. The synthetic potential of most of the transformations was confirmed by the gram-scale production of the target molecules. Some of the reactions have been useful for the synthesis of natural products and other bioactive compounds.