The Chemistry of Nonclassical Taxane Diterpene

The taxane diterpenes are a pharmaceutically vital family of natural products, consisting of more than 550 congeners. All taxane diterpenes are isolated from slow growing evergreen shrubs (genus Taxus) commonly known as "yews" and have a history of over 50 years as potent anticancer compounds. The most prominent congener, taxol (paclitaxel = PTX), has been used in clinics for more than 25 years and is one of the top-selling anticancer drugs worldwide, with annual sales reaching 1.5 billion USD in 1999. Within the taxane diterpene family 11 different scaffolds originating from rearrangements, fragmentations, or transannular C-C bond formations of the "classical taxane core" are known. Among them, five different scaffolds alone belong to the so-called complex or cyclotaxane subfamily, their signature structural feature bearing different types and numbers of transannular C-C bonds across the classical taxane backbone. For synthetic chemists, these five scaffolds represent by far the most challenging of all and have thus evaded total synthesis as well as detailed pharmaceutical evaluation-the latter due to extremely poor sourcing from natural producers. The cousinship of complex taxanes to taxol renders them potentially interesting compounds for drug research in the fight against cancer.This Account specifically summarizes the work on nonclassical taxanes from a biosynthetic, as well as a synthetic, point and provides a synthetic perspective on complex taxanes. Special attention is given to the biosynthetic relationship of complex taxanes and their biological emergence from classical taxanes. The transannular C-C bond forming events in the biosynthesis leading to the five individual scaffolds within this subfamily are structured on the basis of the exact type and number of these specific C-C bond formations. Since functionalization of the classical taxane core in the "oxidase phase" of the biosynthesis precedes the formation of complex taxanes, and is in part prerequisite for these transannular cyclization events, a detailed discussion of these oxidations of the classical taxane backbone is provided. Synthetic efforts toward nonclassical taxanes are scarce in literature and are thus presented in a comprehensive manner for abeotaxanes and complex taxanes. The last part of this Account deals with a synthetic perspective on the synthesis of complex taxanes (cyclotaxanes) and how these most intricate scaffolds can potentially be obtained via a deconvolution strategy. This discussion involves in part unpublished results by our group and is based upon synthetic studies in the literature. The deconvolution strategy we advocate aims for selective fragmentations of the signature transannular C-C bonds of the most intricate scaffold represented by the natural product canataxpropellane, which has recently been synthesized by our group. This strategy represents the converse process of the biosynthesis of complex taxanes (e.g., transannular cyclizations) and is enabled and feasible due to our approach to the canataxpropellane scaffold. We show that, by following this deconvolution strategy, all five scaffolds of complex taxanes can thereby be accessed.