Two-Phase Total Synthesis of Taxanes: Tactics and Strategies.

This Perspective goes into the fine details of our laboratory's quest to answer a longstanding fundamental question: Could any new approach to terpene synthesis, perhaps one patterned on biosynthesis, enable a divergent synthetic approach to the taxane family of natural products? We targeted Taxol, the flagship taxane, as the upper limit of chemical complexity and employed two-phase terpene synthesis logic as the guiding strategy. The first synthesis target was taxadiene, the lowest oxidized member of the taxane family, followed by three site-selective allylic oxidations at C5, C10, and C13, which led to the two-phase synthesis of taxuyunnanine D. Successful C9 oxidation enabled access to a wider range of taxanes, which was demonstrated by the two-phase synthesis of decinnamoyltaxinine E and taxabaccatin III. The final two sp3 C-H oxidations at C1 and C7 were attained by dioxirane-mediated C-H oxidation and an oxidation relay based on judicious substrate design, culminating in a two-phase synthesis of Taxol. The purpose of this Perspective is to articulate strategies and tactics developed for the two-phase synthesis of taxanes, whose lessons can be potentially extrapolated to medicinal chemistry endeavors in the taxane family, as well as to the synthesis of other terpene families.

Utilizing o-Quinone Methide Chemistry: Synthesis of d9-Ivacaftor.

Lead time and cost are important factors for any pharmaceutical API. However, these issues become even more important when the drug substance contains an isotope such as deuterium, which has a natural abundance of only ∼0.016% of all hydrogen. Fewer suppliers and logistical barriers both play a role in driving up the cost. These factors can challenge the supply route used to manufacture d9-ivacaftor (17), requiring investigation into alternative routes. By adapting the work from Pettus et al., a synthetic approach utilizing a transient o-quinone methide allowed access to the deuterium-labeled o-tert-butylphenol moiety. This was developed and proven on pilot scale to significantly reduce the number of deuterated reagents used, leading to an overall reduction in cost by a factor of 10, while also providing the substantial benefit of applying prior process knowledge from the parent, nonisotopically enriched API ivacaftor (7).

Short, Enantioselective Total Synthesis of Highly Oxidized Taxanes.

In the realm of natural product chemistry, few isolates have risen to the level of fame justifiably accorded to Taxol (1) and its chemical siblings. This report describes the most concise route to date for accessing the highly oxidized members of this family. As representative members of taxanes containing five oxygen atoms, decinnamoyltaxinine E (2) and taxabaccatin III (3), have succumbed to enantioselective total synthesis for the first time in only 18 steps from a simple olefin starting material. The strategy holistically mimics nature's approach (two-phase synthesis) and features a carefully choreographed sequence of stereoselective oxidations and a remarkable redox-isomerization to set the key trans-diol present in 2 and 3. This work lays the critical groundwork necessary to access even higher oxidized taxanes such as 1 in a more practical fashion, thus empowering a medicinal chemistry campaign that is not wedded to semi-synthesis.

Two-phase synthesis of (-)-taxuyunnanine D.

The first successful effort to replicate the beginning of the Taxol oxidase phase in the laboratory is reported, culminating in the total synthesis of taxuyunnanine D, itself a natural product. Through a combination of computational modeling, reagent screening, and oxidation sequence analysis, the first three of eight C-H oxidations (at the allylic sites corresponding to C-5, C-10, and C-13) required to reach Taxol from taxadiene were accomplished. This work lays a foundation for an eventual total synthesis of Taxol capable of delivering not only the natural product but also analogs inaccessible via bioengineering.

A New, Short, and Stereocontrolled Synthesis of C2-Symmetric 1,2-Diamines.

The previously unknown 5-spirocyclohexylisoimidazole has been made efficiently and simply by reaction of ammonia, glyoxal hydrate, and cyclohexanone. It is a very useful precursor for the diastereocontrolled synthesis of many C2-symmetric 1,2-diamines, a class which is important for the generation of a variety of C2-symmetric reagents and catalysts for enantioselective synthesis.