Go to the Board: A Journey through the Life of Professor David A. Evans.

By taking a journey through the events that happened during Professor David A. Evans' lifetime in the context of chemical synthesis and drug discovery, this in-focus article reflects upon Professor Evans' lifelong scientific and padegogical impacts on the broader fields influenced by organic chemistry.

Efficient synthesis of antiviral agent uprifosbuvir enabled by new synthetic methods.

An efficient route to the HCV antiviral agent uprifosbuvir was developed in 5 steps from readily available uridine in 50% overall yield. This concise synthesis was achieved by development of several synthetic methods: (1) complexation-driven selective acyl migration/oxidation; (2) BSA-mediated cyclization to anhydrouridine; (3) hydrochlorination using FeCl3/TMDSO; (4) dynamic stereoselective phosphoramidation using a chiral nucleophilic catalyst. The new route improves the yield of uprifosbuvir 50-fold over the previous manufacturing process and expands the tool set available for synthesis of antiviral nucleotides.

Design of an in vitro biocatalytic cascade for the manufacture of islatravir.

Enzyme-catalyzed reactions have begun to transform pharmaceutical manufacturing, offering levels of selectivity and tunability that can dramatically improve chemical synthesis. Combining enzymatic reactions into multistep biocatalytic cascades brings additional benefits. Cascades avoid the waste generated by purification of intermediates. They also allow reactions to be linked together to overcome an unfavorable equilibrium or avoid the accumulation of unstable or inhibitory intermediates. We report an in vitro biocatalytic cascade synthesis of the investigational HIV treatment islatravir. Five enzymes were engineered through directed evolution to act on non-natural substrates. These were combined with four auxiliary enzymes to construct islatravir from simple building blocks in a three-step biocatalytic cascade. The overall synthesis requires fewer than half the number of steps of the previously reported routes.

Scalable Synthesis of Diazeniumdiolates: Application to the Preparation of MK-8150.

Synthetic diazeniumdiolate (DAZD)-based nitric oxide is utilized to modulate the nitric oxide (NO) concentration in cellular environments and to control physiological processes, yet chemists are still struggling to find efficient and scalable methodologies that will enable them to access sufficient quantities of the high-energy diazeniumdiolate intermediates for biological studies. Now, a general, scalable, safer, and high-yielding new methodology adaptable to the large-scale synthesis of DAZDs has been developed.

The importance of synthetic chemistry in the pharmaceutical industry.

Innovations in synthetic chemistry have enabled the discovery of many breakthrough therapies that have improved human health over the past century. In the face of increasing challenges in the pharmaceutical sector, continued innovation in chemistry is required to drive the discovery of the next wave of medicines. Novel synthetic methods not only unlock access to previously unattainable chemical matter, but also inspire new concepts as to how we design and build chemical matter. We identify some of the most important recent advances in synthetic chemistry as well as opportunities at the interface with partner disciplines that are poised to transform the practice of drug discovery and development.

Highly Diastereoselective Synthesis of a HCV NS5B Nucleoside Polymerase Inhibitor.

An asymmetric synthesis of HCV NS5B nucleoside polymerase inhibitor (1) is described. This novel route features several remarkably diastereoselective and high-yielding transformations, including construction of the all-carbon quaternary stereogenic center at C-2 via a thermodynamic aldol reaction. A subsequent glycosylation reaction with activated uracil via C-1 phosphate and installation of the cyclic phosphate group using an achiral phosphorus(III) reagent followed by oxidation provides 1.

Enantioselective Synthesis of Hemiaminals via Pd-Catalyzed C-N Coupling with Chiral Bisphosphine Mono-oxides.

A novel approach to hemiaminal synthesis via palladium-catalyzed C-N coupling with chiral bisphosphine mono-oxides is described. This efficient new method exhibits a broad scope, provides a highly efficient synthesis of HCV drug candidate elbasvir, and has been applied to the synthesis of chiral N,N-acetals.

An experimental and in situ IR spectroscopic study of the lithiation-substitution of N-Boc-2-phenylpyrrolidine and -piperidine: controlling the formation of quaternary stereocenters.

A general and enantioselective synthesis of 2-substituted 2-phenylpyrrolidines and -piperidines, an important class of pharmaceutically relevant compounds that contain a quaternary stereocenter, has been developed. The approach involves lithiation-substitution of enantioenriched N-Boc-2-phenylpyrrolidine or -piperidine (prepared by asymmetric Negishi arylation or catalytic asymmetric reduction, respectively). The combined use of synthetic experiments and in situ IR spectroscopic monitoring allowed optimum lithiation conditions to be identified: n-BuLi in THF at -50 °C for 5-30 min. Monitoring of the lithiation using in situ IR spectroscopy indicated that the rotation of the tert-butoxycarbonyl (Boc) group is slower in a 2-lithiated pyrrolidine than a 2-lithiated piperidine; low yields for the lithiation-substitution of N-Boc-2-phenylpyrrolidine at -78 °C can be ascribed to this slow rotation. For N-Boc-2-phenylpyrrolidine and -piperidine, the barriers to rotation of the Boc group were determined using density functional theory calculations and variable-temperature (1)H NMR spectroscopy. For the pyrrolidine, the half-life (t(1/2)) for rotation of the Boc group was found to be ∼10 h at -78 °C and ∼3.5 min at -50 °C. In contrast, for the piperidine, t(1/2) was determined to be ∼4 s at -78 °C.

Enantioselective, palladium-catalyzed α-arylation of N-Boc pyrrolidine: in situ react IR spectroscopic monitoring, scope, and synthetic applications.

A comprehensive study of the enantioselective Pd-catalyzed α-arylation of N-Boc pyrrolidine has been carried out. The protocol involves deprotonation of N-Boc pyrrolidine using s-BuLi/(-)-sparteine in TBME or Et(2)O at -78 °C, transmetalation with ZnCl(2) and Negishi coupling using Pd(OAc)(2), t-Bu(3)P-HBF(4) and the aryl bromide. This paper reports several new features including in situ React IR spectroscopic monitoring of the process; use of (-)-sparteine and the (+)-sparteine surrogate to access products with opposite configuration; development of a catalytic asymmetric lithiation-Negishi coupling reaction; extension to a wide range of heteroaromatic bromides; total synthesis of (R)-crispine A, (S)-nicotine and (S)-SIB-1508Y via short synthetic routes; and examples of α-vinylation of N-Boc pyrrolidine using vinyl bromides exemplified by the total synthesis of naturally occurring (+)-maackiamine (thus establishing its configuration as (R)). In this way, the full scope and limitations of the methodology are delineated.

Enantioselective, biocatalytic reduction of 3-substituted cyclopentenones: application to the asymmetric synthesis of an hNK-1 receptor antagonist.

A convergent and enantioselective route to the hNK-1 receptor antagonist (1) is described, which sets all six stereogenic centers with high diastereoselectivity and delivers 1 in only 11 steps and 23% overall yield. The process was enabled by the development of the enantioselective enzymatic reduction of 3-functionalized cyclopentenones and stereospecific Pd-catalyzed etherification coupling of fragments 6 and 7.

Diamine-free lithiation-trapping of N-Boc heterocycles using s-BuLi in THF.

A diamine-free protocol for the s-BuLi-mediated lithiation-trapping of N-Boc heterocycles has been developed. In the optimized procedure, lithiation is accomplished using s-BuLi in THF at -30 °C for only 5 or 10 min. Subsequent electrophilic trapping or transmetalation-Negishi coupling delivered a range of functionalized pyrrolidines, imidazolidines, and piperazines in 43-83% yield.

Asymmetric deprotonation of N-boc piperidine: react IR monitoring and mechanistic aspects.

The high yielding asymmetric deprotonation trapping of N-Boc piperidine is successfully realized using s-BuLi and a (+)-sparteine surrogate. Monitoring of the lithiation by in situ React IR allowed the direct observation of a prelithiation complex.

Enantioselective Pd-catalyzed alpha-arylation of N-Boc-pyrrolidine: the key to an efficient and practical synthesis of a glucokinase activator.

A short and practical synthesis of glucokinase activator 1 was achieved utilizing a convergent strategy involving S(N)Ar coupling of activated aryl fluoride 11 with hydroxypyridine 9. The key to the success of the synthesis was the development of a novel method for enantioselective formation of alpha-arylpyrrolidines during the course of the project. In this method, (-)-sparteine-mediated enantioselective lithiation of N-Boc-pyrrolidine was followed by in situ transmetalation to zinc and Pd-catalyzed coupling with aryl bromide 3, proceeding in 92% ee. This transformation allowed the preparation of compound 1 in a 31% overall yield over six steps.

Copper-catalyzed synthesis of enantioenriched tetraarylethanes.

Herein we report the asymmetric synthesis of 1,2-dipyridyl-1,2-diarylethanes via an unusual Cu(I)-catalyzed dimerization reaction. Subjection of a variety of enantioenriched substituted 2-pyridyl alcohols to a one-pot protocol generates the desired products in good yields and diastereoselectivities and with ee's up to >99%.

Direct sp3 C-H bond activation adjacent to nitrogen in heterocycles.

Activation of sp(3) C-H bonds adjacent to nitrogen in heterocycles is an attractive transformation that is emerging as a practical method in organic synthesis. This tutorial review aims to summarize the key examples of direct functionalization of nitrogen-containing heterocycles via metal-mediated and metal-catalyzed processes, which is meant to serve as a foundation for future investigations into this rapidly developing area of research. The review covers functionalization of N-heterocycles via alpha-lithiation with alkyllithium/diamine complexes, alpha-amino radical formation, metal-catalyzed direct C-H activation, C-H oxidations and oxidative couplings, and metal-catalyzed carbene insertions.

Enantioselective, palladium-catalyzed alpha-arylation of N-Boc-pyrrolidine.

This communication discloses the first instance of the enantioselective Pd-catalyzed alpha-arylation of N-Boc-pyrrolidine. The methodology relies on Beak's sparteine-mediated, enantioselective deprotonation of N-Boc-pyrrolidine to form the 2-pyrrolidinolithium specices in high enantiomeric ratio (er). Transmetalation of this intermediate with zinc chloride generates the stereochemically rigid, 2-pyrrolidinozinc reagent, which was readily coupled to a variety of functionalized aryl halides at room temperature using a catalyst generated from Pd(OAc)2 and PtBu3-HBF4. A diverse array of 2-aryl-N-Boc-pyrrolidines was synthesized using this methodology, providing adducts consistently in a 96:4 er. A survey of the stoichiometry revealed that as little as 0.3 equiv of zinc could be used in the coupling reaction, and the 2-pyrrolidinozinc reagent was found to exhibit stereochemical stability up to 60 degrees C. The method allows for the most convergent and reliable preparation of a broad range of functionalized 2-aryl-N-Boc-pyrrolidines in high enantioselectivity, which is highlighted in this report by the enantioselective synthesis of (R)-nicotine.

Mild and practical method for the alpha-arylation of nitriles with heteroaryl halides.

[reaction: see text] A mild and transition-metal-free method for the alpha-arylation of aliphatic nitriles with activated heteroaryl halides was developed using NaHMDS or KHMDS as base at ambient temperature. The key to the success of this method is generation of the nitrile anion in the presence of the heteroaryl halide. The method is applicable to both primary and secondary carbonitriles and a wide range of heteroaryl halides. Selective monoarylation was observed with primary carbonitriles. The operational simplicity and the mild reaction conditions add to the value of this method as a practical alternative to the preparation of alpha-heteroaryl carbonitriles.

Preparation of enamides via palladium-catalyzed amidation of enol tosylates.

[reaction: see text] A Pd-catalyzed coupling of enol tosylates and amides has been developed. Ligand screening revealed dipf as the most general ligand for this transformation. A variety of enol tosylates were coupled to an array of enamides in 58-97% yield.

Asymmetric synthesis of a prostaglandin D2 receptor antagonist.

An asymmetric synthesis was developed for the production of a prostaglandin D(2) receptor antagonist for the treatment of allergic rhinitis. The stereogenic center was set using asymmetric allylic alkylation chemistry, and the core of the structure was constructed via Pd-catalyzed N-cyclization/Heck methodology. The synthesis relies on a late stage indoline oxidation which does not racemize the product.