Guy R. Humphrey

Key green chemistry research areas—a perspective from pharmaceutical manufacturers

In 2005, the ACS Green Chemistry Institute (GCI) and the global pharmaceutical corporations developed the ACS GCI Pharmaceutical Roundtable to encourage the integration of green chemistry and green engineering into the pharmaceutical industry. The Roundtable has developed a list of key research areas. The purpose of this perspective is to summarise how that list was agreed, provide an assessment of the current state of the art in those areas and to highlight areas for future improvement.

Asymmetric synthesis of telcagepant, a CGRP receptor antagonist for the treatment of migraine.

A highly efficient, asymmetric synthesis of telcagepant (1), a CGRP receptor antagonist for the treatment of migraine, is described. This synthesis features the first application of iminium organocatalysis on an industrial scale. The key to the success of this organocatalytic transformation was the identification of a dual acid cocatalyst system, which allowed striking a balance of the reaction efficiency and product stability effectively. As such, via an iminium species, the necessnary C-6 stereogenicity was practically established in one operation in >95% ee. Furthermore, we enlisted an unprecedented Doebner-Knoevenagel coupling, which was also via an iminium species, to efficiently construct the C3-C4 bond with desired functionality. In order to prepare telcagepant (1) in high quality, a practical new protocol was discovered to suppress the formation of desfluoro impurities formed under hydrogenation conditions to <0.2%. An efficient lactamization facilitated by t-BuCOCl followed by a dynamic epimerization-crystallization resulted in the isolation of caprolactam acetamide with the desired C3 (R) and C6 (S) configuration cleanly. Isolating only three intermediates, the overall yield of this cost-effective synthesis was up to 27%. This environmentally responsible synthesis contains all of the elements required for a manufacturing process and prepares telcagepant (1) with the high quality required for pharmaceutical use.

Synthesis of Vaniprevir (MK-7009): Lactamization To Prepare a 22-Membered Macrocycle

Development of a practical synthesis of MK-7009, a 20-membered [corrected] macrocycle, is described. A variety of ring-closing strategies were evaluated, including ring-closing metathesis, intermolecular palladium-catalyzed cross-couplings, and macrolactamization. Ring closure via macrolactamization was found to give the highest yields under relatively high reaction concentrations. Optimization of the ring formation step and the synthesis of key intermediates en route to MK-7009 are reported.

Asymmetric synthesis of a potent, aminopiperidine-fused imidazopyridine dipeptidyl peptidase IV inhibitor.

A practical asymmetric synthesis of a novel aminopiperidine-fused imidazopyridine dipeptidyl peptidase IV (DPP-4) inhibitor 1 has been developed. Application of a unique three-component cascade coupling with chiral nitro diester 7, which is easily accessed via a highly enantioselective Michael addition of dimethyl malonate to a nitrostyrene, allows for the assembly of the functionalized piperidinone skeleton in one pot. Through a base-catalyzed, dynamic crystallization-driven process, the cis-piperidionone 16a is epimerized to the desired trans isomer 16b, which is directly crystallized from the crude reaction stream in high yield and purity. Isomerization of the allylamide 16b in the presence of RhCl(3) is achieved without any epimerization of the acid/base labile stereogenic center adjacent to the nitro group on the piperidinone ring, while the undesired enamine intermediate is consumed to <0.5% by utilizing a trace amount of HCl generated from RhCl(3). The amino lactam 4, obtained through hydrogenation and hydrolysis, is isolated as its crystalline pTSA salt from the reaction solution directly, as such intramolecular transamidation has been dramatically suppressed via kinetic control. Finally, a Cu(I) catalyzed coupling-cyclization allows for the formation of the tricyclic structure of the potent DPP-4 inhibitor 1. The synthesis, which is suitable for large scale preparation, is accomplished in 23% overall yield.

Development of a practical, asymmetric synthesis of the hepatitis C virus protease inhibitor MK-5172.

The development of a practical, asymmetric synthesis of the hepatitis C virus (HCV) protease inhibitor MK-5172 (1), an 18-membered macrocycle, is described.

Regioselective nucleophilic substitutions of fluorobenzene derivatives

Abstract Regioselective nucleophilic substitutions of tri- and di-substituted fluorobenzoates, fluorobenzonitriles, and fluoronitrobenzenes were accomplished by sequential addition of various nucleophiles, such as, potassium N-Boc-4-piperidinyl oxide, potassium methoxide, and piperidine.

A stereoselective synthesis of a key 1β-methylcarbapenem intermediate via a diastereoselective decarboxylation

(3S,4S)-3[(R)-1-(t-Butyldimethylsilyloxy)ethyl]-4-[(R)-1-carboxyethyl]-2-azetidinone 7a was prepared from (3R,4R)-4-acetoxy-3-[(R)-1-t-butyldimethylsilyloxy)ethyl]-2-azetidinone 3 via a sequence involving coupling with 2,2,5-trimethyl-1,3-dioxan-4,6-dione 4, N-silylation, solvolysis of the methylmeldrums acid moiety and a stereoselective acid catalyzed decarboxylation.

Highly Efficient Synthesis of HIV NNRTI Doravirine

The development of an efficient and robust process for the production of HIV NNRTI doravirine is described. The synthesis features a continuous aldol reaction as part of a de novo synthesis of the key pyridone fragment. Conditions for the continuous flow aldol reaction were derived using microbatch snapshots of the flow process.

Stereocomplementary reductions of ring fused enelactams

Abstract Selective reductions of ring fused enelactams to trans - and cis -ring fused lactams is described.

Potassium isopropyl xanthate (PIX): an ultra-efficient palladium scavenger

The increasing employment of palladium-catalyzed reactions in the synthesis of active pharmaceutical ingredients (APIs) has created a pressing need for ultra-efficient palladium removal of the resulting metal contaminants. This communication discusses the identification and development of Potassium Isopropyl Xanthate (PIX) as a simple, readily available and ultra-efficient palladium scavenger capable of removing residual palladium from the API to levels less than 1 ppm. In addition, the discovery of a synergistic effect of iodine, in combination with PIX and other palladium scavengers, to enhance palladium removal has further increased the efficiency of the palladium removal process. The PIX and I2 system has been successfully applied to the ceftolozane sulfate 2nd generation manufacturing chemistry to reduce palladium in the API resulting from a late stage palladium-catalyzed coupling reaction to only 0.1 ppm.