Mild Debenzylation of Aryl Benzyl Ether with BCl3 in the Presence of Pentamethylbenzene as a Non-Lewis-Basic Cation Scavenger

Modular Synthetic Platform for Interior-Functionalized Dendritic Macromolecules Enabled by the Palladium/Norbornene Catalysis

Synthesis of interior-functionalized dendritic macromolecules is generally tedious and labor-intensive, which has been a key factor hampering their practical applications. Here, we have revisited this long-standing challenge and devised a modular and convergent platform to synthesize multifunctional dendrons with all-carbon backbones up to four generations via an in situ functionalization strategy. Enabled by the palladium/norbornene cooperative catalysis, different functional groups can be introduced to each generation of dendrons during the dendron growth, making it convenient for systematic comparison of their properties. The utility of this versatile platform is further showcased in the internal-functionalization-dependent properties of dendrons as organogels and aggregation-induced emission materials.

Total Synthesis of (+)-Silybin A

We report the first total synthesis of silybin A (1). Key synthetic steps include the construction of the 1,4-benzodioxane neolignan skeleton, a modified Julia-Kocienski olefination reaction between m-nitrophenyltetrazole sulfone (m-NPT sulfone) 10 and aldehyde 21, the formation of the flavanol lignan skeleton 28 via a quinomethide intermediate under acidic conditions, and stepwise oxidation of the benzylic position of flavanol 29.

Concise Total Synthesis of (+)-Pleiocarpamine and Convergent Total Syntheses of (+)-Voacalgine A and (+)-Bipleiophylline via an Aerobic Oxidative Coupling

The stereocontrolled total synthesis of (+)-pleiocarpamine and the total syntheses of (+)-voacalgine A and (+)-bipleiophylline have been achieved. The scalable and concise 10-step synthesis of (+)-pleiocarpamine features construction of stereochemistry at the C16 position by radical cyclization and that of the highly strained cage-like structure via Pd-catalyzed intramolecular aromatic C-H functionalization. By modifying the biomimetic aerobic oxidative coupling of tryptophane derivatives catalyzed by FePc(CO₂H)₈, the oxidative coupling of the synthesized (+)-pleiocarpamine with pyrocatechuic acid was established to produce (+)-voacalgine A. The total synthesis of (+)-bipleiophylline was completed by the second coupling of (+)-voacalgine A with (+)-pleiocarpamine or one-pot couplings of 2 equiv of (+)-pleiocarpamine with pyrocatechuic acid.

Chemoenzymatic Synthesis of Novel Cytotoxic Epoxyketones Using the Eponemycin Biosynthetic Enzyme EpnF

Eponemycin is an α,β-epoxyketone natural product that inhibits the proteasome via covalent interaction of the epoxyketone warhead with catalytic N-terminal threonine residues. The epoxyketone warhead is biosynthesized from a β-ketoacid substrate by EpnF, a recently identified flavin-dependent acyl-CoA dehydrogenase-like enyzme. Herein, we report biochemical characterization of EpnF kinetics and substrate scope using a series of synthetic β-ketoacid substrates. These studies indicate that epoxide formation likely occurs prior to other tailoring reactions in the biosynthetic pathway, and have led to the identification of novel epoxyketone analogues with potent anticancer activity.

Development of a Manufacturing Process toward the Convergent Synthesis of the COVID-19 Antiviral Ensitrelvir

We describe the development of the practical manufacturing of Ensitrelvir, which was discovered as a SARS-CoV-2 antiviral candidate. Scalable synthetic methods of indazole, 1,2,4-triazole and 1,3,5-triazinone structures were established, and convergent couplings of these fragments enabled the development of a concise and efficient scale-up process to Ensitrelvir. In this process, introducing a meta-cresolyl moiety successfully enhanced the stability of intermediates. Compared to the initial route at the early research and development stage, the overall yield of the longest linear sequence (6 steps) was improved by approximately 7-fold. Furthermore, 9 out of the 12 isolated intermediates were crystallized directly from each reaction mixture without any extractive workup (direct isolation). This led to an efficient and environmentally friendly manufacturing process that minimizes waste of organic solvents, reagents, and processing time. This practical process for manufacturing Ensitrelvir should contribute to protection against COVID-19.

Protecting group principles suited to late stage functionalization and global deprotection in oligosaccharide synthesis

Chemical synthesis is a powerful tool to access homogeneous complex glycans, which relies on protecting group (PG) chemistry. However, the overall efficiency of chemical glycan assembly is still low when compared to oligonucleotide or oligopeptide synthesis. There have been many contributions giving rise to collective improvement in carbohydrate synthesis that includes PG manipulation and stereoselective glycoside formation and some of this chemistry has been transferred to the solid phase or adapted for programmable one pot synthesis approaches. However, after all glycoside bond formation reactions are completed, the global deprotection (GD) required to give the desired target OS can be challenging. Difficulties observed in the removal of permanent PGs to release the desired glycans can be due to the number and diversity of PGs present in the protected OSs, nature and structural complexity of glycans, etc. Here, we have reviewed the difficulties associated with the removal of PGs from densely protected OSs to obtain their free glycans. In particularly, this review focuses on the challenges associated with hydrogenolysis of benzyl groups, saponification of esters and functional group interconversion such as oxidation/reduction that are commonly performed in GD stage. More generally, problems observed in the removal of permanent PGs is reviewed herein, including benzyl, acyl (levulinoyl, acetyl), N-trichloroacetyl, N-2,2,2-trichloroethoxycarbonyl, N-phthaloyl etc. from a number of fully protected OSs to release the free sugar, that have been previously reported in the literature.

Construction of N-Heterocycles Fused with a Highly Substituted Benzene Ring by a Benzyne-Mediated Cyclization/Functionalization Cascade Reaction and Its Application to the Total Synthesis of Marine Natural Products

This account summarizes the development of a benzyne-mediated cyclization/functionalization protocol for the versatile construction of highly substituted benzene derivatives fused with an N-heterocyclic ring such as indolines, indoles, and related nitrogen-containing heterocycles. The protocol comprises sequential reactions initiated by generating a benzyne species and subsequent cyclization via addition of magnesium amide to the benzyne, followed by trapping of the resultant magnesium compound in situ with various electrophiles. The substituent scope was expanded by conducting a transmetalation on a copper species to introduce alkyl, aryl, and alkenyl substituents. The utility of the sequential reaction was demonstrated in the synthesis of a carbazole natural product (heptaphylline), pyrrolo[4,3,2-de]quinoline alkaloids (batzellines), and pyrrolo[2,3-c]carbazole alkaloids (dictyodendrines).

Highly Selective and Catalytic C-N Bond Cleavage of Tertiary Sulfonamides: Scope and Mechanistic Insight

A highly chemoselective C-N bond cleavage reaction of p-methoxybenzyl- (PMB), 3,4-dimethoxybenzyl- (DMB), or cinnamyl-substituted tertiary sulfonamides in the presence of catalytic Bi(OTf)₃ is presented. A wide range of sulfonamide substrates smoothly furnished the corresponding C-N bond cleavage products in good to excellent yields. Great efforts have been made to obtain insights into the reaction mechanism based on a series of control experiments and mass spectroscopy.

Gold(I)-Catalyzed Intramolecular Hydroarylation of Phenol-Derived Propiolates and Certain Related Ethers as a Route to Selectively Functionalized Coumarins and 2H-Chromenes

Methods are reported for the efficient assembly of a series of phenol-derived propiolates, including the parent system 56, and their Au(I)-catalyzed cyclization (intramolecular hydroarylation) to give the corresponding coumarins (e.g., 1). Simple syntheses of natural products such as ayapin (144) and scoparone (145) have been realized by such means, and the first of these subject to single-crystal X-ray analysis. A related process is described for the conversion of propargyl ethers such as 156 into the isomeric 2H-chromene precocene I (159), a naturally occurring inhibitor of juvenile hormone biosynthesis.

Convergent Total Synthesis of Lamellarins and Their Congeners

A convergent total synthesis of lamellarins S and Z is described. The synthesis features a halogen dance of an easily accessible α,β-dibromopyrrole promoted by an ester moiety. The resultant β,β'-dibromopyrrole undergoes a ligand-controlled Suzuki-Miyaura coupling to provide a range of diarylated pyrrole derivatives. The established synthetic method was also applicable to the synthesis of ningalin B and lukianols A and B.

Synthetic Studies toward Dimeric Indole Alkaloids Based on Convergent Synthetic Strategy

The total syntheses of dimeric indole alkaloids, haplophytine, and T988s are described. These dimeric compounds comprising two structurally different indole units are ubiquitous in nature, and many possess pharmaceutically important activities. To realize an efficient chemical synthesis of these dimeric indole alkaloids, the establishment of convergent synthetic strategies and development of new coupling methods are indispensable. The linkage of two highly functionalized units at a late stage of the synthesis frequently induces synthetic problems such as chemoselectivity and steric repulsion. Moreover, although transition metal-catalyzed reactions are usually an effective method for the cross-coupling of two units, the application of these cross-coupling reactions to bond formation involving a sterically hindered C(sp³) is often difficult. Thus, even with precise modern synthetic methods, it is currently difficult to realize convergent syntheses of dimeric indole alkaloids possessing a quaternary carbon linking two units. To combat these synthetic problems, we developed a synthetic method to link two indole units using an Ag-mediated nucleophilic substitution reaction. In this review, we provide a detailed discussion of convergent synthetic strategies and coupling methods for dimeric indole alkaloids.

Facile Hydrogenative Deprotection of N-Benzyl Groups Using a Mixed Catalyst of Palladium and Niobic Acid-on-Carbon

The palladium-on-carbon (Pd/C)-catalyzed hydrogenative deprotection of the N-benzyl-protecting group was effectively facilitated by the combined use of niobic acid-on-carbon (Nb₂O₅/C). Nb₂O₅/C is an acidic heterogeneous catalyst prepared from NbCl₅ and activated carbon. The catalysts were easily removed from the reaction mixture and reusable. Deprotected amines were obtained in excellent yields without an additional neutralization process. The facilitating effect of Nb₂O₅/C was also observed during the Pd/C-catalyzed hydrogenative deprotection of the N-benzyloxycarbonyl (Cbz) and O-benzyl groups.

Synthesis of Fmoc-Protected Amino Alcohols via the Sharpless Asymmetric Aminohydroxylation Reaction Using FmocNHCl as the Nitrogen Source

The aminohydroxylation of various alkenes using FmocNHCl as a nitrogen source is reported. In general, in the absence of a ligand, the reaction provided racemic Fmoc-protected amino alcohols with excellent regioselectivity but in low to moderate yields. However, in some instances, the yield of an amino alcohol product and the regioselectivity could be altered by the addition of a catalytic amount of triethylamine (TEA). The Sharpless asymmetric variant of this reaction (Sharpless asymmetric aminohydroxylation (SAAH)), using (DHQD)₂PHAL (DHQD) or (DHQ)₂PHAL (DHQ) as chiral ligands, proceeded more readily and in higher yield compared to the same reaction in the absence of a chiral ligand. The enantiomeric ratios (er) of all but two examples exceeded 90:10 with many examples giving er values of 95:5 or higher, making FmocNHCl a highly practical reagent for preparing chiral amino alcohols. The SAAH reaction using FmocNHCl was used for the preparation of d-threo-β-hydroxyasparagine and d-threo-β-methoxyaspartate, suitably protected for Fmoc solid phase peptide synthesis.

α-Amino Diphenyl Phosphonates as Novel Inhibitors of Escherichia coli ClpP Protease

Increased Gram-negative bacteria resistance to antibiotics is becoming a global problem, and new classes of antibiotics with novel mechanisms of action are required. The caseinolytic protease subunit P (ClpP) is a serine protease conserved among bacteria that is considered as an interesting drug target. ClpP function is involved in protein turnover and homeostasis, stress response, and virulence among other processes. The focus of this study was to identify new inhibitors of Escherichia coli ClpP and to understand their mode of action. A focused library of serine protease inhibitors based on diaryl phosphonate warheads was tested for ClpP inhibition, and a chemical exploration around the hit compounds was conducted. Altogether, 14 new potent inhibitors of E. coli ClpP were identified. Compounds 85 and 92 emerged as most interesting compounds from this study due to their potency and, respectively, to its moderate but consistent antibacterial properties as well as the favorable cytotoxicity profile.

Asymmetric Synthesis of Fmoc-Protected β-Hydroxy and β-Methoxy Amino Acids via a Sharpless Aminohydroxylation Reaction Using FmocNHCl

An efficient asymmetric synthesis of l- threo-β-hydroxyasparagine and l- threo-β-methoxyaspartate that are suitably protected for Fmoc solid phase peptide synthesis is described. The key step in these syntheses was a Sharpless asymmetric aminohydroxylation reaction under basic conditions using N-chlorofluorenyl carbamate (FmocNHCl), a readily prepared and storable nitrogen source.

Total Synthesis of (+)-Pochonin D and (+)-Monocillin II via Chemo- and Regioselective Intramolecular Nitrile Oxide Cycloaddition

Asymmetric total syntheses of (+)-pochonin D (1) and (+)-monocillin II (2), Hsp90 inhibitors with potent anticancer activity, have been accomplished where the macrolactone 3 was constructed through a chemo- and regioselective intramolecular nitrile oxide cycloaddition of diene 4.

Study on the synthesis of the cyclopenta[f]indole core of raputindole A

The raputindoles from the rutaceous tree Raputia simulans share a cyclopenta[f]indole partial structure the synthesis of which is subject of this investigation. An efficient route to a series of 1,5-di(indol-6-yl)pentenones was developed via Mo/Au-catalyzed Meyer–Schuster rearrangement of tertiary propargylic alcohol precursors. However, none of the enones underwent the desired Nazarov cyclization to a cyclopenta[f]indole. More suitable were 6-hydroxyallylated indolines which gave good yields of cyclopenta[f]indolines after treatment with SnCl₄, as soon as sterically demanding β-cyclocitral adducts were reacted. Most successful were Pt(II) and Au(I)-catalyzed cyclizations of N-TIPS-protected indolin-6-yl-substituted propargylacetates which provided the hydrogenated tricyclic cyclopenta[f]indole core system in high yield.

Total Synthesis of (±)-Glyceollin II and a Dihydro Derivative

Stressed soybeans produce a group of phytoalexins that belong to the 6a-hydroxypterocarpan family of flavonoids. Certain of the more prominent members, such as the glyceollins I, II, and III, have demonstrated potential antidiabetic properties and promising cytotoxicity in both human breast and prostate cancer cell cultures with preliminary studies in animals further demonstrating antitumor effects in estrogen-dependent, human breast cancer cell implants. Although syntheses of glyceollin I have been reported previously, this work constitutes the first total directed synthesis of (±)-glyceollin II. It involves 12 steps with an overall yield of 7% using practical methods that should be readily scalable to produce quantities needed for advanced biological characterization. Highlights include a novel intramolecular benzoin condensation, a chelation-controlled lithium aluminum hydride-mediated reduction, and an intramolecular cyclization via the formation of a transient epoxide intermediate to cap the construction of the 6a-hydroxypterocarpan system. Additionally, a dihydro analogue has been obtained, and several isolated intermediates have been made available for evaluation of their biological properties and possible contributions toward elaborating key structure-activity relationship data among this family of promising phytoalexins elicited from stressed soybeans.