Synthesis of a Promising Immunosuppressant: FR252921

Ynamide Coupling Reagents: Origin and Advances

Since the pioneering work of Curtius and Fischer, chemical peptide synthesis has witnessed a century's development and evolved into a routine technology. However, it is far from perfect. In particular, it is challenged by sustainable development because the state-of-the-art of peptide synthesis heavily relies on legacy reagents and technologies developed before the establishment of green chemistry. Over the past three decades, a broad range of efforts have been made for greening peptide synthesis, among which peptide synthesis using unprotected amino acid represents an ideal and promising strategy because it does not require protection and deprotection steps. Unfortunately, C → N peptide synthesis employing unprotected amino acids has been plagued by undesired polymerization, while N → C inverse peptide synthesis with unprotected amino acids is retarded by severe racemization/epimerization owing to the iterative activation and aminolysis of high racemization/epimerization susceptible peptidyl acids. Consequently, there is an urgent need to develop innovative coupling reagents and strategies with novel mechanisms that can address the long-standing notorious racemization/epimerization issue of peptide synthesis.This Account will describe our efforts in discovery of ynamide coupling reagents and their application in greening peptide synthesis. Over an eight-year journey, ynamide coupling reagents have evolved into a class of general coupling reagents for both amide and ester bond formation. In particular, the superiority of ynamide coupling reagents in suppressing racemization/epimerization enabled them to be effective for peptide fragment condensation, and head-to-tail cyclization, as well as precise incorporation of thioamide substitutions into peptide backbones. The first practical inverse peptide synthesis using unprotected amino acids was successfully accomplished by harnessing such features and taking advantage of a transient protection strategy. Ynamide coupling reagent-mediated ester bond formation enabled efficient intermolecular esterification and macrolactonization with preservation of α-chirality and the configuration of the conjugated α,β-C-C double bond. To make ynamide coupling reagents readily available with reasonable cost and convenience, we have developed a scalable one-step synthetic method from cheap starting materials. Furthermore, a water-removable ynamide coupling reagent was developed, offering a column-free purification of the target coupling product. In addition, the recycle of ynamide coupling reagent was accomplished, thereby paving the way for their sustainable industrial application.As such, this Account presents the whole story of the origin, mechanistic insights, preparation, synthetic applications, and recycle of ynamide coupling reagents with a perspective that highlights their future impact on peptide synthesis.

Stereoselective synthesis of backbone extended π-conjugated amino esters

Utilization of the Wittig reaction to synthesize conjugative multiple double bonds is rare. We examined the utility of the Wittig reaction to construct conjugative two and three carbon-carbon double bonds on the N-protected amino acid backbone. The ethyl esters of N-Boc amino acids with multiple carbon-carbon double bonds in the backbone were isolated in excellent yields with exceptional E-selectivity of the double bonds. The allylic alcohols of α,β-unsaturated γ-amino esters were selectively synthesized from the DIBAL-H and BF₃·OEt₂. The allylic alcohols were transformed into aldehydes using IBX oxidation. Using this protocol, we synthesized ethyl esters of N-Boc-(E,E)-α,β,γ,δ-unsaturated ε-amino acids with various side-chain functionalities and ethyl esters of N-Boc-(E,E,E)-α,β,γ,δ,ε,ζ-unsaturated η-amino acids with excellent yields. We speculated the exceptional E-selectivity is probably due to the stabilization of the planar transition state of the Wittig reaction with the double bond p-orbitals. No racemization was observed in the synthesis of amino acids. The reported process may serve as an excellent route to synthesize multiple conjugative carbon-carbon double bonds.

Synthesis of Polyene Bioactive Natural Products: FR252921 and Vitamin A

A formal synthesis of FR252921, a potent macrocyclic immunosuppressive agent, and a six-step synthesis of vitamin A have been demonstrated. The application of a ruthenium-catalyzed step-economic and environmentally benign strategy for the highly stereo- and chemoselective construction of valuable polyene motifs of FR252921 and vitamin A highlights the syntheses. The key features for the synthesis FR252921 include preparation of the triene moiety followed by two consecutive peptide couplings of the three fragments.

Unconventional Macrocyclizations in Natural Product Synthesis

Over the past several decades, macrocyclic compounds have emerged as increasingly significant therapeutic candidates in drug discovery. Their pharmacological activity hinges on their rotationally restricted three-dimensional orientation, resulting in a unique conformational preorganization and a high enthalpic gain as a consequence of high-affinity macrocycle-protein binding interactions. Synthetic access to macrocyclic drug candidates is therefore crucial. From a synthetic point of view, the efficiency of macrocyclization events commonly suffers from entropic penalties as well as undesired intermolecular couplings (oligomerization). Although over the past several decades ring-closing metathesis, macrolactonization, or macrolactamization have become strategies of choice, the toolbox of organic synthesis provides a great number of versatile transformations beyond the aforementioned. This Outlook focuses on a selection of examples employing what we term unconventional macrocyclizations toward the synthesis of natural products or analogues.

Asymmetric Total Syntheses of (+)-Penostatins A and C

Penostatins A and C are cytotoxic natural products that show promising selective inhibitory activity against PTP1B. Here the first asymmetric total syntheses of (+)-penostatins A and C are reported. Our strategy features (i) a new method for the synthesis of 6-alkyl-3-hydroxy-2-pyrones, (ii) a cascade involving the intramolecular Diels-Alder reaction of 2-pyrone and a retro-hetero-Diels-Alder (decarboxylation) reaction, (iii) Ando-Horner-Wadsworth-Emmons olefination/lactonization, and (iv) selenoxide elimination. Our study confirmed the absolute configurations of penostatins A and C and laid the groundwork for further bioactivity studies.

A Domino 10-Step Total Synthesis of FR252921 and Its Analogues, Complex Macrocyclic Immunosuppressants

FR252921, FR252922, and FR256523 are a family of potent macrocyclic polyene immunosuppressive agents with a novel mode of action. However, the lack of an efficient and flexible synthesis has hindered further biological studies, mostly due to the fact that the natural products appear to be kinetic isomers regarding the triene moiety. Herein, we report the development and application of an unprecedented, unique domino Suzuki–Miyaura/4π-electrocyclic ring-opening macrocyclization, resulting in a concise, unified, and stereoselective synthetic route to these complex targets in only 10 steps. This in turn enables ready access to a range of unnatural analogues, among which several compounds showed inhibition of T-lymphocyte proliferation at levels equal or superior to those of the natural products themselves.

An enantioselective synthesis of the C3-C21 segment of the macrolide immunosuppressive agent FR252921

An enantioselective synthesis of the C3-C21 segment of the novel immunosuppressant FR252921 is described. The C12 and C13 stereogenic centers were constructed by a non-aldol process utilizing optically active 4-phenylbutyrolactone. The C18 stereogenic center was installed using Braun's highly diastereoselective acetate aldol reaction. Other key steps involved Curtius rearrangement and Horner-Wadsworth-Emmons olefination reactions.

Catalytic dehydrogenative N-((triisopropylsilyl)oxy)carbonyl (Tsoc) protection of amines using iPr3SiH and CO2

Heterogeneous Pd/C efficiently catalyzes the dehydrogenative N-((triisopropylsilyl)oxy)carbonyl (Tsoc) protection of amines using iPr₃SiH and CO₂ without the liberation of any salts.

STILLE CROSS-COUPLING REACTIONS OF ARYL MESYLATES AND TOSYLATES USING A BIARYLPHOSPHINE BASED CATALYST SYSTEM

A catalyst system for the Stille cross-coupling reactions of aryl mesylates and tosylates is reported. Using the combination of Pd(OAc)2, XPhos, and CsF in t-BuOH an array of aryl and heteroaryl sulfonates were successfully employed in these reactions. Morever, heteroarylstannanes, such as furyl, thiophenyl, and N-methylpyrrole, which are often prone to decomposition, were efficiently coupled under these conditions. Ortho-substitution on the stannane coupling partner was well tolerated; however, the presence of ortho substituents on the aryl sulfonates greatly reduced the proficiency of these reactions.

Total synthesis of (-)-zampanolide and questionable existence of (-)-dactylolide as the elusive biosynthetic precursor of (-)-zampanolide in an Okinawan sponge

A new and concise total synthesis of (-)-zampanolide, (-)-1, and (-)-dactylolide, (-)-2, is described. Synthetic highlights include (i) a mild Horner-Wadsworth-Emmons reaction providing the seco acid, (ii) an unusual stepwise cross-coupling reaction of a 1,1-dibromodiene with inversion of olefin geometry, and (iii) specific O-Michael reaction conditions using catalytic LHMDS with TMEDA for the synthesis of functionalized 2,6-cis-tetrahydropyran. A marine sponge extract was analyzed for the presence of (-)-2 as the biosynthetic precursor of (-)-zampanolide.

Synthesis of two bioactive natural products: FR252921 and pseudotrienic acid B

Concise and highly convergent syntheses of the immunosuppressive agent FR252921 and the related antimicrobial natural product pseudotrienic acid B were achieved from a common intermediate by using optically active titanium complexes to control the configuration of the stereogenic centers, a highly stereo- and regioselective cross-metathesis to generate the triene moieties, and a Stille cross-coupling to install the dienic units.