Enabling Closed-Loop Circularity of "Non-Polymerizable" α, β-Conjugated Lactone Towards High-Performance Polyester with the Assistance of Cyclopentadiene

Chemical recycling of polymers to monomers presents a promising solution to the escalating crisis associated with plastic waste. Despite considerable progress made in this field, the primary efforts have been focused on redesigning new monomers to produce readily recyclable polymers. In contrast, limited research into the potential of seemingly "non-polymerizable" monomers has been conducted. Herein, we propose a paradigm that leverages a "chaperone"-assisted strategy to establish closed-loop circularity for a "non-polymerizable" α, β-conjugated lactone, 5,6-dihydro-2H-pyran-2-one (DPO). The resulting PDPO, a structural analogue of poly(δ-valerolactone) (PVL), exhibits enhanced thermal properties with a melting point (Tm) of 114 °C and a decomposition temperature (Td,5%) of 305 °C. Notably, owing to the structural similarity between DPO and δ-VL, the copolymerization generates semi-crystalline P(DPO-co-VL)s irrespective of the DPO incorporation ratio. Intriguingly, the inherent C=C bonds in P(DPO-co-VL)s enable their convenient post-functionalization via Michael-addition reaction. Lastly, PDPO was demonstrated to be chemically recyclable via ring-closing metathesis (RCM), representing a significant step towards the pursuit of enabling the closed-loop circularity of "non-polymerizable" lactones without altering the ultimate polymer structure.

Identification of a short ACE2-derived stapled peptide targeting the SARS-CoV-2 spike protein

The design and synthesis of a series of peptide derivatives based on a short ACE2 α-helix 1 epitope and subsequent [i - i+4] stapling of the secondary structure resulted in the identification of a 9-mer peptide capable to compete with recombinant ACE2 towards Spike RBD in the micromolar range. Specifically, SARS-CoV-2 Spike inhibitor screening based on colorimetric ELISA assay and structural studies by circular dichroism showed the ring-closing metathesis cyclization being capable to stabilize the helical structure of the 9-mer ³⁴HEAEDLFYQ⁴² epitope better than the triazole stapling via click chemistry. MD simulations showed the stapled peptide being able not only to bind the Spike RBD, sterically interfering with ACE2, but also showing higher affinity to the target as compared to parent epitope.

Design and synthesis of hetero-steroids via ring-closing metathesis: Biological studies towards in vitro anticancer activity

Here, we report a synthetic approach to hetero-steroids and also studied their biological activities as anticancer agents. A novel class of oxacycles containing estrone moiety were synthesized in this report. Allyl ether derived from estrone underwent Claisen rearrangement (CR) and again O-allylation and subsequent ring-closure gave A-ring-furan and oxepine fused derivatives in high yields. We used double bond isomerization and ring-closing metathesis (RCM) as key steps to assemble hetero steroids containing a mixture of regio isomers like benzofurans and benzoxepine moieties. The novel benzofuran and benzoxepine-based hybrid steroid derivatives were subjected to in vitro cytotoxicity analysis and were found to exert cancer cell-specific activity.

Synthesis of a tricyclic hexapeptide -via two consecutive ruthenium-catalyzed macrocyclization steps- with a constrained topology to mimic vancomycin's binding properties toward D-Ala-D-Ala dipeptide

A ring-closing metathesis (RCM) - peptide coupling - ruthenium-catalyzed azide alkyne cycloaddition (RuAAC) strategy was developed to synthesize a tricyclic hexapeptide in which the side chain to side chain connectivity pattern resulted in a mimic with a topology that effectively mimics the bioactivity of vancomycin as a potent binder of the bacterial cell wall D-Ala-D-Ala dipeptide sequence and more importantly being an effective inhibitor of bacterial growth.

Asymmetric Total Synthesis of Indole Diterpenes Paspalicine, Paspalinine, and Paspalinine-13-ene

Paspaline-derived indole diterpenes (IDTs) are structurally complex mycotoxins with unique tremorgenic activity. Reported are asymmetric total syntheses of three paspaline-derived IDTs paspalicine, paspalinine and paspalinine-13-ene. Our synthesis features a green Achmatowicz rearrangement/bicycloketalization for the efficient construction of FG rings (75 % yield) and a cascade ring-closing metathesis of dienyne for highly regioselective formation of CD rings (72 % yield). Other highlights include four palladium-mediated reactions (Stille, aza-Wacker, Suzuki, and Heck) to forge the BE rings and the installation of two continuous all-carbon quaternary stereocenters via reductive ring-opening of cyclopropane and α-methylation of the conjugate ester. Our new synthetic strategy is expected to be applicable to the chemical synthesis of other paspaline-derived IDTs and will facilitate the bioactivity studies of these agriculturally and pharmacologically important IDTs.

Helix stabilization by stapled N-capping box

The N-capping box is a distinct helix-stabilizing motif frequently found in proteins. In this study, we examined a ruthenium-mediated intramolecular backbone to side chain macrocyclization as a rigidified mimicry of the N-capping box. Experimental data indicate that the 15-membered macrocycle formed by a hept-4-enoyl staple, which directly tethers the α-amino group of N1 residue and the α-carbon of N3 residue, is highly effective in stabilizing helical structures of short peptides.

A concise route to virginiamycin M2

Modular, fully synthetic routes to structurally complex natural products provide useful avenues to access chemical diversity. Herein we report a concise route to virginiamycin M2, a member of the group A streptogramin class of natural products that inhibits bacterial protein synthesis. Our approach features a longest linear sequence of six steps from 7 simple building blocks, and is the shortest and highest yielding synthesis of any member of the streptogramin class reported to date. We believe this route will enable access to unexplored structural diversity and may serve as a useful tool to improve the therapeutic potential of the streptogramin class of antibiotics.

Design and Synthesis of Polycycles, Heterocycles, and Macrocycles via Strategic Utilization of Ring-Closing Metathesis

In this perspective, we summarize new synthetic approaches for the construction of various polycyclic compounds involving ring-closing metathesis as a key step. In this regard, we used ring-closing metathesis in combination with other popular reactions like Suzuki-Miyaura coupling, Claisen rearrangement, Fischer indolization, Grignard addition, Diels-Alder reaction, and [2+2] cycloaddition reaction etc. To this end, a variety of functional molecules such as α-amino acids, cyclophanes, heterocycles, propellanes, spirocycles, and macrocycles have been prepared. The strategies developed and the molecules prepared here play a key role in designing new materials and also act as lead compounds in drug design. The strategies and tactics developed here are useful to design polycycles, macrocycles, and heterocycles of diverse ring systems.

Design, synthesis, and opioid activity of arodyn analogs cyclized by ring-closing metathesis involving Tyr(allyl)

Kappa (κ) opioid receptor selective antagonists are useful pharmacological tools in studying κ opioid receptors and have potential to be used as therapeutic agents for the treatment of a variety of diseases including mood disorders and drug addiction. Arodyn (Ac[Phe^1-3,Arg⁴,d-Ala⁸]Dyn A-(1-11)NH₂) is a linear acetylated dynorphin A (Dyn A) analog that is a potent and selective κ opioid receptor antagonist (Bennett et al. J Med Chem 2002;45:5617-5619) and prevents stress-induced reinstatement of cocaine-seeking behavior following central administration (Carey et al. Eur J Pharmacol 2007;569:84-89). To restrict its conformational mobility, explore possible bioactive conformations and potentially increase its metabolic stability we synthesized cyclic arodyn analogs on solid phase utilizing a novel ring-closing metathesis (RCM) reaction involving allyl-protected Tyr (Tyr(All)) residues. This approach preserves the aromatic functionality and directly constrains the side chains of one or more of the Phe residues. The novel cyclic arodyn analog 4 cyclized between Tyr(All) residues incorporated in positions 2 and 3 exhibited potent κ opioid receptor antagonism in the [³⁵S]GTPγS assay (K_B = 3.2 nM) similar to arodyn. Analog 3 cyclized between Tyr(All) residues in positions 1 and 2 also exhibited nanomolar κ opioid receptor antagonist potency (K_B = 27.5 nM) in this assay. These are the first opioid peptides cyclized via RCM involving aromatic residues, and given their promising pharmacological activity represent novel lead peptides for further exploration.

A-Ring Synthons of 19-Nor Type Vitamin D Derivatives

Various 19-nor vitamin D analogs, which lack the methylene group at C19, exhibit significant vitamin D (VD)-related biological activities, but generally show a reduced calcemic side-effect compared with VD itself. Among them, paricalcitol is already used clinically for treatment and prevention of secondary hyperparathyroidism associated with chronic renal failure. Therefore, considerable synthetic efforts have been directed towards 19-nor VD analogs, focusing especially on A-ring synthons suitable for use in convergent synthetic strategies based on coupling of CD-ring synthons and A-ring synthons. For example, we have recently developed a new synthetic route to A-ring synthons from linear dienes based upon a ring-closing olefin metathesis strategy. Here, we review recent synthetic approaches to A-ring synthons for 19-nor VD derivatives.

A mild and efficient approach to enantioenriched α-hydroxyethyl α,β-unsaturated δ-lactams

A straightforward approach toward enantioenriched α-substituted α,β-unsaturated δ-lactams is described. Although a considerable number of approches toward α,β-unsaturated δ-lactams have been reported, there are relatively few examples of enantioenriched α,δ-disubstituted α,β-unsaturated δ-lactams formation. The δ-stereocenter was formed by addition of allylmagnesium bromide to an N-tert-butylsulfinyl imine. The α,β-unsaturated δ-lactam was furnished by ring-closing metathesis. Although Baylis-Hillman chemistry failed on this cyclic compound, introduction of the hydroxyethyl group prior to ring-closing metathesis was successful. A Baylis-Hillman reaction was used to introduce the substituent at the α-position of the α,β-unsaturated lactam.

Enantioselective synthesis of metacycloprodigiosin via the "Wasserman pyrrole"

A concise, nine-step enantioselective total synthesis of metacycloprodigiosin is reported. The synthesis provides increased step-efficiency over the previous racemic and enantioselective syntheses of this compound. Key features of the work include investigations into a convergent oxidative coupling reaction and subsequent ring-closing metathesis to deliver an advanced pyrrole intermediate we name the "Wasserman pyrrole" that can be converted to metacycloprodigiosin in one step.

Synthesis of novel vitamin D3 analog with an additional ring annulated to A and seco-B rings

A simple method for the synthesis of yet unknown 5E-vitamin D3 analogs with an additional six-membered ring connecting C-6 and C-19 was developed. Ring-closing metathesis (RCM) was used for efficient formation thereof from the corresponding 5E-isomers of 6-alkenyl vitamin D3 compounds which in turn were prepared from the 6-oxo-3,5-cyclovitamin D3. Reinvestigation of the Grignard reactions of this latter compound as well as the following acid-catalyzed cycloreversions showed discrepancies with the literature data describing the course of such processes.

Bicyclic tetrapeptides as potent HDAC inhibitors: effect of aliphatic loop position and hydrophobicity on inhibitory activity

Several histone deacetylase (HDAC) inhibiting bicyclic tetrapeptides have been designed and synthesized through intramolecular ring-closing metathesis (RCM) reaction and peptide cyclization. We designed bicyclic tetrapeptides based on CHAP31, trapoxin B and HC-toxin I. The HDAC inhibitory and p21 promoter assay results showed that the aliphatic loop position as well as the hydrophobicity plays an important role toward the activity of the bicyclic tetrapeptide HDAC inhibitors.