Stereospecific Syntheses of Olefins from 1,2-Thionocarbonates and 1,2-Trithiocarbonates. trans-Cycloheptene

Advances in the Synthesis of Bioorthogonal Reagents: s-Tetrazines, 1,2,4-Triazines, Cyclooctynes, Heterocycloheptynes, and trans-Cyclooctenes

Aligned with the increasing importance of bioorthogonal chemistry has been an increasing demand for more potent, affordable, multifunctional, and programmable bioorthogonal reagents. More advanced synthetic chemistry techniques, including transition-metal-catalyzed cross-coupling reactions, C-H activation, photoinduced chemistry, and continuous flow chemistry, have been employed in synthesizing novel bioorthogonal reagents for universal purposes. We discuss herein recent developments regarding the synthesis of popular bioorthogonal reagents, with a focus on s-tetrazines, 1,2,4-triazines, trans-cyclooctenes, cyclooctynes, hetero-cycloheptynes, and -trans-cycloheptenes. This review aims to summarize and discuss the most representative synthetic approaches of these reagents and their derivatives that are useful in bioorthogonal chemistry. The preparation of these molecules and their derivatives utilizes both classical approaches as well as the latest organic chemistry methodologies.

Design and synthesis of a tetracyclic tripeptide mimetic frozen in a polyproline type II (PP2) helix conformation

A synthesis of the new tetracyclic scaffold ProM-19, which represents a XPP tripeptide unit frozen in a PPII helix conformation, was developed. As a key building block, N-Boc-protected ethyl (1S,3S,4R)-2-azabicyclo[2.2.1]hept-5-ene-2-carboxylate was prepared through a diastereoselective aza-Diels-Alder reaction and subsequent hydrogenolytic removal of the chiral N-1-phenylethyl substituent under temporary protection of the double bond through dihydroxylation and reconstitution by Corey-Winter olefination. The target compound Boc-[ProM-19]-OMe was then prepared via subsequent peptide coupling and Ru-catalyzed ring-closing metathesis steps employing (S)-N-Boc-allylgylcine and cis-5-vinyl-proline methyl ester as additional building blocks. In addition, Ac-[2-Cl-Phe]-[Pro]-[ProM-19]-OMe was prepared by solution phase peptide synthesis as a potential ligand for the ena-VASP EVH1 domain.

Visible-light-mediated alkylation of 4-alkyl-1,4-dihydropyridines with alkenyl sulfones

Herein we report a mild, general protocol for visible-light-mediated alkylation of 4-alkyl-1,4-dihydropyridines with alkenyl sulfones. The protocol permits efficient functionalization of sulfones with a broad range of cyclic and acyclic secondary and tertiary alkyl groups and is scalable to the gram level. Its excellent functional group tolerance and mildness make it suitable for late-stage functionalization of natural products and drug molecules.

Chemical trigger-enabled bioconjugation reaction

Development of conceptually novel and practically useful bioconjugation reactions has been an intense pursuit of chemical biology research. Herein, we report an unprecedented bioconjugation reaction that hinges on a chemical trigger-enabled inverse-electron-demand Diels-Alder (IEDDA) cycloaddition of trans-cycloheptene (TCH) with tetrazine. Unlike the conventional strain-promoted bioconjugation reactions that utilize built-in strained alkenes as reactants, the current one features a "trigger-release-conjugate" reaction model, in which a highly strained TCH species is released from a bench-stable bicyclic N-nitrosourea (BNU) derivative upon treatment with an external stimulus. It is noteworthy that the reactivity-stability balance of BNU derivatives could be tuned by manipulating their N-1 substituents. As a proof-of-concept case, this new chemical trigger-enabled IEDDA reaction has been applied to in vitro protein labeling and pretargeted cell imaging. This work opens a new avenue to utilize BNU derivatives as a new class of chemical reporters in bioconjugate chemistry.

Visible-Light-Mediated Alkenylation of Alkyl Boronic Acids without an External Lewis Base as an Activator

Herein we report a protocol for the direct visible-light-mediated alkenylation of alkyl boronic acids at room temperature without an external Lewis base as an activator, and we propose a mechanism involving benzenesulfinate activation of the alkyl boronic acids. The protocol permits the efficient functionalization of a broad range of cyclic and acyclic primary and secondary alkyl boronic acids with various alkenyl sulfones. We demonstrated its utility by preparing or functionalizing several pharmaceuticals and natural products.

Flow Photochemical Syntheses of trans-Cyclooctenes and trans-Cycloheptenes Driven by Metal Complexation

trans-Cyclooctenes and trans-cycloheptenes have long been the subject of physical organic study, but the broader application had been limited by synthetic accessibility. This account describes the development of a general, flow photochemical method for the preparative synthesis of trans-cycloalkene derivatives. Here, photoisom erization takes place in a closed-loop flow reactor where the reaction mixture is continuously cycled through Ag(I) on silica gel. Selective complexation of the trans-isomer by Ag(I) during flow drives an otherwise unfavorable isomeric ratio toward the trans-isomer. Analogous photoreactions under batch-conditions are low yielding, and flow chemistry is necessary in order to obtain trans-cycloalkenes in preparatively useful yields. The applications of the method to bioorthogonal chemistry and stereospecific transannulation chemistry are described.

Electrochemical Corey-Winter reaction. Reduction of thiocarbonates in aqueous methanol media and application to the synthesis of a naturally occurring α-pyrone

An electrochemical version of the Corey-Winter reaction was developed giving excellent results in aqueous methanol media (MeOH/H₂O (80:20) with AcOH/AcONa buffer 0.5 M as supporting electrolyte), using a reticulated vitreous carbon as cathode in a divided cell. The electrochemical version is much more environmentally friendly than the classical reaction, where a large excess of trialkyl phosphite as reducing agent and high temperatures are required. Thus, cathodic reduction at room temperature of two cyclic thiocarbonates (-1.2 to -1.4 V vs Ag/AgCl) afforded the corresponding alkenes, trans-6-(pent-1-enyl)-α-pyrone and trans-6-(pent-1,4-dienyl)-α-pyrone, which are naturally occurring metabolites isolated from Trichoderma viride and Penicillium, in high chemical yield and with excellent stereo selectivity.

Photochemical syntheses, transformations, and bioorthogonal chemistry of trans-cycloheptene and sila trans-cycloheptene Ag(i) complexes

A photochemical synthesis of AgNO3 complexes of trans-cycloheptene (TCH) and trans-1-sila-4-cycloheptene (Si-TCH) derivatives is described. A low temperature flow photoreactor was designed to enable the synthesis of carbocyclic TCH derivatives due to their thermal sensitivity in the absence of metal coordination. Unlike the free carbocycles, TCH·AgNO3 complexes can be handled at rt and stored for weeks in the freezer (-18 °C). Si-TCH·AgNO3 complexes are especially robust, and are bench stable for days at rt, and for months in the freezer. X-ray crystallography was used to characterize a Si-TCH·AgNO3 complex for the first time. With decomplexation of AgNO3in situ, metal-free TCO and Si-TCH derivatives can engage in a range of cycloaddition reactions as well as dihydroxylation reactions. Computation was used to predict that Si-TCH would engage in bioorthogonal reactions that are more rapid than the most reactive trans-cyclooctenes. Metal-free Si-TCH derivatives were shown to display good stability in solution, and to engage in the fastest bioorthogonal reaction reported to date (k2 1.14 × 107 M-1 s-1 in 9 : 1 H2O : MeOH). Utility in bioorthogonal protein labeling in live cells is described, including labeling of GFP with an unnatural tetrazine-containing amino acid. The reactivity and specificity of the Si-TCH reagents with tetrazines in live mammalian cells was also evaluated using the HaloTag platform. The cell labeling experiments show that Si-TCH derivatives are best suited as probe molecules in the cellular environment.

Dearomative cycloadditions of a silylene with pyrazine and quinoxaline

The dearomative cycloaddition of a dialkylsilylene with pyrazine affords an eight-membered heterocyclic compound with a trans-cycloalkene moiety.

Enantioselective Total Synthesis of Otteliones A and B, Novel and Powerful Antitumor Agents from the Freshwater Plant Ottelia alismoides

Otteliones A and B isolated from the freshwater plant Ottelia alismoides have attracted significant attention because of their potential as novel anticancer agents. In this review four independent enantioselective total syntheses and one formal synthesis of these natural products are presented with particular focus on their methodology and strategy.

3-Substituted pyrazinone nucleosides--a new family of D4T analogues

The synthesis of a new family of D4T analogues is described to study the influence of pyrazinone base on antiretroviral power. Substitution of 3H by methyl or n-decyl increases the lipophilic character and may facilitate diffusion across cell membranes. The compounds were characterized by (1)H NMR and infrared spectroscopy. Antiviral (HIV-1) properties of these compounds were examined.

Chemistry of the cortistatins--a novel class of anti-angiogenic agents

Synthetic efforts culminating the construction of several highly advanced intermediates, and completed syntheses of the recently disclosed cortistatin family of anti-proliferative agents are described in this perspective.

Recent Progress on the Stereoselective Synthesis of Cyclic Quaternary alpha-Amino Acids

The most recent papers describing the stereoselective synthesis of cyclic quaternary alpha-amino acids are collected in this review. The diverse synthetic approaches are classified according to the size of the ring and taking into account the bond that is formed to complete the quaternary skeleton.

Enantioselective Total Synthesis of (+)-Ottelione A, (−)-Ottelione B, (+)-3-epi-Ottelione A and Preliminary Evaluation of Their Antitumor Activity

Enantioselective total synthesis of (+)-ottelione A (1) and (-)-ottelione B (2), novel and potent antitumor agents from a freshwater plant, and (+)-3-epi-ottelione A (3), the earlier proposed stereostructure of 1, was efficiently achieved starting from the known tricyclic compound 10. The synthesis involved the following key steps: i) coupling reactions of aldehydes 8 and 9 with the aromatic portion 7 (8+7-->15 and 9+7-->27), ii) base-induced hemiacetal-opening/epimerization reactions of the cyclic hemiacetals 6 and 27 (6-->17 and 27 a-->26 a), and iii) Corey-Winter's reductive olefination of the cyclic thiocarbonates 21 and 36 (21-->22 and 36-->37). The present total synthesis fully established the absolute configuration of these natural products. The cell growth inhibition profile, COMPARE analysis, and tubulin inhibitory assay of (+)-3-epi-ottelione A (3) and its O-acetyl derivative 24 demonstrated that these unnatural substances could be prominent lead compounds for the development of anticancer agents with a novel mode of action.

Benzotriazole-assisted thioacylation

[Chemical reaction: See text] Benzotriazole reagents for thioacylation (RCSBt), thiocarbamoylation (RR'NCSBt), aryl/alkoxythioacylation (ROCSBt), and aryl/alkylthiothioacylation (RSCSBt) are synthesized, and their utility is assessed by syntheses of representative heteroaryl thioureas 3a-g, thioamides 15a-s, thionoesters 16a-h, thiocarbamates 17a-e, dithiocarbamates 18a-d, thiocarbonates 19a-c, and dithiocarbonates 20a-c.

How stable is trans-cycloheptene?

There is a discrepancy between the observed and calculated stability of trans-cycloheptene (t-CHP). Generation of t-CHP has always led to its low-temperature (-40 degrees C) isomerization to cis-cycloheptene (c-CHP). However, force field and semiempirical calculations on the energy difference between the two isomers have suggested that t-CHP should be stable at room temperature. We performed a series of ab initio calculations, which predicted that the simple process of double bond rotation leading from t-CHP to c-CHP would have an activation barrier too high to permit isomerization below 100 degrees C (35 kcal/mol). The validity of our calculation method on this very strained system was supported by the agreement between the calculation and the dynamics of the ring flip of the unsymmetrical t-CHP ring and the observed NMR shifts and coupling constants for the system. This incompatibility between the experimental behavior of t-CHP and our calculations led to our reexamining the decay kinetics of t-CHP. We find that this decay is second order and represents an "interrupted" dimerization, where an initially formed 1,4-biradical rapidly changes its geometry and cleaves back to produce two c-CHP molecules. This mechanism was supported by calculations of the 1,4-biradical potential energy surface.