Generation, Stability, and Utility of Lithium 4,4′-Di-tert-butylbiphenylide (LiDBB)

Convergent Total Synthesis of (-)-Cyclopamine

A concise and enantioselective total synthesis of the Veratrum alkaloid cyclopamine is disclosed. This highly convergent synthesis with a 16-step longest linear sequence (LLS) was enabled by a de novo synthesis of the trans-6,5-heterobicycle via a strain-inducing halocyclization process, a key Tsuji-Trost cyclization to construct the fully substituted, spirocyclic THF motif with exquisite diastereocontrol, and a late-stage ring-closing metathesis (RCM) reaction to forge the central tetrasubstituted olefin.

Recent Advances in Chemical Synthesis of Amino Sugars

Amino sugars are a kind of carbohydrates with one or more hydroxyl groups replaced by an amino group. They play crucial roles in a broad range of biological activities. Over the past few decades, there have been continuing efforts on the stereoselective glycosylation of amino sugars. However, the introduction of glycoside bearing basic nitrogen is challenging using conventional Lewis acid-promoted pathways owing to competitive coordination of the amine to the Lewis acid promoter. Additionally, diastereomeric mixtures of O-glycoside are often produced if aminoglycoside lack a C2 substituent. This review focuses on the updated overview of the way to stereoselective synthesis of 1,2-cis-aminoglycoside. The scope, mechanism, and the applications in the synthesis of complex glycoconjugates for the representative methodologies were also included.

Enantioselective Total Synthesis of (+)-Sieboldine A and Analogues Thereof

An 11-step enantioselective total synthesis of (+)-sieboldine A (1) has been accomplished from (5R)-methylcyclohex-2-en-1-one (16), in which an intramolecular ketone/ester reductive coupling followed by one-pot acidic treatment to quickly construct the unique oxa-spiroacetal and a TsOH-catalyzed displacement to directly form the characteristic N-hydroxyazacyclononane ring successfully served as the key methodologies. Moreover, several full-skeleton analogues of 1 were also synthesized on the basis of the advanced intermediates, and their inhibitory effects on electric eel acetylcholinesterase were examined.

Synthesis of 2-Deoxyglycosides Bearing Free Hydroxyl Substituents on the Glycosyl Donor

Recent efforts in the field of carbohydrate chemistry have focused on the site- and stereocontrolled synthesis of O-glycosides derived from acceptors bearing multiple hydroxyl substituents. By comparison, there are few examples of the site-selective synthesis of O-glycosides bearing free hydroxyl substituents on the donor reagent. Here, we report the application of an umpolung glycosylation strategy to the synthesis of O-glycosides derived from donors bearing free hydroxyl substituents. The reaction proceeds via prior deprotonation of one or more free hydroxyl groups on a thiophenylglycoside donor, reductive lithiation to generate an anomeric anion intermediate, and addition of this anion to an alkyl 2-(2-methyltetrahydropyranyl) peroxide. By this approach, α-linked glycosides were obtained in 39-84% yields and with >50:1 α/β selectivities. In many instances, β-linked products could be obtained by thermal equilibration of the anomeric anion intermediate (selectivities = 3.8-8:1 β/α; yields = 33-68%). The strategy is applicable to polyhydroxyl donors bearing up to three free hydroxyl groups, N-acylated carbohydrates, and the single-flask syntheses of oligosaccharides.

Unified Total Synthesis of Tetracyclic Diquinane Lycopodium Alkaloids (+)-Paniculatine, (-)-Magellanine, and (+)-Magellaninone

A unified route for the total synthesis of three tetracyclic diquinane Lycopodium alkaloids (+)-paniculatine, (-)-magellanine, and (+)-magellaninone has been accomplished in 13-14 overall steps based on late-stage diverse transformations from an advanced tetracyclic common intermediate. In the established synthesis, quick formation of the two five-membered rings was efficiently achieved by an intramolecular reductive coupling of ketone-carbonyl and ester-carbonyl and an organocatalytic intramolecular Michael addition of aldehyde-derived enamine to an internal enone functionality with satisfactory redox and step economies and excellent stereoselectivities, providing the requisite tricyclic carbo-framework possessing multiple dense stereogenic centers, and an intramolecular reductive amination finally furnished the essential piperidine ring.

Enantioselective Synthesis and Biological Evaluation of Sanglifehrin A and B and Analogs

Sanglifehrin A and B are immunosuppressive macrocyclic natural products endowed with and differentiated by a unique spirocyclic lactam. Herein, we report an enantioselective total synthesis and biological evaluation of sanglifehrin A and B and analogs. Access to the spirocyclic lactam was achieved through convergent assembly of a key pyranone intermediate followed by a stereo-controlled spirocyclization. The 22-membered macrocyclic core was synthesized by ring-closing metathesis in the presence of 2,6-bis(trifluoromethyl) benzeneboronic acid (BFBB). The spirocyclic lactam and macrocycle fragments were united by a Stille coupling to furnish sanglifehrin A and B. Additional sanglifehrin B analogs with variation at the C40 position were additionally prepared. Biological evaluation revealed that the 2-CF₃ analog of sanglifehrin B exhibited higher anti-proliferative activity than the natural products sanglifehrin A and B in Jurkat cells. Both natural products induced higher-order homodimerization of cyclophilin A (CypA), but only sanglifehrin A promoted CypA complexation with inosine-5'-monophosphate dehydrogenase 2 (IMPDH2). The synthesis reported herein will enable further evaluation of the spirolactam and its contribution to sanglifehrin-dependent immunosuppressive activity.

Concise Unified Access to (-)-8-Deoxy-13-dehydroserratinine, (+)-Fawcettimine, (+)-Fawcettidine, and (-)-8-Deoxyserratinine Using a Direct Intramolecular Reductive Coupling

A short, scalable, and collective total synthesis of four fawcettimine-type Lycopodium alkaloids in eight or nine steps is disclosed. A dense multi-small-ring spiro-α-aminocyclopentanone successfully served as the key intermediate, which was directly accessed by a LiDBB-mediated intramolecular reductive coupling of the aliphatic imine and an ester-carbonyl. Compared to those that employ classical Heathcock intermediate(s) containing a nine-membered ring, the new strategy shows the significant improvement of the synthetic step and redox economies as well as excellent stereochemical control.

Synthesis and Structural Implication of the JKLMN-Ring Fragment of Caribbean Ciguatoxin C-CTX-1

Synthesis of the JKLMN-ring fragment of Caribbean ciguatoxin C-CTX-1, the causative toxin of ciguatera fish poisoning in the Caribbean Sea and the Northeast Atlantic areas, is described in detail. Key to the synthesis are a [2,3]-sigmatropic rearrangement to construct a seven-membered α-hydroxy exo-enol ether, stereoselective construction of an angular tetrasubstituted stereogenic center on the seven-membered M-ring by a hydrogen atom transfer-based reductive olefin coupling, Suzuki-Miyaura coupling of the KLMN-ring enol phosphate with a highly congested M-ring, and silica gel-mediated epoxide ring opening to form the J-ring. Comparison of the nuclear magnetic resonance spectroscopic data for the synthesized fragment with those for the natural product provided support for the formerly assigned structure of the N-ring in the right-hand terminal of C-CTX-1.

General Method for the Synthesis of α- or β-Deoxyaminoglycosides Bearing Basic Nitrogen

The introduction of glycosides bearing basic nitrogen is challenging using conventional Lewis acid-promoted pathways owing to competitive coordination of the amine to the Lewis acid promoter. Additionally, because many aminoglycosides lack a C2 substituent, diastereomeric mixtures of O-glycosides are often produced. Herein, we present a method for the synthesis of α- or β- 2,3,6-trideoxy-3-amino- and 2,4,6-trideoxy-4-amino O-glycosides from a common precursor. Our strategy proceeds by the reductive lithiation of thiophenyl glycoside donors and trapping of the resulting anomeric anions with 2-methyltetrahydropyranyl peroxides. We apply this strategy to the synthesis of α- and β-forosamine, pyrrolosamine, acosamine, and ristosamine derivatives using primary and secondary peroxides as electrophiles. α-Linked products are obtained in 60-96% yield and with >50:1 selectivity. β-Linked products are obtained in 45-94% yield and with 1.7->50:1 stereoselectivity. Contrary to donors bearing an equatorial amine substituent, donors bearing an axial amine substituent favored β-products at low temperatures. This work establishes a general strategy to synthesize O-glycosides bearing a basic nitrogen.

Evaluation and Development of Methodologies for the Synthesis of Thiophosphinic Acids

Thiophosphorus acids R¹R²P(S)OH constitute an important class of organophosphorus compounds, in which the phosphorus atom is intrinsically chiral if R¹ ≠ R². In connection with a project aimed at the preparation of chiral thiophosphorus acids, various available literature methods were considered, but few fit the requirement of odorless reagents. Herein, the results of our studies on the synthesis of thiophosphinic acids are reported. Ultimately, two major approaches were selected: (1) the Stec reaction of phosphorus amides with carbon disulfide; and (2) the one-pot synthesis of thiophosphorus acids from H-phosphinates, an organometallic nucleophile, and quenching with elemental sulfur. An application to the preparation of a potential chiral phosphorus organocatalyst is also reported.

Pharmacophore-Directed Retrosynthesis Applied to Ophiobolin A: Simplified Bicyclic Derivatives Displaying Anticancer Activity

Pharmacophore-directed retrosynthesis applied to ophiobolin A led to bicyclic derivatives that were synthesized and display anticancer activity. Key features of the ultimate defensive synthetic strategy include a Michael addition/facially selective protonation sequence to set the critical C6 stereocenter and a ring-closing metathesis to form the cyclooctene. Cytotoxicity assays toward a breast cancer cell line (MDA-MB-231) confirm the anticipated importance of structural complexity for selectivity (vs MCF10A cells) while C3 variations modulate stability.

Generation of Alkyl Radical through Direct Excitation of Boracene-Based Alkylborate

The generation of tertiary, secondary, and primary alkyl radicals has been achieved by the direct visible-light excitation of a boracene-based alkylborate. This system is based on the photophysical properties of the organoboron molecule. The protocol is applicable to decyanoalkylation, Giese addition, and nickel-catalyzed carbon-carbon bond formations such as alkyl-aryl cross-coupling or vicinal alkylarylation of alkenes, enabling the introduction of various C(sp³) fragments to organic molecules.

Enantioselective synthesis of cis-hydrobenzofurans bearing all-carbon quaternary stereocenters and application to total synthesis of (‒)-morphine

(‒)-Morphine, which is selected as an essential medicine by World Health Organization, is widely applied in the treatment of the pain-related diseases. Due to its synthetically challenging molecular architecture and important clinical role, extensive synthetic studies of morphine-type alkaloids have been conducted. However, catalytic asymmetric total synthesis of (‒)-morphine remains a long-standing challenge. Here, we disclose an efficient enantioselective total synthesis of (‒)-morphine in a longest linear sequence of 16 steps. The key transformation features a highly enantioselective Robinson annulation enabled by our spiro-pyrrolidine catalyst to rapidly construct the densely functionalized cis-hydrodibenzofuran framework containing vicinal stereocenters with an all-carbon quaternary center. This asymmetric approach provides an alternative strategy for the synthesis of (‒)-morphine and its analogues.

(‒)-Morphine is an essential medicine selected by the World Health Organization, however its catalytic asymmetric syntheses have been rarely reported. Here, the authors developed an intramolecular enantioselective Michael addition leading to (‒)-morphine in a longest linear sequence of 16 steps.