Au-Catalyzed [2 + 3] Annulation of Enamides with Propargyl Esters: Total Synthesis of Cephalotaxine and Cephalezomine H

α,β-Epoxy Ketone Rearrangements for the Practical Synthesis of Cephalotaxine and the Stereodivergent Synthesis of Azaspiro Allylic Alcohols

We report herein that three facile rearrangements of epoxy ketones can be employed for the efficient and practical synthesis of cephalotaxine, the parent member of the family of Cephalotaxus alkaloids. The Meinwald rearrangement of epoxy ketone (90%) was used for the preparation of the dense functional groups in the cyclopentane ring of cephalotaxine. A novel acid-catalyzed umpolung SN2' rearrangement and the Wharton transposition reaction of epoxy ketones were also developed to synthesize the Mori intermediate via the key azaspiro allylic alcohols in a stereodivergent manner.

Dearomative (3 + 2) Cycloaddition of Indoles for the Stereoselective Assembly of Fully Functionalized Cyclopentanoids

The gold(I)-catalyzed dearomative cyclopentannulation of various indoles with 2-ethynyl-1,3-dithiolane is reported. The method generates three new stereocenters with excellent control of relative stereochemistry and is tolerant of diverse functionalization and substitution patterns on the indoles. The obtained cyclopentane-fused indolines allow for interesting subsequent synthetic manipulations, giving rise to fully substituted cyclopentanes with control of the relative stereochemistry of all five stereocenters. The reported reaction illustrates and elucidates a mechanistic dichotomy underlying gold(I)-catalyzed reactions of 2-ethynyl-1,3-dithiolane.

Reductive Oxy-Nazarov Cyclization toward Expedient Construction of a Cyclopenta[1,2-b]pyrrolo[1,2-a]azepine Ring System: Formal Total Syntheses of Stemonamine and Cephalotaxine

Formal total syntheses of stemonamine and cephalotaxine bearing the core cyclopenta[1,2-b]pyrrolo[1,2-a]azepine ring skeleton were achieved. The general synthetic strategy in the synthesis features the reductive oxy-Nazarov cyclization as key step, leading to the versatile construction of N-substituted spiro quaternary stereogenic centers from readily available starting materials.

Concise Synthesis of (±)-Fortuneicyclidins and (±)-Cephalotine B Enabled by Pd-Catalyzed Dearomative Spirocyclization

Total syntheses of C11-oxygenated Cephalotaxus alkaloids, fortuneicyclidins A and B, and cephalotine B, were achieved. The key for the synthesis is a Pd-catalyzed dearomative spirocyclization of bromofurans with N-tosylhydrazones, followed by acid-mediated tandem transformation to construct the tetracyclic skeleton with the C11-oxygen functional group. Chemo-selective and catalytic functional group conversions of the tetracyclic intermediate completed the synthesis of fortuneicyclidins and cephalotine B in 8 and 9 steps, respectively.

Enantioselective Total Synthesis of the Cephalotaxus Alkaloid (-)-Cephalotine A

Cephalotine A, a recently isolated Cephalotaxus alkaloid, was first synthesized enantioselectively through three critical reactions. SmI2 -mediated radical cyclization of lactone and aldehyde to forge the final ring system, Chang's iridium-catalyzed C-H amidation to construct pyrrolidone stereoselectively, and Carreria's dual Ir/amine catalyzed allylation to install the vicinal tertiary stereocenters.

Concise Synthesis of (±)-Myrioneurinol Enabled by Sequential [2+2] Cycloaddition/Retro-Mannich Fragmentation/Mannich Reaction

A concise total synthesis of (±)-myrioneurinol has been achieved in 14 steps. An efficient AgSbF₆ /t-BuCl-catalyzed intramolecular [2+2] cycloaddition reaction of the alkynone-tethered enamine was developed to prepare the highly strained cyclobutene. It was used in combination with a subsequent retro-Mannich fragmentation/Mannich reaction to efficiently construct the tricyclic core of myrioneurinol.

Construction of azaspirocyclic skeletons mediated by the carbonyl of the Weinreb amide: formal total synthesis of (±)-cephalotaxine

A facile Stevens rearrangement of the Weinreb amide and the subsequent key steps mediated by the carbonyl of the Weinreb amide led to the construction of azaspirocyclic skeletons of some typical alkaloids. And the formal total synthesis of (±)-cephalotaxine was completed via a shorter synthetic route by this efficient method. Further studies on the development and asymmetric synthesis application of this strategy are underway.

Gold(I)-Catalyzed Cyclization-3-Aza-Cope-Mannich Cascade and Its Application to the Synthesis of Cephalotaxine

The discovery of a new gold(I)-catalyzed cascade reaction involving cyclization onto a vinylammonium, 3-aza-Cope rearrangement, and Mannich cyclization is reported. A variety of fused nitrogen heterocycles were prepared from simple cyclic tertiary amines using 1-5 mol % of a AuCl(PPh₃)/Ag[C₅(CN)₅] cocatalyst system. The developed reaction was used in a study aimed at synthesizing cephalotaxine. A five-step operation from norhydrastinine provided demethylcephalotaxinone in 39.1% overall yield, which was transformed to (-)-cephalotaxine in two steps.

Stereoselective Gold(I)-Catalyzed Vinylcyclopropanation via Generation of a Sulfur-Substituted Vinyl Carbene Equivalent

A stereoselective gold(I)-catalyzed vinylcyclopropanation of alkenes has been developed. A gold-coordinated cationic vinyl carbene species, readily generated via a rearrangement of the ethylenedithioacetal of propargyl aldehyde, reacts with a wide range of alkenes to afford thio-substituted vinylcyclopropanes. The gold-catalyzed vinyl cyclopropanation proceeds under mild conditions at room temperature and is generally selective for the formation of cis-substituted cyclopropanes. The reaction allows the formal introduction of a "naked" vinyl carbene, by subsequent chemoselective hydrodesulfurisation of the ethylenedithio-bridge. The synthetic utility of the new method is demonstrated by a short, racemic formal synthesis of the alkaloid cephalotaxin, hinging on a key vinyl cyclopropane-cyclopentene rearrangement.

Proline Ester Enolate Claisen Rearrangement and Formal Total Synthesis of (-)-Cephalotaxine

A concise formal total synthesis of (-)-cephalotaxine was achieved via an ester enolate Claisen rearrangement (EECR). A series of EECRs of proline allyl esters were examined to obtain the desired relative stereochemistry of an azaspiranic tetracyclic backbone. An unexpected reversal or low diastereoselectivity of ( Z)-cinnamyl ester was observed. The diastereoselectivity was controlled by substitution patterns of a styrene region. This result represents a useful guide in aiding the prediction of stereochemical outcome of EECR of α-amino allylic esters.

Total Synthesis and Structural Elucidation of (-)-Cephalezomine G

The first asymmetric synthesis and configurational elucidation of (-)-cephalezomine G was achieved. The highly functionalized Cα-substituted proline derivative was prepared from d-proline as the only chiral source via a C → N → C chirality transfer method consisting of stereoselective N-allylation and [2,3]-Stevens rearrangement. The azaspiranic tetracyclic backbone was constructed using ring-closing metathesis and the Friedel-Crafts reaction. Two contiguous hydroxyl groups were introduced in the later stages.

Synthesis of benzoazepine derivatives via Rh(iii)-catalyzed inert C(sp2)–H functionalization and [4 + 3] annulation

A novel synthesis of benzoazepine derivatives via Rh(iii)-catalyzed inert C(sp²)–H bond functionalization and [4 + 3] annulation is presented.

Formal Synthesis of ent-Cephalotaxine Using a One-Pot Parham-Aldol Sequence

A short formal synthesis of ent-Cephalotaxine is achieved. The approach features a new Lewis acid-mediated [2,3]-Stevens rearrangement of N-allylated prolineamide to generate a key quaternary stereogenic center. Additionally, a one-pot Parham-aldol sequence was developed to rapidly assemble two of the four rings in the cephalotaxine core.

Selenolactams as Synthetic Intermediates for the Synthesis of Polycyclic Amines via Seleno-Claisen Rearrangements

A highly diastereoselective α-allylation of selenolactams with allyl halides is reported. DFT analyses and experimental observations suggested that this reaction proceeds via a Se-allylation of the eneselenolates of the lactams followed by a seleno-Claisen rearrangement. The thus-obtained products could be efficiently transformed into polycyclic amines using a previously developed sequential addition of organometallic reagents and ring-closing metathesis.

Stereoselective Dynamic Cyclization of Allylic Azides: Synthesis of Tetralins, Chromanes, and Tetrahydroquinolines

This report describes the stereoselective synthesis of 3-azido-tetralins, -chromanes, and -tetrahydroquinolines via a tandem allylic azide rearrangement/Friedel-Crafts alkylation. Exposure of allylic azides with a pendant trichloroacetimidate to catalytic quantities of AgSbF6 proved optimal for this transformation. This cascade successfully differentiates the equilibrating azide isomers, providing products in excellent yield and selectivity (>25 examples, up to 94% yield and >25:1 dr). In many cases, the reactive isomer is only a trace fraction of the equilibrium mixture, keenly illustrating the dynamic nature of these systems. We demonstrate the utility of this process via a synthesis of hasubanan.

Cephalotaxus Alkaloids

Cephalotaxus alkaloids represent a family of plant secondary metabolites known for 60 years. Significant activity against leukemia in mice was demonstrated for extracts of Cephalotaxus. Cephalotaxine (CET) (1), the major alkaloid of this series was isolated from Cephalotaxus drupacea species by Paudler in 1963. The subsequent discovery of promising antitumor activity among new Cephalotaxus derivatives reported by Chinese, Japanese, and American teams triggered extensive structure elucidation and biological studies in this family. The structural feature of this cephalotaxane family relies mainly on its tetracyclic alkaloid backbone, which comprises an azaspiranic 1-azaspiro[4.4]nonane unit (rings C and D) and a benzazepine ring system (rings A and B), which is linked by its C3 alcohol function to a chiral oxygenated side chain by a carboxylic function alpha to a tetrasubstituted carbon center. The botanical distribution of these alkaloids is limited to the Cephalotaxus genus (Cephalotaxaceae). The scope of biological activities of the Cephalotaxus alkaloids is mainly centered on the antileukemic activity of homoharringtonine (HHT) (2), which in particular demonstrated marked benefits in the treatment of orphan myeloid leukemia and was approved as soon as 2009 by European Medicine Agency and by US Food and Drug Administration in 2012. Its exact mechanism of action was partly elucidated and it was early recognized that HHT (2) inhibited protein synthesis at the level of the ribosome machinery. Interestingly, after a latency period of two decades, the topic of Cephalotaxus alkaloids reemerged as a prolific source of new natural structures. To date, more than 70 compounds have been identified and characterized. Synthetic studies also regained attention during the past two decades, and numerous methodologies were developed to access the first semisynthetic HHT (2) of high purity suitable for clinical studies, and then high grade enantiomerically pure CET (1), HHT (2), and analogs.