Electrocyclic ring-opening/pi-allyl cation cyclization reaction sequences involving gem-dihalocyclopropanes as substrates: application to syntheses of (+/-)-, (+)-, and (-)-gamma-lycorane

Synthetic Approaches to α-, β-, γ-, and δ-lycoranes

Lycorane is a pentacyclic core presented in alkaloids isolated from the Amaryllidaceae family of herbaceous flowering plants. Members of this class of natural products have shown to display important biological properties including analgesic, antiviral, and antiproliferative activities. This review presents the known synthetic routes toward α-, β-, γ-, and δ-lycoranes. α-(19 routes), β-(10 routes), γ-(38 routes), and δ-(6 routes).

Extension of hydrogen borrowing alkylation reactions for the total synthesis of (-)-γ-lycorane

The total synthesis of (-)-γ-lycorane (10 steps) and synthesis of (±)-γ-lycorane (8 steps) was completed from cyclohexenone. A new two step hydrogen borrowing alkylation of an aziridinyl alcohol, coupled with a Ph* (Me₅C₆) deprotection/cyclisation procedure was developed for de novo formation of the fused 6,5 heterocyclic ring. This work is one of the first examples of hydrogen borrowing C-C bond formation being used as a key step in a total synthesis project.

Catalytic, Asymmetric Total Synthesis of (+)-α-, (+)-β-, (+)-γ-, and (-)-δ-Lycorane

The first collectively asymmetric total synthesis of all members of lycorane, including (+)-α, (+)-β, (+)-γ, and (-)-δ, in a catalytic manner has been achieved. The cornerstone of this synthesis features an asymmetric, stereodivergent Ir/amine dual catalytic α-allylation of 2-phthalimidoacetaldehyde.

Electrocyclic Ring-Opening of 6,6-Dichlorobicyclo[3.1.0]-hexanes and Trapping of the Resulting π-Allyl Cations by C-1 Tethered Hydroxyamine Derivatives: Formation of 2-Oxa-1-azaspiro[4.5]decan-3-ones

The C-1 substituted 6,6-dichlorobicyclo[3.1.0]hexanes 1a–c have been prepared and shown to undergo electrocyclic ring-opening to give the corresponding π-allyl cations 2 that cyclise to afford the spirocyclic products 3b–d, each of which has been subjected to single-crystal X-ray analysis.

An Approach to the 9-Oxo-10-oxabicyclo[5.3.0]dec-2-ene Core of the Guaianolide and Pseudoguaianolide Sesquiterpenes via a Domino Electrocyclic Ring-Opening/Carboxylic Acid Trapping of a gem-Dibromocyclopropane

A domino silver(I)-promoted electrocyclic 2π-disrotatory electrocyclic ring-opening/intramolecular nucleophilic trapping of [ n.1.0]-dibromocyclopropanes by tethered carboxylic acids results in cyclization to butyrolactones fused to six- and seven-membered carbocycles. In the case of bicyclic [4.3.0] lactones the cis-fused stereoisomer was formed, whereas for the bicyclic [5.3.0] lactones the trans-fused stereoisomer was formed. Optimal conditions for the reaction used silver(I) trifluoroacetate (2.0 equiv) in trifluoroethanol or with added pyridine (2.0 equiv) and NaPF₆ (5.0 equiv). The dibromocyclopropane precursors were made through cyclopropanation with in situ-generated dibromocarbene. The trans-fused lactones are potentially useful building blocks for pseudoguaianolide, guaianolide, and xanthanolide total synthesis. A computational study on the conformational preferences of these systems indicates that the trans-fused bicyclic [5.3.0] butyrolactones are lower in energy than the corresponding cis-fused lactones at the B3LYP/cc-pVTZ level of theory.

Recent Advances in Steroid Synthesis: A Tribute to Sir Derek Barton

The synthesis of steroids and gaining an ultimate understanding of their reactivity was one of Sir Derek Barton’s most notable research areas. This highlight will focus on the construction of the steroid ring system from 2016 to 2018, and will include pathways that eventually led to natural product synthesis. For example, efficient syntheses of ent-pregnanolone sulfate and oestradiol methyl ether will be explained along with the total synthesis of cannogenol-3-O-α-l-rhamnoside.

Synthetic nat- or ent-steroids in as few as five chemical steps from epichlorohydrin

Today, more than 100 Food and Drug Administration-approved steroidal agents are prescribed daily for indications including heart failure, inflammation, pain and cancer. While triumphs in organic chemistry have enabled the establishment and sustained growth of the steroid pharmaceutical industry, the production of highly functionalized synthetic steroids of varying substitution and stereochemistry remains challenging, despite the numerous reports of elegant strategies for their de novo synthesis. Here, we describe an advance in chemical synthesis that has established an enantiospecific means to access novel steroids with unprecedented facility and flexibility through the sequential use of two powerful ring-forming reactions: a modern metallacycle-mediated annulative cross-coupling and a new acid-catalysed vinylcyclopropane rearrangement cascade. In addition to accessing synthetic steroids of either enantiomeric series, these steroidal products have been selectively functionalized within each of the four carbocyclic rings, a synthetic ent-steroid has been prepared on a multigram scale, the enantiomer of a selective oestrogen has been synthesized, and a novel ent-steroid with growth inhibitory properties in three cancer cell lines has been discovered.

Short Total Synthesis of (±)-γ-Lycorane by a Sequential Intramolecular Acylal Cyclisation (IAC) and Intramolecular Heck Addition Reaction

An intramolecular acylal cyclisation (IAC) approach to the synthesis of a range of bicyclic heterocycles is reported. As an example of the utility of the IAC reaction, the methodology was applied in a protecting-group-free five-step total synthesis of (±)-γ-lycorane, incorporating a new intramolecular Heck addition reaction to generate the pentacyclic core structure of the natural product in good yield.

Synthesis of functionalised azepanes and piperidines from bicyclic halogenated aminocyclopropane derivatives

Reductive amination of aldehydes or ketones with suitable dihalogenated bicyclic aminocyclopropanes gives functionalised ring-expanded products in a single step.

Concise construction of the tetracyclic core of lycorine-type alkaloids and the formal synthesis of α-lycorane based on asymmetric bifunctional thiourea-catalyzed cascade reaction

A concise and stereoselective construction of the tetracyclic core of lycorine-type alkaloids and the formal synthesis of α-lycorane has been achieved. The feature of the current method is the employment of a bifunctional thiourea-catalyzed cascade reaction as a powerful tool to construct the skeleton of the natural product, which is a challenging yet very rarely explored strategy. As a result, the tetracyclic core is efficiently synthesized in just three simple operations involving two consecutive cascade reactions.

Total synthesis of (±)-γ-lycorane via the electrocyclic ring closure of a divinylpyrroline

A concise total synthesis of (±)-γ-lycorane is described. The key step in the synthesis is the 6π-electrocyclic ring closure of a divinylpyrroline to give a tetrahydroindole, which is subsequently hydrogenated to give the all-cis indolizidine core.

gem-Dihalocyclopropanes as Building Blocks in Natural-Product Synthesis: Enantioselective Total Syntheses ofent-Erythramine and 3-epi-Erythramine

ent-Erythramine ((-)-1), the enantiomer of the alkaloid erythramine, was prepared in 15 steps from known compounds. The first of three pivotal bond-forming steps in the synthesis was a Suzuki-Miyaura cross-coupling reaction of the starting materials to give a bis-silyl ether. The second involved silver(I)-induced electrocyclic ring opening of the gem-dichlorocyclopropane formed in the next step and trapping of the ensuing pi-allyl cation by the tethered nitrogen atom to give, following cleavage of the allyloxycarbonyl protecting group, an approximately 5:6 mixture of the chromatographically separable diastereoisomeric spirocyclic products. In the third critical bond-forming reaction, the iodide formed from one of the diastereoisomers underwent a radical-addition/elimination reaction sequence that led to (-)-1 in 89 % yield. The application of the same sequence of transformations to the other diastereoisomer afforded 3-epi-(+)-erythramine (3-epi-(+)-1).

Concise Assembly of the Polycyclic Frameworks Associated with the Hapalindole and Fischerindole Alkaloids

[reaction: see text] Reaction of N-methylindole (4) with 6,6-dibromobicyclo[3.1.0]hexane (5) in the presence of silver tetrafluoroborate affords conjugate 7 in 67% yield. This product can be readily elaborated to compounds 12b and 13b which embody the polycyclic frameworks associated with members of the hapalindole and fischerindole classes of alkaloids. The chiral-auxiliary-substituted 6,6-dibromobicyclo[3.1.0]hexanes 21 and 22 react with indole to give adducts likely to be useful in the enantioselective total synthesis of the title alkaloids.

New Protocols for the Assembly of the Tetracyclic Framework Associated with the Aromatic Erythrina Alkaloids

[reaction: see text] Treatment of the anion derived from the ring-fused gem-dichlorocyclopropane 4c with silver tetrafluoroborate afforded the spirocyclic compound 17 in 74% yield. Product 17 was readily converted, over three steps, into the beta-iodoethyl derivative 20 and treatment of this latter compound with n-Bu(3)SnH then afforded, in 93% yield and via a radical addition/elimination sequence, compound 2 incorporating the ABCD framework of the aromatic erythrina alkaloids.

Intramolecular bromo-amination of 1,4-cyclohexadieneaminal: one-pot discrimination of two olefins and concise asymmetric synthesis of (−)-γ-lycorane

The reaction of cyclohexa-2,5-dienyl-1-methylaldehyde and optically pure 1,2-diaryl-1,2-diamine followed by intramolecular bromo-amination produced a one-pot discrimination of two olefins in the cyclohexane system, which was used for the asymmetric synthesis of (-)-gamma-lycorane.

Solvolytic Ring-Opening Reactions of Cyclopropyl Bromides. An Assessment of the Woodward−Hoffmann−DePuy Rule

A theoretical study on the torquoselectivity of the electrocyclic ring opening of 2,3-disubstituted-mono and dibromo cyclopropanes was carried out at the B3LYP/6-311++G* level. As a result of charge donation from the dissociating C-C bond to the antibonding orbital of the breaking C-Br bond, only one of the two allowed disrotatory paths for the disrotatory ring opening, which takes place in concert with the C-Br bond cleavage, is energetically accessible, both in the gas phase and in solution. The general trends in torquoselectivity for these systems are not significantly affected by stereoelectronic effects of the substituents or the introduction of a second bromine in C1. For the Woodward-Hoffmann-DePuy-allowed processes, the remarkable lowering in energy when the OH or NH(2) groups rotate inward can be attributed to the formation of an intramolecular X-H-Br hydrogen bond in the transition state. Aromaticity in reactants and transition states for the model systems is evaluated using a ring current model to shed light on the reaction mechanism.

A photochemical approach to the Galanthan ring system

A five-step, atom-efficient synthesis of the Galanthan tetracyclic skeleton has been developed. The key step is an unusual intramolecular de Mayo reaction using an isocarbostyril substrate with a functionalized tether on nitrogen. The target molecule is produced in 35% overall yield from isocarbostyril.

Development of an Efficient Procedure for Indole Ring Synthesis from 2-Ethynylaniline Derivatives Catalyzed by Cu(II) Salts and Its Application to Natural Product Synthesis

The development of efficient methods for the indole synthesis catalyzed by Cu(II) salts and its applications were investigated. Cu(OAc)(2) has been proved to be the best catalyst for the synthesis of various 1-p-tolylsulfonyl or 1-methylsulfonylindoles, which have both electron-withdrawing and electron-donating groups on the aromatic ring and C2 position of indoles. For the primary aniline derivatives, Cu(OCOCF(3))(2) showed good activities, while Cu(OAc)(2) was a good catalyst for the cyclization of secondary anilines. This methodology could be applied to the sequential cyclization reaction for the compounds which have the electrophilic part in the same molecule. By prior treatment with KH, the sequential cyclization was realized to provide the tricyclic ring systems, but it was limited to five- and six-membered rings for the second cyclization. Finally, formal and total synthesis of hippadine with the Cu(II)-promoted indole synthesis as the key step was accomplished.

Total Synthesis of (+)-Acanthodoral by the Use of a Pd-Catalyzed Metal-ene Reaction and a Nonreductive 5-exo-Acyl Radical Cyclization

[reaction: see text] The first total synthesis of the antibiotic acanthodoral (1) has been achieved from 3-methyl-2-cyclohexen-1-one in 19 steps in 2.1% overall yield. The synthesis features the use of a Pd-ene reaction in the presence of CO to form the endocyclic alkene 8, a nonreductive acyl radical cyclization reaction, and a ring contraction reaction by the Wolff rearrangement. (+)-Acanthodoral has also been synthesized starting from (+)-S-2,2-dimethyl-6-methylenecyclohexanecarboxylic acid.

Synthesis of Nitrogen-Functionalized Cyclohexanes Using Chemoselective Conjugate Addition of Phenyllithium to Linear .OMEGA.-Nitro-.ALPHA.,.BETA.,.PSI.,.OMEGA.-Unsaturated Ester and Subsequent Stereoselective Intramolecular Nitro-Michael Cyclization

Nitrogen-functionalized cyclohexane derivatives with three contiguous chiral centers were synthesized by nitroalkene-selective conjugate addition of phenyllithium to a omega-nitro-alpha,beta,psi,omega-unsaturated ester and subsequent stereocontrolled intramolecular nitro-Michael cyclization with cesium fluoride and a quaternary ammonium bromide. The cyclohexanes were applicable to the total synthesis of alpha-, beta- and gamma-lycoranes.