Short Access to (+)-Lupinine and (+)-Epiquinamide via Double Hydroformylation

Gilman reagent toward the synthesis of natural products

With the ever-increasing scope of organocuprates, a well-established Gilman reagent has been considered as an unprecedented synthetic tool in modern organic chemistry. The broad research profile of the Gilman reagent (R2CuLi in THF or Et2O) is owing to its propensity to carry out three kinds of reactions, i.e., epoxide ring opening reactions, 1,4-conjugate addition reactions, and SN2 reactions in a regioselective manner. This review examines the applications of Gilman reagent in the total synthesis of both abundant and scarce natural products of remarkable synthetic pharmaceutical profile reported since 2011. The presented insights will be of a vital roadmap to general organic synthesis and it will contribute to the development of new natural products and their analogues in future drug discovery.

An aza-Robinson Annulation Strategy for the Synthesis of Fused Bicyclic Amides: Synthesis of (±)-Coniceine and Quinolizidine

An aza-Robinson annulation strategy is described using a NaOEt-catalyzed conjugate addition of cyclic imides onto vinyl ketones, followed by a TfOH-mediated intramolecular aldol condensation to afford densely functionalized fused bicyclic amides. The potential use of these amides in the synthesis of alkaloids is demonstrated by the sequential conversion of appropriate precursors to (±)-coniceine and quinolizidine in two additional steps, thus allowing their preparation in overall 40 and 44% yields, respectively.

Stereoselective Synthesis of Pyrrolo/Pyrido[2,1-a]isoindoles via Alkyne Iminium Ion Cyclization of Vinylogous Carbamates

An efficient, acid-mediated, intramolecular alkyne iminium ion cyclization of oxoisoindolidene for the diastereoselective synthesis of pyrrolo/pyridoisoindole is described. This protocol features broad substrate scope and easy scalability. An unusual N to C-1,3-alkyl shift is observed with substrates bearing strong electron donating group at the phenyl ring attached to alkyne with concomitant hydration of alkyne to the ketone.

Regioselective Asymmetric Alkynylation of N-Alkyl Pyridiniums

Disclosed in this Letter is a novel asymmetric addition of alkynyl nucleophiles to N-alkylpyridinium electrophiles. The coupling is effected under mild and simple reaction conditions, affording dihydropyridine products with complete regiochemical and stereochemical control. In addition to several manipulations of the dihydropyridine products, the utility of this transformation is demonstrated through a concise, dearomative, and asymmetric synthesis of (+)-lupinine, a natural acetylcholine esterase inhibitor.

Sequential One-Pot Vilsmeier-Haack and Organocatalyzed Mannich Cyclizations to Functionalized Benzoindolizidines and Benzoquinolizidines

The development of new one-pot sequential cyclizations involving a Vilsmeier-Haack reaction followed by an organocatalyzed Mannich reaction is reported. This synthetic strategy gives access to functionalized indolizidines and quinolizidines in one operation from readily synthesized precursors. Yields and diastereoselectivities are good to excellent when formamides are used to trigger the key step, bearing either an electron-rich aryl or a pyrrole as the nucleophilic partner in the first cyclization.

Rh-Catalyzed Hydroformylation-Initiated Bicyclization: Construction of Azabicyclic Systems

Here, we describe the recent progress toward construction of 1-azabicyclic structures using a domino hydroformylation double cyclization strategy of an amide bearing the trisubstituted alkene functionality. The method provides a rapid and atom-economic access to alkaloid structures under mild conditions, especially for quinolizidine and pyrrolidine-fused azepane skeletons with yields up to 82% and good diastereoselectivity. Subsequent oxidative cleavage conditions are developed for the synthesis of Dendrobatid alkaloid epi-epiquinamide.

Activating Imides with Triflic Acid: A General Intramolecular Aldol Condensation Strategy Toward Indolizidine, Quinolizidine, and Valmerin Alkaloids

A simple, inexpensive, step economic, and highly modular synthetic strategy to access izidine alkaloids is described. The key step is a TfOH-promoted intramolecular aldol condensation between enol and cyclic imide moieties. This cyclization strategy can be employed within an aza-Robinson annulation framework and represents a general tool to build fused bicyclic amines. To illustrate the power of this method, we describe the preparation of (±)-coniceine, (±)-quinolizidine, (±)-tashiromine, (±)-epilupinine, and the core of (±)-valmerins.

Stereoselective Addition of a Lithium Anion of 1,1-Diphenyl-2-aza-pentadiene to Sulfinimines: Application to the Synthesis of (-)-Epiquinamide

The addition of a lithium anion of diphenylallylimine to nonracemic sulfinimines was investigated. It was found that the reaction with sulfinimines derived from aliphatic aldehydes afforded the products with excellent diastereoselectivity (>99:1), furnishing the product vicinal diamines in very good yields. Application of the formed product vicinal diamines was demonstrated in the total synthesis of the natural product (-)-epiquinamide.

Alkylative Ring-Opening of Bicyclic Aziridinium Ion and Its Application for Alkaloid Synthesis

Alkylative ring-opening of bicyclic aziridinium ion generated from 4-hydroxybutylaziridine with organocopper reagent was achieved successfully to afford 2-alkylsubstituted piperidine in high or moderate yield. This method allowed carbon-carbon bond formation of "non-activated" aziridine via aziridinium ion ring-opening in regio- and stereo-selective manner for the first time. This newly developed reaction was applied for an efficient synthesis of alkaloid with the representative example of conine and epiquinamide.

Decarboxylative Annulation of α-Amino Acids with γ-Nitroaldehydes

Indolizidine and quinolizidine derivatives are readily assembled from proline or pipecolic acid and γ-nitroaldehydes by means of a decarboxylative annulation process. These reactions are promoted by simple acetic acid and involve azomethine ylides as reactive intermediates. The method was applied to the synthesis of an epiquinamide analog.

Simple Indolizidine and Quinolizidine Alkaloids

This review of simple indolizidine and quinolizidine alkaloids (i.e., those in which the parent bicyclic systems are in general not embedded in polycyclic arrays) is an update of the previous coverage in Volume 55 of this series (2001). The present survey covers the literature from mid-1999 to the end of 2013; and in addition to aspects of the isolation, characterization, and biological activity of the alkaloids, much emphasis is placed on their total synthesis. A brief introduction to the topic is followed by an overview of relevant alkaloids from fungal and microbial sources, among them slaframine, cyclizidine, Steptomyces metabolites, and the pantocins. The important iminosugar alkaloids lentiginosine, steviamine, swainsonine, castanospermine, and related hydroxyindolizidines are dealt with in the subsequent section. The fourth and fifth sections cover metabolites from terrestrial plants. Pertinent plant alkaloids bearing alkyl, functionalized alkyl or alkenyl substituents include dendroprimine, anibamine, simple alkaloids belonging to the genera Prosopis, Elaeocarpus, Lycopodium, and Poranthera, and bicyclic alkaloids of the lupin family. Plant alkaloids bearing aryl or heteroaryl substituents include ipalbidine and analogs, secophenanthroindolizidine and secophenanthroquinolizidine alkaloids (among them septicine, julandine, and analogs), ficuseptine, lasubines, and other simple quinolizidines of the Lythraceae, the simple furyl-substituted Nuphar alkaloids, and a mixed quinolizidine-quinazoline alkaloid. The penultimate section of the review deals with the sizable group of simple indolizidine and quinolizidine alkaloids isolated from, or detected in, ants, mites, and terrestrial amphibians, and includes an overview of the "dietary hypothesis" for the origin of the amphibian metabolites. The final section surveys relevant alkaloids from marine sources, and includes clathryimines and analogs, stellettamides, the clavepictines and pictamine, and bis(quinolizidine) alkaloids.

Synthesis of the tetracyclic skeleton of the Lycopodium alkaloid lycopladine H via a pivotal double hydroformylation/intramolecular reductive amination sequence

A synthesis of the complete tetracyclic framework of the structurally unique Lycopodium alkaloid lycopladine H has been accomplished using a strategy involving a double alkene hydroformylation/intramolecular reductive amination to form the azocane and spiro-piperidine moieties of the natural product.

The total synthesis of (−)-cryptocaryol A

A stereoselective total synthesis of (−)-cryptocaryol A (1) is described. Key features of the 17-step route include the use of three boron-mediated aldol reaction–reduction sequences to control all stereocenters and an Ando modification of the Horner–Wadsworth–Emmons olefination that permitted the installation of the Z double bond of the α-pyrone ring.

A highly stereo-controlled protocol to prepare pipecolic acids based on Heck and cyclohydrocarbonylation reactions

A consecutive series of metal-catalyzed reactions for the preparation of enantiomerically pure piperidine derivatives.

Total synthesis of alkaloid 205B

Concise and highly stereocontrolled total syntheses of racemic and enantiopure frog alkaloid 205B (1) were accomplished in 11 steps from 4-methoxypyridines 6 and 7 in overall yields of 8 and 8%, respectively. The assembly of the core of the natural product relies on a stereoselective Tsuji-Trost allylic amination reaction and a ring-closing metathesis. The synthesis features the use of an N-acylpyridinium salt reaction to introduce the first stereocenter and an unprecedented trifluoroacetic anhydride-mediated addition of an allylstannane to a vinylogous amide with complete facial selectivity. Deoxygenation of the C4 ketone proved difficult but was accomplished via a modified Barton-McCombie reaction in the presence of a catalytic amount of diphenyl diselenide.

A diastereoselective oxa-Pictet-Spengler-based strategy for (+)-frenolicin B and epi-(+)-frenolicin B synthesis

An efficient diastereoselective oxa-Pictet-Spengler reaction strategy was developed to construct benzoisochroman diastereomers. The utility of the reaction was demonstrated in the context of both the total synthesis of naturally occurring pyranonaphthoquinones (+)-frenolicin B and epi-(+)-frenolicin B as well as a range of frenolicin precursor analogs. The method is versatile and offers exquisite stereocontrol and, as such, offers a synthetic advance for the synthesis of pyranonaphthoquinone analogs.

Hypervalent iodine/TEMPO-mediated oxidation in flow systems: a fast and efficient protocol for alcohol oxidation

Hypervalent iodine(III)/TEMPO-mediated oxidation of various aliphatic, aromatic and allylic alcohols to their corresponding carbonyl compounds was successfully achieved by using microreactor technology. This method can be used as an alternative for the oxidation of various alcohols achieving excellent yields and selectivities in significantly shortened reaction times.

(+)-Laburnamine, a natural selective ligand and partial agonist for the α4β2 nicotinic receptor subtype

(+)-Laburnamine (1), a rare alkaloid extracted from Laburnum anagyroides seeds (∼4 mg from 1 kg), was shown to bind with high affinity (Ki, 293 nM) to the α4β2 nicotinic receptor subtype, which is, respectively, 126 and 136 times higher than to the α3β4 (Ki 37 μM) and α7 subtypes (Ki 40 μM). When its ability to release [(3)H]-dopamine from striatal slices was tested in a functional assay, compound 1 behaved as a partial agonist with an EC50 of 5.8 μM and an Emax that was 43% that of nicotine. When incubated with nicotine in the same assay, 1 prevented a maximal effect from being reached.

Multi-Bond Forming Processes in Efficient Synthesis

An increasing number of synthetic organic chemists are embracing the philosophy of efficiency. Herein we highlight multi-bond forming processes, which form two or more new covalent bonds in a single synthetic operation. Such processes, which have the ability to rapidly increase structural complexity, are preeminent in contemporary synthetic organic chemistry. In this short review we classify, analyse, and contrast contemporary multi-bond forming processes, frame these cutting edge contributions within a historical context, and speculate on likely future developments in the area.

Diastereoselective access to nonracemic 2-cis-substituted and 2,6-cis-disubstituted piperidines

Access to nonracemic amino ketones via a hydrozirconation/transmetalation/acylation sequence applied to Boc-protected 1-aminobut-3-enes is presented. This method was applied to the stereoselective synthesis of cyclic imines (or iminiums) which were diastereoselectively converted into 2-cis-substituted and 2,6-cis-disubstituted piperidines. The potential of this approach in the field of alkaloid synthesis was illustrated by the synthesis of (-)-coniine and (-)-indolizidine 209D. Furthermore, access to indolizidines bearing a quaternary center could also be envisioned through this strategy.