Samarium(ii) iodide-mediated reactions applied to natural product total synthesis

Natural product synthesis remains a field in which new synthetic methods and reagents are continually being evaluated. Due to the demanding structures and complex functionality of many natural products, only powerful and selective methods and reagents will be highlighted in this proceeding. Since its introduction by Henri Kagan, samarium(ii) iodide (SmI₂, Kagan's reagent) has found increasing use in chemical synthesis. Over the years, many reviews have been published on the application of SmI₂ in numerous reductive coupling procedures as well as in natural product total synthesis. This review highlights recent advances in SmI₂-mediated synthetic strategies, as applied in the total synthesis of natural products since 2004.

Natural product synthesis remains a field in which new synthetic methods and reagents are continually being evaluated.

Enantioselective Total Synthesis of (+)-Corymine and (-)-Deformylcorymine

We report herein the first enantioselective total synthesis of akuammiline alkaloids (+)-corymine and (-)-deformylcorymine. Starting from commercially available N-nosyltryptamine, the target molecules are both achieved in 11 steps. Key elements of the design include (a) a copper-catalyzed enantioselective addition of dimethyl malonate to a 3-bromooxindole to secure the C7 all-carbon quaternary stereocenter, (b) a one-step construction of cyclohexyl and pyrrolidinyl rings via intramolecular nucleophilic C- and N-addition, and (c) a nickel-promoted 7-endo cyclization of alkenyl bromide to furnish the azepanyl ring. The strategy is further extended to the synthesis of another three members of the akuammiline family, namely, (-)-10-demethoxyvincorine, (-)-2(S)-cathafoline, and (-)-3-epi-dihydrocorymine 17-acetate.

Stereoselective Cascade Cyclizations with Samarium Diiodide to Tetracyclic Indolines: Precursors of Fluorostrychnines and Brucine

A series of γ-indolylketones with fluorine, cyano or alkoxy substituents at the benzene moiety was prepared and subjected to samarium diiodide-promoted cyclization reactions. The desired dearomatizing ketyl cascade reaction forming two new rings proceeded in all cases with high diastereoselectivity, but with differing product distribution. In most cases, the desired annulated tetracyclic compounds were obtained in moderate to good yields, but as second product tetracyclic spirolactones were isolated in up to 29 % yield. The reaction rate was influenced by the substituents at the benzene moiety of the substrate as expected, with electron-accepting groups accelerating and electron-donating groups decelerating the cyclization process. In case of a difluoro-substituted γ-indolylketone a partial defluorination was observed. The intermediate samarium enolate of the tetracyclic products could be trapped by adding reactive alkylating agents as electrophiles delivering products with quarternary carbons. In the case of a dimethoxy-substituted tetracyclic cyclization product a subsequent reductive amination stereoselectively provided a pentacyclic compound that was subsequently N-protected and subjected to a regioselective elimination. The obtained functionalized pentacyclic product should be convertible into the alkaloid brucine by four well-established steps. Overall, the presented report shows that functionalized tetracyclic compounds with different substituents are rapidly available with the samarium diiodide cascade cyclization as crucial step. Hence, analogues of the landmark alkaloid strychnine, for example, with specific fluorine substitutions, should be easily accessible.

TBHP promoted demethylation of α-amino carbonyl compounds: a concise approach to substituted γ-lactams

A novel tert-butyl hydroperoxide (TBHP) promoted CH₂-extrusion reaction of α-amino carbonyl compounds has been developed, which is driven by a demethylenation process to give various ring contraction products γ-lactams under radical conditions.

CBr4-mediated cross-coupling reactions of α-amino carbonyl compounds with alcohols and thiols to build C–O and C–S bonds, respectively

The direct C(sp³)–H bond functionalization of α-amino carbonyl compounds with CBr₄ to construct C–O and C–S bonds.

Synthesis of Nitrogen Heterocycles Using Samarium(II) Iodide

Nitrogen heterocycles represent vital structural motifs in biologically-active natural products and pharmaceuticals. As a result, the development of new, convenient and more efficient processes to N-heterocycles is of great interest to synthetic chemists. Samarium(II) iodide (SmI₂, Kagan’s reagent) has been widely used to forge challenging C–C bonds through reductive coupling reactions. Historically, the use of SmI₂ in organic synthesis has been focused on the construction of carbocycles and oxygen-containing motifs. Recently, significant advances have taken place in the use of SmI₂ for the synthesis of nitrogen heterocycles, enabled in large part by the unique combination of high reducing power of this reagent (E_1/2 of up to −2.8 V) with excellent chemoselectivity of the reductive umpolung cyclizations mediated by SmI₂. In particular, radical cross-coupling reactions exploiting SmI₂-induced selective generation of aminoketyl radicals have emerged as concise and efficient methods for constructing 2-azabicycles, pyrrolidines and complex polycyclic barbiturates. Moreover, a broad range of novel processes involving SmI₂-promoted formation of aminyl radicals have been leveraged for the synthesis of complex nitrogen-containing molecular architectures by direct and tethered pathways. Applications to the synthesis of natural products have highlighted the generality of processes and the intermediates accessible with SmI₂. In this review, recent advances involving the synthesis of nitrogen heterocycles using SmI₂ are summarized, with a major focus on reductive coupling reactions that enable one-step construction of nitrogen-containing motifs in a highly efficient manner, while taking advantage of the spectacular selectivity of the venerable Kagan’s reagent.

Unified Total Syntheses of Structurally Diverse Akuammiline Alkaloids

The unified total syntheses of structurally diverse akuammiline alkaloids deformylcorymine (1), strictamine (2), and calophyline A (3) are reported. The strategy mimics the biosynthesis in nature at a strategic level, which allows for structural diversification from a common synthetic precursor by late-stage ring migrations.

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.

Collective synthesis of Lycopodium alkaloids and tautomer locking strategy for the total synthesis of (-)-lycojapodine A

The collective total synthesis of Lycopodium alkaloids (+)-fawcettimine (1), (+)-fawcettidine (2), (+)-alopecuridine (4), (-)-lycojapodine A (6), and (-)-8-deoxyserratinine (7) has been accomplished from a common precursor (15) based on a highly concise route inspired by the proposed biosynthesis of the fawcettimine- and serratinine-type alkaloids. An intramolecular C-alkylation enabled efficient installation of the challenging spiro quaternary carbon center and the aza-cyclononane ring. The preparation of the tricyclic skeleton as well as the establishment of the correct relative stereochemistry of the oxa-quaternary center were achieved by hydroxyl-directed SmI(2)-mediated pinacol couplings. An unprecedented tandem transannular N-alkylation and removal of a Boc group was discovered to realize a biosynthesis-inspired process to furnish the desired tetracyclic skeleton. Of particular note is the unique and crucial tautomer locking strategy employed to complete the enantioselective total synthesis of (-)-lycojapodine A (6). The central step in this synthesis is the late-stage hypervalent iodine oxidant (IBX or Dess-Martin periodinane)/TFA-mediated tandem process, which constructed the previously unknown carbinolamine lactone motif and enabled a biomimetic transformation to generate (-)-lycojapodine A (6) in a single operation.