Multicomponent Linchpin Coupling of Silyl Dithianes Employing an N-Ts Aziridine as the Second Electrophile: Synthesis of (−)-Indolizidine 223AB

Stereoselective synthesis of 2,6-disubstituted piperidine alkaloids

Among the large number of structurally diverse alkaloids, 2,6-disubstituted piperidine and its analogs have often been targeted when exploiting new synthetic techniques perhaps because of their strong pharmacological properties. This review outlines synthetic strategies to build the 2,6-disubstituted piperidine structural motif with a focus on stereochemical control of two substituents at C2 and C6. The key reactions in this process are then classified on the basis of how the piperidine rings were built with specific examples of natural products that control the stereochemical outcomes and their transition states.

Stereoselective Tandem Bis-Electrophile Couplings of Diborylmethane

A copper-catalyzed three-component linchpin coupling method for the stereoselective union of readily available epoxides and allyl electrophiles is disclosed. Transformations employ [B(pin)]₂-methane as a conjunctive reagent, resulting in the formation of two C-C bonds at a single carbon center bearing a C(sp³) organoboron functional group. Products are obtained in 42-99% yield, and up to >20:1 dr. The utility of the approach is highlighted by stereospecific transformations entailing allylation, tandem cross coupling, and application to the synthesis 1,3-polyol motifs.

Collective formal synthesis of (±)-rhynchophylline and homologues

A collective formal synthesis approach to six bioactive oxindole alkaloids, including (±)-rhynchophylline and its five homologues, is completed in a highly efficient way.

Total Synthesis of (-)-Secu'amamine A Exploiting Type II Anion Relay Chemistry

A total synthesis of (-)-secu'amamine A has been achieved exploiting Type II Anion Relay Chemistry (ARC) to provide the full linear carbon and nitrogen skeleton in a single flask with the requisite stereochemistry and functionality. A mechanistic rationale is also proposed to account for the stereochemical outcome of the key aldol reaction leading to the advanced aza tricyclic core.

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.

Cross-coupling of cyclopropanols: concise syntheses of indolizidine 223AB and congeners

A new synthetic method for indolizidine or pyrrolizidine alkaloids based on readily available and attractively functionalized cyclopropanols, as exemplified in concise syntheses of indolizidine (-)-223AB, its 3-epimer, (-)-indolizidine 239AB, and (-)-indolizidine 239CD, is reported. This work highlights the applications of SN2' alkylation and C-acylation of cyclopropanols to meet stereochemical challenges in natural product synthesis. Also included is diastereoselective cyclization of the resulting aminoallene adduct for bicyclic ring formation.

Through-bond/through-space anion relay chemistry exploiting vinylepoxides as bifunctional linchpins

The development of new bifunctional linchpins that permit the union of diverse building blocks is essential for the synthetic utility of anion relay chemistry (ARC). The design, synthesis, and validation of three vinylepoxide linchpins for through-bond/through-space ARC are now reported. For negative charge migration, this class of bifunctional linchpins employs initial through-bond ARC by an SN 2' reaction, followed by through-space ARC exploiting a 1,4-Brook rearrangement. The trans-disubstituted vinylepoxide linchpin yields a mixture of E/Z isomers, whereas the cis-disubstituted and the trans-trisubstituted vinylepoxide linchpins proceed to deliver three-component adducts with excellent E selectivity.

Diversity-oriented synthesis of 2,4,6-trisubstituted piperidines via type II anion relay chemistry

An effective, general protocol for the Diversity-Oriented Synthesis (DOS) of 2,4,6-trisubstituted piperidine congeners has been designed and validated. The successful strategy entails a modular approach to all possible stereoisomers of the selected piperidine scaffold, exploiting Type II Anion Relay Chemistry (ARC), followed in turn by intramolecular S(N)2 cyclization, chemoselective removal of the dithiane moieties and carbonyl reductions.

Enantioselective rhodium-catalyzed [2 + 2 + 2] cycloadditions of terminal alkynes and alkenyl isocyanates: mechanistic insights lead to a unified model that rationalizes product selectivity

This manuscript describes the development and scope of the asymmetric rhodium-catalyzed [2 + 2 + 2] cycloaddition of terminal alkynes and alkenyl isocyanates leading to the formation of indolizidine and quinolizidine scaffolds. The use of phosphoramidite ligands proved crucial for avoiding competitive terminal alkyne dimerization. Both aliphatic and aromatic terminal alkynes participate well, with product selectivity a function of both the steric and electronic character of the alkyne. Manipulation of the phosphoramidite ligand leads to tuning of enantio- and product selectivity, with a complete turnover in product selectivity seen with aliphatic alkynes when moving from Taddol-based to biphenol-based phosphoramidites. Terminal and 1,1-disubstituted olefins are tolerated with nearly equal efficacy. Examination of a series of competition experiments in combination with analysis of reaction outcome shed considerable light on the operative catalytic cycle. Through a detailed study of a series of X-ray structures of rhodium(cod)chloride/phosphoramidite complexes, we have formulated a mechanistic hypothesis that rationalizes the observed product selectivity.

(+)-Rimocidin synthetic studies: construction of the C(1-27) aglycone skeleton

Assembly of the C(1-27) macrocyclic skeleton of rimocidinolide, the aglycone of (+)-rimocidin (1), has been achieved in convergent fashion. Key features of the synthetic strategy entail application of multicomponent Type I Anion Relay Chemistry (ARC), in conjunction with the S(N)2/S(N)2' reaction manifolds of vinyl epoxides, both employing 2-substituted 1,3-dithianes to construct the C(1-19) carbon backbone. Yamaguchi union of a C(20-27) vinyl borate ester, possessing the all-trans triene, with an advanced C(1-19) vinyl iodide followed by macrocyclization via Suzuki-Miyaura cross-coupling completed construction of the C(1-27) rimocidinolide skeleton.

Evolution of multi-component anion relay chemistry (ARC): construction of architecturally complex natural and unnatural products

Efficient construction of architecturally complex natural and unnatural products is the hallmark of organic chemistry. Anion relay chemistry (ARC)-a multi-component coupling protocol-has the potential to provide the chemist with a powerful synthetic tactic, enabling efficient, rapid elaboration of structurally complex scaffolds in a single operation with precise stereochemical control. The ARC tactic can be subdivided into two main classes, comprising the relay of negative charge either through bonds or through space, the latter with aid of a transfer agent. This review will present the current state of through-space anion relay, in conjunction with examples of natural and unnatural product syntheses that illustrate the utility of this synthetic method.