Concise formal synthesis of (-)-peduncularine via ring-closing metathesis

Asymmetric total synthesis strategies of halichlorine and pinnaic acid

Halichlorine and pinnaic acid are structurally related natural alkaloids isolated from different marine organisms. These two marine alkaloids bearing a 6-azaspiro[4.5]decane skeleton demonstrate a wide range of biological effects. It is this kind of unique structure and potentially valuable biological activity that have prompted strong synthetic interest, making it a research focus in recent years. Since the first total synthesis of halichlorine and pinnaic acid completed by Danishefsky's group, many groups have reported their outstanding synthesis methods especially the asymmetric synthesis strategies. This review summarizes the asymmetric synthesis strategies of halichlorine and pinnaic acid using a 6-azaspiro[4.5]decane skeleton as the key intermediate, which can provide some guidance for related work.

Syntheses of Aristotelia Alkaloids: Reflections in the Chiral Pool

The Aristotelia alkaloids are a family of monoterpene indole alkaloids possessing a characteristic azabicyclononane scaffold, which has been assembled by several synthetic methods. Herein we review those approaches that have adopted a biomimetic approach to unite heterocyclic synthons with chiral pool monoterpenes. Throughout this discussion, the tendency of monoterpenes like α-pinene and limonene to undergo racemization is highlighted, revealing the challenges in developing stereospecific syntheses of these alkaloids. Finally, we provide a brief discussion of how these synthetic efforts have enabled the structural confirmation and elucidation of the Aristotelia alkaloids’ absolute configurations, including our own recent efforts to employ bioactivity data to deduce the naturally occurring configuration of the quinoline alkaloid aristoquinoline.

Natural Products and Their Mimics as Targets of Opportunity for Discovery

Diverse structural types of natural products and their mimics have served as targets of opportunity in our laboratory to inspire the discovery and development of new methods and strategies to assemble polyfunctional and polycyclic molecular architectures. Furthermore, our efforts toward identifying novel compounds having useful biological properties led to the creation of new targets, many of which posed synthetic challenges that required the invention of new methodology. In this Perspective, selected examples of how we have exploited a diverse range of natural products and their mimics to create, explore, and solve a variety of problems in chemistry and biology will be discussed. The journey was not without its twists and turns, but the unexpected often led to new revelations and insights. Indeed, in our recent excursion into applications of synthetic organic chemistry to neuroscience, avoiding the more-traveled paths was richly rewarding.

Applications of Ring Closing Metathesis. Total Synthesis of (±)-Pseudotabersonine

A novel approach to the Aspidosperma family of alkaloids was developed and applied to a concise total synthesis of (±)-pseudotabersonine that was accomplished in 11 steps. Key transformations include a stepwise variant of a Mannich-like multicomponent assembly process, a double ring-closing metathesis sequence, and a one-pot deprotection/cyclization reaction.

Carbamoyl radical-mediated synthesis and semipinacol rearrangement of β-lactam diols

In an approach to the biologically important 6-azabicyclo[3.2.1]octane ring system, the scope of the tandem 4-exo-trig carbamoyl radical cyclization-dithiocarbamate group transfer reaction to ring-fused β-lactams is evaluated. β-Lactams fused to five-, six-, and seven-membered rings are prepared in good to excellent yield, and with moderate to complete control at the newly formed dithiocarbamate stereocentre. No cyclization is observed with an additional methyl substituent on the terminus of the double bond. Elimination of the dithiocarbamate group gives α,β- or β,γ-unsaturated lactams depending on both the methodology employed (base-mediated or thermal) and the nature of the carbocycle fused to the β-lactam. Fused β-lactam diols, obtained from catalytic OsO4-mediated dihydroxylation of α,β-unsaturated β-lactams, undergo semipinacol rearrangement via the corresponding cyclic sulfite or phosphorane to give keto-bridged bicyclic amides by exclusive N-acyl group migration. A monocyclic β-lactam diol undergoes Appel reaction at a primary alcohol in preference to semipinacol rearrangement. Preliminary investigations into the chemo- and stereoselective manipulation of the two carbonyl groups present in a representative 7,8-dioxo-6-azabicyclo[3.2.1]octane rearrangement product are also reported.

Modular enantioselective synthesis of 8-aza-prostaglandin E1

We report herein for the first time the enantioselective synthesis of 8-aza-PGE1. The synthesis used the cross olefin metathesis reaction to connect the 5-vinyl-γ-lactam subunit, prepared from (R)-malic acid via the Ley's sulfone-based α-amidalkylation protocol (dr = 6.8:1), with the chiral pre-ω-chain. The latter was synthesized in high enantioselectivity from (E)-2-octenol by the Sharpless asymmetric epoxidation and the titanocene-mediated epoxide opening. This modular approach is quite concise and flexible, and requires only eight steps from commercially available reagents.

Facile Syntheses of Substituted, Conformationally-Constrained Benzoxazocines and Benzazocines via Sequential Multicomponent Assembly and Cyclization

A multicomponent assembly process (MCAP) was utilized to prepare versatile intermediates that are suitably functionalized for subsequent cyclizations via Ullmann and Heck reactions to efficiently construct substituted 2,6-methanobenzo[b][1,5]oxazocines and 1,6-methanobenzo[c]azocines, respectively. The intramolecular Ullmann cyclization was conducted in tandem with an intermolecular arylation that enabled the rapid syntheses of a number of O-functionalized methanobenzoxazocines.

Novel approach to the lundurine alkaloids: synthesis of the tetracyclic core

The tetracyclic core of the lundurine family of alkaloids has been synthesized by a novel approach that features a double ring-closing olefin metathesis to form the five-and eight-membered rings.

Concise total synthesis of (+/-)-pseudotabersonine via double ring-closing metathesis strategy

A concise synthesis of (+/-)-pseudotabersonine from commercially available 1-(phenylsulfonyl)-3-indolecarboxaldehyde has been accomplished. This synthesis features the convergent assembly of a key intermediate via a stepwise variant of a Mannich-type multicomponent coupling process, a double ring-closing metathesis, and a one-pot deprotection/cyclization reaction.

Enantioselective synthesis of (+)-isolysergol via ring-closing metathesis

The first enantioselective synthesis of (+)-isolysergol was completed in 12 steps from commercially available materials by a novel approach that features a late stage microwave-mediated, diastereomeric ring-closing metathesis catalyzed by a chiral molybdenum catalyst to simultaneously form the D ring and set the stereocenter at C(8).

Recent Applications of Imines as Key Intermediates in the Synthesis of Alkaloids and Novel Nitrogen Heterocycles

One of the major challenges in contemporary synthetic organic chemistry is the design and development of new tactics and strategies and their application to concise and efficient syntheses of biologically active natural products. Strategies that utilize reactions that enable the rapid assembly of the skeletal framework of such targets are thus especially attractive. In this context, we have developed novel applications of imine chemistry in Mannich and related reactions, cascade processes, and multicomponent reactions to rapidly assemble structural subunits common to diverse families of alkaloids. The practical utility of these chemistries is evidenced by their use in the execution of facile total syntheses of (±)-epilupinine (1), (±)-tashiromine (2), (-)-epimyrtine (3), and (±)-roelactamine (4) as well as other nitrogen heterocycles of potential biological interest.

The Pauson-Khand Reaction as a New Entry to the Synthesis of Bridged Bicyclic Heterocycles: Application to the Enantioselective Total Synthesis of (-)-Alstonerine

The first application of the Pauson-Khand reaction (PKR) to the synthesis of azabridged bicyclic structures is described. Compounds containing azabicyclo[3.3.1]nonane and azabicyclo[3.2.1]octane rings fused to cyclopentenones were efficiently constructed via the PKR of cis-2,6-disubstituted N-acyl piperidine enyne substrates, many of which can be readily prepared from 4-methoxypyridine in a few steps. Moreover, the PKR of cis-2,6-disubstituted piperazine enynes allowed the preparation of diazabicyclo[3.3.1]nonanes fused to cyclopentenones. This new strategy for the synthesis of azabridged bicyclic frameworks was exploited as a key step in a concise, enantioselective total synthesis of the macroline alklaoid (-)-alstonerine.

Synthesis of Functionalized Isoindolinones: Addition of In Situ Generated Organoalanes to Acyliminium Ions

Addition of in situ generated di- or trisubstituted alkenylalanes to N-acyliminium ions provides rapid access to functionalized isoindolinones. Subsequent ring closing metathesis leads to tricyclic products. These transformations proceed under mild conditions and allow for the convergent synthesis of biologically significant scaffolds from readily available starting materials.

Toward the total synthesis of FR901483: concise synthesis of the azatricyclic skeleton

A concise synthesis of the azatricyclic core of FR901483 has been accomplished using a novel strategy that involves a nucleophilic addition to an N-acyl iminium ion, a ring-closing metathesis, a diastereoselective hydroboration, and a lactone-lactam rearrangement that worked well in a preliminary model study. Extension of this approach to the synthesis of a more highly functionalized intermediate that could be transformed into (-)-FR901483 first required the development of a new protecting group, the 1-ethylallyloxycarbamate group, for amines that may be removed under mild conditions. However, because the stereoselectivity in a key step in which a functionalized allyl zinc reagent was added to an intermediate hydroxy-substituted imine was low, this route to (-)-FR901483 is no longer being pursued.

Concise, enantioselective total synthesis of (-)-alstonerine

A novel enantioselective total synthesis of (-)-alstonerine has been completed that requires only 15 steps from L-tryptophan. The synthesis features the first application of a Pauson-Khand reaction to synthesize an azabridged bicyclic skeleton. [reaction: see text]

Formal syntheses of (+/-)-pinnaic acid and (+/-)-halichlorine

[chemical reaction: see text]. Concise formal syntheses of marine alkaloids (+/-)-pinnaic acid (1) and (+/-)-halichlorine (2) have been accomplished from a common intermediate. The syntheses illustrate the utility of selective olefin cross metathesis methodologies for the elaboration of advanced synthetic intermediates in complex molecule synthesis.

Simple indole alkaloids and those with a non-rearranged monoterpenoid unit

This review covers newly isolated simple indole alkaloids, structure determinations, total syntheses and biological activities reported in the literature in 2003.