N,N-Acetals as N-Acyliminium Ion Precursors: Synthesis and Absolute Stereochemistry of Epiquinamide

AgNTf2 catalyzed cycloaddition of N-acyliminium ions with alkynes for the synthesis of the 3,4-dihydro-1,3-oxazin-2-one skeleton

A catalyzed process for the synthesis of the 4,6-substituted 3,4-dihydro-1,3-oxazin-2-one skeleton has been developed through cycloaddition of in situ generated acyliminium intermediates with alkynes. A range of chain N,O-acetals and terminal alkynes were amenable for this mild transformation. As a result, a series of desired cycloaddition products were obtained in moderate to good yields.

Stereodivergent routes in organic synthesis: carbohydrates, amino acids, alkaloids and terpenes

The natural occurrence of enantiomers and diastereomers is often encountered.

Chemical synthesis of febrifugine and analogues

The quinazolinone-containing 2,3-disubstituted piperidines febrifugine and isofebrifugine have been the subject of significant research efforts since their occurrence in Dichroa febrifuga and their anti-malarial actions were first described in the late 1940s. Subsequently they have also been shown to be present in other plants belonging to the hydrangea family and various analogues of febrifugine have been prepared in attempts to tune biological properties. The most notable analogue is termed halofuginone and a substantial body of work now demonstrates that this compound possesses potent human disease relevant activities. This review focuses on the literature associated with efforts dedicated towards uncovering the structures of febrifugine and isofebrifugine, the development of practical methods for their synthesis and the syntheses of structural analogues.

Photoredox Approach to N-Acyl-N'-aryl-N,N'-aminals Using Enamides and Their Conversion to γ-Lactams

A photoredox catalytic approach to synthetically valuable N-acyl-N'-aryl-N,N'-aminals is described. This method uses the addition of a radical precursor to enamides, with subsequent interception of the cationic iminium intermediate with an arylamine. The reaction has been shown to be compatible with electron-rich and electron-deficient arylamines, and moderate to good levels of diastereoselectivity can be attained using a chiral enamide. Furthermore, the N-acyl-N'-aryl-N,N'-aminal reaction products can be readily cyclized, providing a novel synthetic route to valuable γ-lactams.

Enantioselective synthesis of cyanohydrins catalysed by hydroxynitrile lyases - a review

The first enantioselective synthesis was the selective addition of cyanide to benzaldehyde catalysed by a hydroxynitrile lyase (HNL). Since then these enzymes have been developed into a reliable tool in organic synthesis. HNLs to prepare either the (R)- or the (S)-enantiomer of the desired cyanohydrin are available and a wide variety of reaction conditions can be applied. As a result of this, numerous applications of these enzymes in organic synthesis have been described. Here the examples of the last decade are summarised, the enzyme catalysed step is discussed and the follow-up chemistry is shown. This proves HNLs to be part of main stream organic synthesis. Additionally the newest approaches via immobilisation and reaction engineering are introduced.

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.

Total synthesis of gonytolides C and G, lachnone C, and formal synthesis of blennolide C and diversonol

The total syntheses of gonytolide C and related compounds have been accomplished from the aldol reaction between acetophenone derived from orcinol and butyrolactone containing ketone, followed by the diastereoselective intramolecular cyclization.

Enantioselective chemoenzymatic synthesis of trans-aziridines

A straightforward, five-step procedure for the synthesis of enantiomerically pure 2,3-disubstituted trans-aziridines has been developed starting from commercially available aldehydes. Hydroxynitrile lyase-mediated cyanohydrin formation provided cyanohydrins in excellent enantioselectivities and good yields. Subsequent formation of diastereomerically pure anti-amino alcohols via a one-pot Grignard addition-reduction sequence, Cu(II)-catalyzed diazotransfer, and triphenylphosphine-mediated reductive cyclization provided the corresponding trans-aziridines in good yields and excellent diastereoselectivities.

Epiquinamide: a poison that wasn't from a frog that was

In 2003, we reported the isolation, structure elucidation, and pharmacology of epiquinamide (1), a novel alkaloid isolated from an Ecuadoran poison frog, Epipedobates tricolor. Since then, several groups, including ours, have undertaken synthetic efforts to produce this compound, which appeared initially to be a novel, beta2-selective nicotinic acetylcholine receptor agonist. Based on prior chiral GC analysis of synthetic and natural samples, the absolute structure of this alkaloid was established as (1S,9aS)-1-acetamidoquinolizidine. We have synthesized the (1R*,9aS*)-isomer (epi-epiquinamide) using an iminium ion nitroaldol reaction as the key step. We have also synthesized ent-1 semisynthetically from (-)-lupinine. Synthetic epiquinamide is inactive at nicotinic receptors, in accord with recently published reports. We have determined that the activity initially reported is due to cross-contamination from co-occurring epibatidine in the isolated material.