Synthesis of the Lycopodium Alkaloid (+)-Lycoflexine

Total synthesis of (+)-sieboldine a: evolution of a pinacol-terminated cyclization strategy

This article describes synthetic studies that culminated in the first total synthesis of the Lycopodium alkaloid sieboldine A. During this study, a number of pinacol-terminated cationic cyclizations were examined to form the cis-hydrindanone core of sieboldine A. Of these, a mild Au(I)-promoted 1,6-enyne cyclization that was terminated by a semipinacol rearrangement proved to be most efficient. Fashioning the unprecedented N-hydroxyazacyclononane ring embedded within the bicyclo[5.2.1]decane-N,O-acetal moiety of sieboldine A was a formidable challenge. Ultimately, the enantioselective total synthesis of (+)-sieboldine A was completed by forming this ring in good yield by cyclization of a protected-hydroxylamine thioglycoside precursor.

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.

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.

Unified total syntheses of fawcettimine class alkaloids: fawcettimine, fawcettidine, lycoflexine, and lycoposerramine B

The total syntheses of the lycopodium alkaloids fawcettimine, fawcettidine, lycoflexine, and lycoposerramine B have been accomplished through an efficient, unified, and stereocontrolled strategy that relies on a Diels-Alder reaction to construct the cis-fused 6,5-carbocycles with one all-carbon quaternary center. Access to the enantioselective syntheses of both antipodes of those alkaloids can be achieved by kinetic resolution of the earliest intermediate via a Sharpless asymmetric dihydroxylation (Sharpless AD). Compared to existing approaches to these alkaloids, our synthetic route possesses superior stereocontrol over the C-4 and C-15 stereogenic centers as well as allowing for more functional variation on the 6-membered ring.

High-yielding oxidation of β-hydroxyketones to β-diketones using o-iodoxybenzoic acid

The oxidation of β-hydroxyketones to β-diketones was systematically investigated. o-Iodoxybenzoic acid (IBX) was found to be efficient, operationally easy, and superior to other common oxidants. The reaction is suitable for milligram- to gram-scale oxidations.

Efficient and scalable one-pot synthesis of 2,4-dienols from cycloalkenones: optimized total synthesis of valerenic acid

A mild and selective one-pot procedure to provide 2,4-dienols from simple cycloalkenones in high yields is described. This transformation is based on the in situ formation of acid-labile allylic alcohols, which on treatment with trifluoroacetic acid undergo a formal [1,3]-hydroxy migration to form diastereo- and enantiomerically enriched 2,4-dienols. The usefulness of this protocol is demonstrated in a short synthesis of valerenic acid.

A simple recipe for sophisticated cocktails: organocatalytic one-pot reactions--concept, nomenclature, and future perspectives

Asymmetric organocatalysis has been successfully incorporated in many multistep one-pot sequences to provide simple access to structurally complex target molecules in a highly stereoselective fashion. The key feature behind this success is the ability of organocatalyzed reactions to proceed efficiently in the presence of large amounts of spectator reagents. Additionally, owing to their organic nature and substoichiometric presence, organocatalysts are also expected to become innocent bystanders in subsequent transformations. In this Minireview, an easy-to-use classification and nomenclatural system that is capable of systematically and informatively describing each one-pot reaction is introduced, and selected important contributions within the field of organocatalytic one-pot reactions are reviewed according to this new system. Finally, future developments and perspectives in the field are discussed.

Asymmetric total synthesis of a pentacyclic Lycopodium alkaloid: huperzine-Q

Right on Q: the first asymmetric total synthesis of (-)-huperzine-Q, which possesses six stereogenic centers and a spiroaminal moiety, has been achieved in 19 steps and 16.4 % overall yield. This synthesis involved a novel stereoselective Pauson-Khand reaction, a vinyl Claisen rearrangement, and a biomimetic spiroaminal formation. TBDPS=tert-butyldiphenylsilyl.

Metathesis in total synthesis

This short account discusses the awesome power of metathesis which has profoundly changed the practice of natural product synthesis during the last decade. Since a comprehensive coverage is beyond the scope of this highlight article, I shall focus on the conceptual framework and on significant advances since the turn of the century.

Lycopodium alkaloids: isolation and asymmetric synthesis

Lycopodium alkaloids have attracted the attention of many natural product chemists and synthetic organic chemists due to their important biological activities and unique skeletal characteristics. In this review we describe isolation and asymmetric syntheses of several new alkaloids such as lycoposerramines-C, -V, -W, and cernuine, and show that asymmetric total synthesis played a key role in elucidating the structures of these complex natural products.