Asymmetric Total Synthesis of Propindilactone G, Part 2: Enantioselective Construction of the Fully Functionalized BCDE Ring System

Enantioselective Total Synthesis of (+)-Heilonine

Chemical transformations that rapidly and efficiently construct a high level of molecular complexity in a single step are perhaps the most valuable in total synthesis. Among such transformations is the transition metal catalyzed [2 + 2 + 2] cycloisomerization reaction, which forges three new C-C bonds and one or more rings in a single synthetic operation. We report here a strategy that leverages this transformation to open de novo access to the Veratrum family of alkaloids. The highly convergent approach described herein includes (i) the enantioselective synthesis of a diyne fragment containing the steroidal A/B rings, (ii) the asymmetric synthesis of a propargyl-substituted piperidinone (F ring) unit, (iii) the high-yielding union of the above fragments, and (iv) the intramolecular [2 + 2 + 2] cycloisomerization reaction of the resulting carbon framework to construct in a single step the remaining three rings (C/D/E) of the hexacyclic cevanine skeleton. Efficient late-stage maneuvers culminated in the first total synthesis of heilonine (1), achieved in 21 steps starting from ethyl vinyl ketone.

Ten-Step Asymmetric Total Synthesis of (+)-Pepluanol A

The first asymmetric synthesis of pepluanol A (1) is presented. The synthesis route is very concise (10 steps) and features a Curtin-Hammett-driven stereoconvergent intramolecular Diels-Alder reaction. A Nozaki-Hiyama-Kishi reaction comprises the connective step, bringing together the seven-membered enone system bearing the dienophile and the diene in the side chain. Subsequent stereoconvergent IMDA reaction furnishes the carboskeleton of the natural product in only 7 steps. The reactions were carried out on a gram scale up to an advanced intermediate and including the stereoconvergent intramolecular Diels-Alder reaction.

New Developments of the Principle of Vinylogy as Applied to π-Extended Enolate-Type Donor Systems

The principle of vinylogy states that the electronic effects of a functional group in a molecule are possibly transmitted to a distal position through interposed conjugated multiple bonds. As an emblematic case, the nucleophilic character of a π-extended enolate-type chain system may be relayed from the legitimate α-site to the vinylogous γ, ε, ..., ω remote carbon sites along the chain, provided that suitable HOMO-raising strategies are adopted to transform the unsaturated pronucleophilic precursors into the reactive polyenolate species. On the other hand, when "unnatural" carbonyl ipso-sites are activated as nucleophiles (umpolung), vinylogation extends the nucleophilic character to "unnatural" β, δ, ... remote sites. Merging the principle of vinylogy with activation modalities and concepts such as iminium ion/enamine organocatalysis, NHC-organocatalysis, cooperative organo/metal catalysis, bifunctional organocatalysis, dicyanoalkylidene activation, and organocascade reactions represents an impressive step forward for all vinylogous transformations. This review article celebrates this evolutionary progress, by collecting, comparing, and critically describing the achievements made over the nine year period 2010-2018, in the generation of vinylogous enolate-type donor substrates and their use in chemical synthesis.

Recent progress in the synthesis of limonoids and limonoid-like natural products

Recent progress in syntheses of limonoids and limonoid-like natural products is reviewed. The current “state-of-art” advance on novel synthetic strategy are summarized and future outlook will be presented.

Total Synthesis of the Schisandraceae Nortriterpenoid Rubriflordilactone A

Full details of the total synthesis of the Schisandraceae nortriterpenoid natural product rubriflordilactone A are reported. Palladium- and cobalt-catalyzed polycyclizations were employed as key strategies to construct the central pentasubstituted arene from bromoendiyne and triyne precursors. This required the independent assembly of two AB ring aldehydes for combination with a common diyne component. A number of model systems were explored to investigate these two methodologies, and also to establish routes for the installation of the challenging benzopyran and butenolide rings.

Total syntheses of schilancidilactones A and B, schilancitrilactone A, and 20-epi-schilancitrilactone A via late-stage nickel-catalyzed cross coupling

The first total syntheses of schilancidilactones A and B, schilancitrilactone A, and 20-epi-schilancitrilactone A have been accomplished using a nickel-catalyzed cross coupling of alkyl bromide with vinyl stannane as the final step. The other key steps include late-stage C(sp3)-H bromination, the oxidative cleavage of a diol to provide the requisite ketone and ester for schilancidilactones A and B, and Dieckmann-type condensation to generate the A ring of schilancitrilactone A and 20-epi-schilancitrilactone A.