Silica gel enables Achmatowicz rearrangement with KBr/oxone under “anhydrous” condition for one-pot functionalization

Aza-Achmatowicz rearrangement coupled with intermolecular aza-Friedel-Crafts enables total syntheses of uleine and aspidosperma alkaloids

Aspidosperma and uleine alkaloids belong to the large family of monoterpene indole alkaloids with diverse biological activities and thus have attracted extensive synthetic interest. Reported is the development of a new synthetic strategy that allows direct C3-C2' linkage of indoles with functionalized 2-hydroxypiperidines to construct the core common to all aspidoserma and uleine alkaloids. Such indole-piperidine linkage is enabled by coupling aza-Achmatowicz rearrangement (AAR) with indoles via an intermolecular aza-Friedel-Crafts (iAFC) reaction. This AAR-iAFC reaction proceeds under mild acidic conditions with wide tolerance of functional groups (33 examples). The synthetic application of the AAR-iAFC method was demonstrated with collective total syntheses of 3 uleine-type and 6 aspidosperma alkaloids: (+)-3-epi-N-nor-dasycarpidone, (+)-3-epi-dasycarpidone, (+)-3-epi-uleine, 1,2-didehydropseudoaspidospermidine, 1,2-dehydroaspidospermidine, vincadifformine, winchinine B, aspidospermidine, and N-acetylaspidospermidine. We expect that this AAR-iAFC strategy is applicable to other monoterpene indole alkaloids with the C3-C2' linkage of indoles and piperidines.

BiBr3 -Mediated Intramolecular Aza-Prins Cyclization of Aza-Achmatowicz Rearrangement Products: Asymmetric Total Synthesis of Suaveoline and Sarpagine Alkaloids

An intramolecular aza-Prins cyclization of aza-Achmatowicz rearrangement products was developed in which bismuth tribromide (BiBr3 ) plays a dual role as an efficient Lewis acid and source of the bromide nucleophile. This approach enables the facile construction of highly functionalized 9-azabicyclo[3.3.1]nonanes (9-ABNs), which are valuable synthetic building blocks and a powerful platform for the synthesis of a variety of alkaloid natural products and drug molecules. Suitable substrates for the aza-Prins cyclization include 1,1-disubstituted alkenes, 1,2-disubstituted alkenes, alkynes, and allenes, with good to excellent yields observed. Finally, we showcase the application of this new approach to the enantioselective total synthesis of six indole alkaloids: (-)-suaveoline (1), (-)-norsuaveoline (2), (-)-macrophylline (3), (+)-normacusine B (4), (+)-Na -methyl-16-epipericyclivine (5) and (+)-affinisine (6) in a total of 9-14 steps. This study significantly expands the synthetic utility of the aza-Achmatowicz rearrangement, and the strategy (aza-Achmatowicz/aza-Prins) is expected to be applicable to the total synthesis of other members of the big family of macroline and sarpagine indole alkaloids.

Achmatowicz Rearrangement-Inspired Development of Green Chemistry, Organic Methodology, and Total Synthesis of Natural Products

The six-membered heterocycles containing oxygen and nitrogen (tetrahydropyrans, pyrans, piperidines) are among the most common heterocyclic structures ubiquitously present in bioactive molecules such as carbohydrates, small-molecule drugs, and natural products. Chemical synthesis of fully functionalized pyrans and piperidines is a research theme of practical importance and scientific significance and, thus, has attracted continuous interest from synthetic chemists. Among the numerous synthetic approaches, Achmatowicz rearrangement (AchR) represents a general and unique strategy that uses biomass-derived furfuryl alcohols as the renewable starting material to obtain fully functionalized six-membered oxygen/nitrogen heterocycles, which provides golden opportunities for organic chemists to address various synthetic challenges.This Account summarizes our 10 years of work on exploiting AchR to address some challenges in organic synthesis ranging from green chemistry and organic methodology to the total synthesis of natural products. We enabled the sustainable and safe use of AchR in a small (academia) or large (industrial) scale by developing two generations of green approaches for AchR (oxone-halide and Fenton-halide), which largely eliminate the use of the most popular, but more toxic and expansive, NBS and m-CPBA. This triggered our intensive interest in developing new green chemistry for important organic reactions, in particular, halogenation/oxidation reactions involving reactive halogenating species with the aim of eliminating the use of commonly used toxic halogen agents such as elemental bromine, chlorine gas, and various N-haloamide reagents (NBS, NCS, and NIS). We successfully employed oxone-halide and Fenton-halide as green alternatives to several mechanistically related organic reactions including arene/alkene halogenation, oxidation or oxidative rearrangement of indoles, oxidation of alcohols/thioacetals, and oxidative halogenation of aldoximes for the in situ generation of nitrile oxide. These green reactions are expected to have a solid impact on the future of organic synthesis in academia and industries.We expanded the synthetic utility of AchR by exploring several new transformations of AchR products and developed a cascade reductive ring expansion, reductive deoxygenation/Heck-Matsuda arylation, palladium-catalyzed C-arylation, and regiodivergent [3 + 2] cycloaddition with 1,3-dicarbonyls. These methodologies offer a new avenue to fully functionalized six-membered heterocycles.The synthetic utility of AchR was demonstrated in our total synthesis of 28 natural products with a pyran/piperidine moiety. The AchR-based strategy endows the total synthesis with scalability, sustainability, and flexibility. The green and scalable approaches developed in our lab for AchR allow us to easily obtain decagrams of synthetically valuable pyrans and/or piperidines with low risk and low cost from biomass-derived furfuryl alcohol/aldehyde.

1,5-Allyl Shift by a Sequential Achmatowicz/Oxonia-Cope/Retro-Achmatowicz Rearrangement

1,3-Allyl and 1,2-allyl shifts through [3,3]- and [2,3]-sigmatropic rearrangements are well-established and widely used in organic synthesis. In contrast, 1,5-allyl shift through related [3,5]-sigmatropic rearrangement is unknown because [3,5]-sigmatropic rearrangement is thermally Woodward-Hoffmann forbidden. Herein, we report an unexpected discovery of a formal 1,5-allyl shift of allyl furfuryl alcohol through a 2-step sequential rearrangement. Mechanistically, this formal 1,5-allyl shift is achieved through a sequential ring expansion/contraction rearrangement: 1) Achmatowicz rearrangement (ring expansion), and 2) cascade oxonia-Cope rearrangement/retro-Achmatowicz rearrangement (ring contraction). This new 1,5-allyl shift method is demonstrated with >20 examples and expected to find applications in organic synthesis and materials chemistry.

Synthetic Studies toward the Berkeleyacetal Core Architecture

Berkeleyacetals are structurally complex natural products that have shown potent anti-inflammatory activity. The presence of a highly dense oxygen functionality and a polycyclic ring system presents significant synthetic challenges. Herein, we report an efficient strategy for the construction of the tetracyclic core system of berkeleyacetal. Our synthetic strategy features two cycloadditions ([4+2] and [5+2]) to forge the tetracyclic core and Achmatowicz rearrangement for the preparation of the cyclization substrates containing B and E rings.

Collective Asymmetric Total Syntheses of Marine Decahydroquinoline Alkaloid Lepadins A-E, H, and ent-I

Lepadins are cis-fused decahydroquinoline (DHQ) marine alkaloids that have shown diverse biological activities and have attracted extensive synthetic interest. A new collective synthetic strategy is reported that features a green chemistry approach for constructing the common cis-fused DHQ core, which is achieved through green oxone-halide oxidation for both the aza-Achmatowicz rearrangement and the intramolecular [3 + 2] cycloaddition of nitrile oxide-alkene. Collective total syntheses of lepadins A-E and H are accomplished from the common DHQ core within 10 steps.

Asymmetric Total Syntheses of (+)-Penostatins A and C

Penostatins A and C are cytotoxic natural products that show promising selective inhibitory activity against PTP1B. Here the first asymmetric total syntheses of (+)-penostatins A and C are reported. Our strategy features (i) a new method for the synthesis of 6-alkyl-3-hydroxy-2-pyrones, (ii) a cascade involving the intramolecular Diels-Alder reaction of 2-pyrone and a retro-hetero-Diels-Alder (decarboxylation) reaction, (iii) Ando-Horner-Wadsworth-Emmons olefination/lactonization, and (iv) selenoxide elimination. Our study confirmed the absolute configurations of penostatins A and C and laid the groundwork for further bioactivity studies.

C-Aryl Glycosylation: Palladium-Catalyzed Aryl-Allyl Coupling of Achmatowicz Rearrangement Products with Arylboronic Acids

The first Pd-catalyzed arylation of Achmatowicz rearrangement products with arylboronic acids under mild conditions (rt) to provide the synthetically versatile C-aryl dihydropyranones is reported. It is found that the 4-keto group of Achmatowicz products is essential to increase the reactivity of the Pd-π-allyl complex toward arylboronic acids and that phosphine as the palladium ligand would be destructive to the reaction. This new coupling method addresses the major limitations of previous Pd-catalyzed allyl-aryl couplings of 2,3-unsaturated glycosides with an aryl Grignard or aryl zinc reagent.

Synthesis of 2-Arylpiperidines via Pd-Catalyzed Arylation of Aza-Achmatowicz Rearrangement Products with Arylboronic Acids

The first Pd-catalyzed arylation of aza-Achmatowicz rearrangement products with arylboronic acids is achieved, providing versatile 2-aryldihydropyridinones for facile synthesis of highly functionalized 2-arylpiperidines. Key to this arylation is the use of non-phosphine-ligand palladium precatalyst. The substrate scope is demonstrated with >26 examples, and the utility of 2-aryldihydropyridinones is illustrated by the synthesis of a small collection of 2-arylpiperidines with substituents or functional groups at any carbon (C2-C6) as well as two NK₁ receptor antagonists (+)-CP-999,94 and (+)-L-733,060.