The synthesis of compounds related to the indole–indoline core of the vinca alkaloids (+)-vinblastine and (+)-vincristine

Total Synthesis of Gastrodinol via Photocatalytic 6π Electrocyclization

The first total synthesis of gastrodinol, an unprecedented poly-p-cresol-substituted natural product with a rearranged and reconstructed C ring moiety, is reported. Our synthesis features a convergent fragment approach. The Sonogashira coupling reaction forges the two segments together to furnish the conjugated ene-yne. Photocatalytic 6π electrocyclization followed by spontaneous aromatization is used to construct the tetrasubstituted B ring at the late stage. Further study shows that gastrodinol exhibits significant cytotoxic activity against five human cancer cell lines in vitro (IC₅₀ 2.5-3.8 μM).

Zn(II)-Catalyzed Addition of Aromatic/Heteroaromatic C(sp2)-H to Azoalkenes: A Polarity-Reversed Arylation of Carbonyl Compounds

An umpolung α-(hetero)arylation strategy that involves the Michael-type reaction between electron-rich (hetero)aromatic substrates and azoalkenes is developed. The reaction proceeds under very mild conditions at room temperature and in the presence of inexpensive, nontoxic ZnCl₂ catalyst to provide access to otherwise inaccessible hydrazone structures. Subsequent hydrolysis of these latter to ketones as well as other valuable synthetic transformations to a variety of heterocyclic scaffolds demonstrate the usefulness of this protocol.

Enantioselective Formal [4+1] Cycloaddition of Diazoarylacetates and the Danishefsky's Diene: Stereoselective Synthesis of (-)-1,13-Herbertenediol

Rodium chiral diene complex-catalyzed enantioselective cycloaddition of aryl α-diazoarylacetates and electron-enriched Danishefsky-type dienes afforded highly functionalized and optically enriched cyclopentenones in excellent yields (up to 97% yield) and with good to excellent enantioselectivities (60-92% ee). (-)-1,13-Herbertenediol was successfully synthesized in an overall 25% yield employing the optically enriched cyclopentenone with an all-carbon quaternary center as the key intermediate.

Reductive Cyclization of o-Nitroarylated-α,β-unsaturated Aldehydes and Ketones with TiCl3/HCl or Fe/HCl Leading to 1,2,3,9-Tetrahydro-4 H-carbazol-4-ones and Related Heterocycles

Compounds such as 3, the product of a palladium[0]-catalyzed Ullmann cross-coupling of o-iodonitrobenzene and 2-iodocyclohex-2-en-1-one, undergo complementary modes of reductive cyclization depending upon the conditions employed. Thus, on treatment with hydrogen in the presence of palladium on carbon, the tetrahydrocarbazole 4 is formed, while reaction of the same substrate (3) with TiCl₃ in acetone affords the 1,2,3,9-tetrahydro-4 H-carbazol-4-one 6.

The Palladium-Catalyzed Ullmann Cross-Coupling Reaction: A Modern Variant on a Time-Honored Process

Cross-coupling reactions, especially those that are catalyzed by palladium, have revolutionized the way in which carbon-carbon bonds can be formed. The most commonly deployed variants of such processes are the Suzuki-Miyaura, Mizoroki-Heck, Stille, and Negishi cross-coupling reactions, and these normally involve the linking of an organohalide or pseudohalide (such as a triflate or nonaflate) with an organo-metallic or -metalloid such as an organo-boron, -magnesium, -tin, or -zinc species. Since the latter type of coupling partner is often prepared from the corresponding halide, methods that allow for the direct cross-coupling of two distinct halogen-containing compounds would provide valuable and more atom-economical capacities for the formation of carbon-carbon bonds. While the venerable Ullmann reaction can in principle achieve this, it has a number of drawbacks, the most significant of which is that homocoupling of the reaction partners is a competitive, if not the dominant, process. Furthermore, such reactions normally occur only under forcing conditions (viz., often at temperatures in excess of 250 °C). As such, the Ullmann reaction has seen only limited application in this regard, especially as a mid- to late-stage feature of complex natural product synthesis. This Account details the development of the palladium-catalyzed Ullmann cross-coupling reaction as a useful method for the assembly of a range of heterocyclic systems relevant to medicinal and/or natural products chemistry. These couplings normally proceed under relatively mild conditions (<100 °C) over short periods of time and, usually, to the exclusion of (unwanted) homocoupling events. The keys to success are the appropriate choice of coupling partners, the form of the copper metal employed, and the choice of reaction solvent. At the present time, the cross-coupling partners capable of engaging in the title reaction are confined to halogenated and otherwise electron-deficient arenes and, as complementary reactants, α- or β-halogenated, α,β-unsaturated aldehydes, ketones, esters, lactones, lactams, and cycloimides. Nitro-substituted (and halogenated) arenes, in particular, serve as effective participants in these reactions, and the products of their coupling with the above-mentioned carbonyl-containing systems can be manipulated in a number of different ways. Depending on the positional relationship between the nitro and carbonyl groups in the cross-coupling product, the reduction of the former group, which can be achieved under a range of different conditions, provides, through intramolecular nucleophilic addition reactions, including Schiff base condensations, access to a diverse range of heterocyclic systems. These include indoles, quinolines, quinolones, isoquinolines, carbazoles, and carbolines. Tandem variants of such cyclization processes, in which Raney cobalt is used as a catalyst for the chemoselective reduction (by dihydrogen) of nitro and nitrile groups (but not olefins), allow for the assembly of a range of structurally challenging natural products, including marinoquinoline A, (±)-1-acetylaspidoalbidine, and (±)-gilbertine.

Conversion of the Enzymatically Derived (1S,2S)-3-Bromocyclohexa-3,5-diene-1,2-diol into Enantiomerically Pure Compounds Embodying the Pentacyclic Framework of Vindoline

The enzymatically derived and enantiomerically pure (1S,2S)-3-bromocyclohexa-3,5-diene-1,2-diol (7) has been elaborated over 17 steps into compounds 8 and 32, each of which embodies the pentacyclic framework and much of the functionality associated with the alkaloid vindoline (3). This work sets the stage for effecting the conversion of the related metabolite (1S,6R)-5-ethyl-1,6-dihydroxycyclohexa-2,4-diene-1-carboxylic acid (4) into compound 3, the latter being a biogenetic precursor to the clinically significant anticancer agents vinblastine and vincristine.

Hypervalent iodine(III)-promoted intermolecular C-C coupling of vindoline with β-ketoesters and related substrates

The regioselective intermolecular coupling reaction of vindoline with a wide range of substrates including β-ketoesters, β-diketones, β-ketoaldehydes, β-ketonitriles, malononitriles, and β-cyanoesters provides an opportunity for the synthesis of vinblastine analogues containing deep-seated changes in the upper velbanamine subunit. The transition-metal-free hypervalent iodine(III)-promoted intermolecular sp(3)/sp(2) coupling, representing a special class of selective C-H activation with direct carbon-carbon bond formation, proceeds with generation of a quaternary center capable of incorporation of the vinblastine C16' methyl ester and functionalized for subsequent divergent heterocycle introduction.