A Cascade Synthesis of Indoles

We report herein an expedient method for the regioselective synthesis of indoles from o-haloanilines and α-ketol-derived N-tosylhydrazones. This two-step, modular synthesis of N-H indoles can be carried out conveniently without purification of intermediates.

Nonbiomimetic total synthesis of indole alkaloids using alkyne-based strategies

Biomimetic natural product synthesis is generally straightforward and efficient because of its established feasibility in nature and utility in comprehensive synthesis, and the cost-effectiveness of naturally derived starting materials. On the other hand, nonbiomimetic strategies can be an important option in natural product synthesis since (1) nonbiomimetic synthesis offers more flexibility and can demonstrate the originality of chemists, and (2) the structures of derivatives accessible by nonbiomimetic synthesis can be considerably different from those that are synthesised in nature. This review summarises nonbiomimetic total syntheses of indole alkaloids using alkyne chemistry for constructing core structures, including ergot alkaloids, monoterpene indole alkaloids (mainly corynanthe, aspidosperma, strychnos, and akuammiline), and pyrroloindole and related alkaloids. To clarify the differences between alkyne-based strategies and biosynthesis, the alkynes in nature and the biosyntheses of indole alkaloids are also outlined.

Piloty's acid and its hydrazide analogue: Insights from the density functional theory and vibrational spectroscopy on the conformational stability and chemical reactivity

N-hydroxybenzenesulfonamide (commonly known as Piloty's acid) is considered a major source for nitroxyl (HNO) species which has potential biological and medicinal applications. In the present study, the conformational preferences and chemical reactivity of Piloty's acid (PA) and its hydrazide analogue (benzenesulfonylhydrazide, BSH) were studied using spectroscopic and computational tools. Six stable conformations of each molecule were theoretically identified, and their structures were fully optimized at the DFT-B3LYP and MP2 levels. Both molecules in their most stable forms adopt the anti configuration with the NH bond of the secondary amine pointing away from the terminal hydroxyl and amine moieties in the acid and hydrazide molecules, respectively. Three stable gauche states facilitated by weak intramolecular interactions of the SO⋯HO and SO⋯HN types arise due to the internal rotation about the SN linkage. Reliable assignments of the vibrational modes and the calculated reaction coordinates support a two-step mechanistic pathway of the Piloty's acid dissociation leading to the production of the nitroxyl (HNO) intermediate with moderate transition state barriers. Frontier molecular orbitals distributions, molecular electrostatic potential maps and condensed Fukui functions analysis of the molecules were employed to elucidate the agility of PA to dissociate to produce HNO and the absence of such a dissociation of BSH that would produce diazene (N₂H₂).

Synthesis of Illudinine from Dimedone and Identification of Activity as a Monoamine Oxidase Inhibitor

The fungal metabolite illudinine is prepared in seven steps and ca. 55% overall yield from dimedone using an "open and shut" (ring-opening and ring-closing) strategy. Tandem ring-opening fragmentation and olefination of dimedone establishes alkyne and vinylarene functionality linked by a neopentylene tether. Oxidative cycloisomerization then provides the illudinine framework. The key innovation in this second-generation synthesis of illudinine is the use of the nitrile functional group, rather than an ester, as the functional precursor to the carboxylic acid of illudinine. The small, linear nitrile (C≡N) is associated with improved selectivity, π-conjugation, and reactivity at multiple points in the synthetic sequence relative to the carboxylic acid ester. Preliminary assays indicate that illudinine and several related synthetic analogues are monoamine oxidase inhibitors, which is the first reported indication of biological activity associated with this natural product. Illudinine was found to inhibit monoamine oxidase B (MAO-B) with an IC₅₀ of 18 ± 7.1 μM in preliminary assays.

C-C fragmentation: origins and recent applications

It has been 60 years since Eschenmoser and Frey disclosed the archetypal CC fragmentation reaction. New fragmentations and several variants of the original quickly followed. Many of these variations, which include the Beckmann, Grob, Wharton, Marshall, and Eschenmoser-Tanabe fragmentations, have been reviewed over the intervening years. A close examination of the origins of fragmentation has not been described. Recently, useful new methods have flourished, particularly fragmentations that give alkynes and allenes, and such reactions have been applied to a range of complex motifs and natural products. This Review traces the origins of fragmentation reactions and provides a summary of the methods, applications, and new insights of heterolytic CC fragmentation reactions advanced over the last 20 years.

Direct access to tetrahydro[1,2]diazepinones from α,β-epoxy-N-aziridinylimines via anionic rearrangement

A novel one-step synthetic approach to tetrahydro[1,2]diazepinones via base-promoted rearrangement of α,β-epoxy-N-aziridinylimines, derived from α,β-epoxyketones and N-aminoaziridines, has been developed.

A ring fragmentation approach to medium-sized cyclic 2-alkynones

Bicyclic γ-silyloxy-β-hydroxy-α-diazoketones, in which the Cβ-Cγ bond is the ring fusion bond, productively fragment when treated with tin(IV) chloride to provide medium-sized cyclic 2-alkynones. This method provides good to excellent yields of 10-, 11-, and 12-membered alkynone products.

Fluorine conformational effects in organocatalysis: an emerging strategy for molecular design

Molecular design strategies that profit from the intrinsic stereoelectronic and electrostatic effects of fluorinated organic molecules have mainly been restricted to bio-organic chemistry. Indeed, many fluorine conformational effects remain academic curiosities with no immediate application. However, the renaissance of organocatalysis offers the possibility to exploit many of these well-described phenomena for molecular preorganization. In this minireview, we highlight examples of catalyst refinement by introduction of an aliphatic C-F bond which functions as a chemically inert steering group for conformational control.

α,β-Unsaturated and cyclopropyl acyl radicals, and their ketene alkyl radical equivalents. Ring synthesis and tandem cyclisation reactions

Treatment of the alpha,beta-unsaturated selenyl esters 12 and 14 with Bu(3)SnH-AIBN produces the corresponding 2-cyclohexenones 13 and 15 respectively via presumed alpha-ketene alkyl radical intermediates, viz. 10. By contrast, the 2,7-diene esters 34 and 39 undergo tandem radical cyclisations producing diquinanes, e.g.(76%), and the corresponding allene-substituted alpha,beta-unsaturated selenyl ester 48 gives the cyclooctadienone 56 on treatment with Bu(3)SnH-AIBN in refluxing benzene. The selenyl ester 19 derived from chrysanthemic acid produces a mixture of the gamma,delta-unsaturated aldehyde 22 and the corresponding dimer 25a on treatment with Bu(3)SnH-AIBN. Furthermore, in the presence of methanol the only product from this reaction was the bis(methyl ester) dimer 25b, thereby lending further credence to the involvement of ketene alkyl radical intermediates in these reactions, and in the aforementioned reactions involving 2,6- and 2,7-diene selenyl esters. Treatment of the cyclopropane selenyl esters and , containing keto- and oxy-group functionality in their side-chains, with Bu(3)SnH-AIBN led to excellent syntheses of the enol lactone 66 (76%) and the trans-fused bicyclo[6.1.0]nonane 67 (80-95%) respectively.

A one-pot sequence of Stille and Heck couplings: synthesis of various 1,3,5-hexatrienes and their subsequent 6pi-electrocyclizations

Palladium-catalyzed cross-coupling reactions of 2-bromocyclohex-1-enyl triflates 7 and 11 with a variety of alkenylstannanes occurred chemoselectively at the site of the triflate leaving group to give bromobutadienes which readily underwent Heck reactions with acrylates and styrene. Both steps could be performed in the same flask to give differentially functionalized hexatrienes in up to 88% overall yield. With simple stannanes, the same catalyst precursor could be used for both coupling steps making it possible to perform the whole sequence with only one portion of catalyst. For some of the functionally substituted stannanes, specifically adjusted catalyst systems had to be used. The 1,3,5-hexatrienes obtained were further transformed, in particular the methoxy-substituted compounds 14a-c were converted to bicyclo[4.4.0]decenones 30 (71-97%), bicyclo[4.3.0]nonenones 35 (74-93%), cyclodecynone 37a (47%), and cyclononynone 39a (15%). Thermal electrocyclizations of the other hexatrienes gave tetrahydronaphthalines 31 (60-61%), the tricyclic lactone 32 (72-75%) and decahydrophenanthrene 33 (75 %) in good yields.