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.

Divergent total syntheses of five illudalane sesquiterpenes and assignment of the absolute configuration

Concise, divergent total syntheses of five bioactive illudalane sesquiterpenes have been achieved. Our synthesis features an intermolecular [2+2+2] cycloaddition, and a lactone-directed aromatic C-H oxygenation to generate a temporary phenolic hydroxyl group which enables regioselective methylation. Furthermore, the absolute configuration of radulactone was assigned by chemical synthesis.

Chemically-activatable alkyne-tagged probe for imaging microdomains in lipid bilayer membranes

A chemically-activatable alkynyl steroid analogue probe has been synthesized for visualizing the lipid raft membrane domains by Raman microscopy. The Raman probe, in which ring A of its steroid backbone is replaced with an alkynyl group, was designed to enable activation of the alkyne signal through the Eschenmoser-Tanabe fragmentation reaction of the oxidized cholesterol precursor in lipid bilayer membranes. The alkynyl steroid analogue was observed to form liquid-ordered raft-like domains on a model giant-liposome system in a similar manner as cholesterol, and the large alkyne signal of the accumulated probe at 2120 cm⁻¹ was mapped on the microdomains with a Raman microscope. The alkyne moiety of the probe was confirmed to be converted from the α,β-epoxy ketone group of its precursor by reaction with p-toluensulfonyl hydrazine under a mild condition. Through the reaction, the alkyne signal of the probe was activated on the lipid bilayer membrane of liposomes. Furthermore, the signal activation of the probe was also detected on living cells by stimulated Raman scattering microscopy. The ring-A-opened alkyne steroid analogue, thus, provides a first chemically-activatable Raman probe as a promising tool for potentially unravelling the intracellular formation and trafficking of cholesterol-rich microdomains.

Catalytic Enantioselective Total Synthesis of Hypocrolide A

The first and catalytic enantioselective total synthesis of hypocrolide A (>99% ee) in 12 steps, as well as other botryanes, is described. The absolute configurations of these compounds have been unambiguously confirmed or reassigned accordingly. The key reactions in this study include an unusual rhodium-catalyzed intramolecular [4 + 2] cycloaddition and a biomimetic oxidative [3 + 2] cycloaddition based on our revised biogenetic pathway.

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.

Aziridinyl imines in organic synthesis: Development of tandem reaction strategies and application to total synthesis of natural products

Aziridinyl imines are well-known carbene equivalents because they are precursors of diazo compounds from which reactive intermediates can be produced. These carbene equivalents can be utilized as zwitterionic species, diradicals, or 4π system for cycloaddition reactions. Thus, the intermediates derived from aziridinyl imines have been used in the sulfur-ylide-mediated epoxide formation, tandem free-radical reactions, or cyclopropanation reaction via carbene intermediates to form trimethylenemethane (TMM) diyls, which undergo [2 + 3] cycloaddition reactions to form cyclopentanoids. Diazo compounds generated from aziridinyl imines also react with allenes to form TMM diyls. This reaction was utilized in tandem cycloaddition reactions of linear substrates to form polyquinanes. These tandem reaction strategies were successfully applied to the total synthesis of various cyclopentanoid natural products.

Stereoelectronic effects in the fragmentation of γ-silyloxy-β-hydroxy-α-diazocarbonyl compounds

A series of γ-silyloxy-β-hydroxy-α-diazocarbonyl compounds were prepared as fragmentation substrates to probe the hypothesis that steric interactions between the diazo ester and adjacent silyloxy group can play an important role in determining the success of fragmentations. Proper stereoelectronic alignment of the diazo ester and the departing hydroxyl group is necessary for productive fragmentation to occur.

Generation of medium-ring cycloalkynes by ring expansion of vinylogous acyl triflates

Reductive cyclization of aryl and vinyl iodides tethered to vinylogous acyl triflates (VATs) induces a ring-expanding fragmentation to provide cyclic alkynyl ketones, including strained nine-membered cycloalkynes, in fair to excellent yield. The tandem cyclization/C-C bond-cleavage is initiated under carefully optimized conditions by halogen-metal exchange in the presence of carbonyl and vinyl triflate functionality. A modified protocol for alkylation of 1,3-cyclohexanedione is described for preparing the relevant VAT substrates.

Tandem Nucleophilic Addition/Fragmentation Reactions and Synthetic Versatility of Vinylogous Acyl Triflates

A thorough analysis of the chemistry of vinylogous acyl triflates provides insight into important chemical processes and opens new directions in synthetic technology. Tandem nucleophilic addition/C-C bond cleaving fragmentation reactions of cyclic vinylogous acyl triflates 1 yield a variety of acyclic acetylenic compounds. Full details are disclosed herein. A wide array of nucleophiles, such as organolithium and Grignard reagents, lithium enolates and their analogues, hydride reagents, and lithium amides, are applied. The respective reactions produce ketones 2, 1,3-diketones and their analogues 3, alcohols 4, and amides 5. The present reactions are proposed to proceed through a 1,2-addition of the nucleophile to the carbonyl group of starting triflates 1 to form tetrahedral alkoxide intermediates C, followed by Grob-type fragmentation, which effects C-C bond cleavage to yield acyclic acetylenic compounds 2-5 and 7. The potent nucleofugacity of the triflate moiety is channeled through the sigma-bond framework of 1, providing direct access to the fragmentation pathway without denying other typical reactions of cyclic vinylogous esters. The synthetic versatility of vinylogous acyl triflates, including functionalization reactions of the cyclic enone core (1 --> 6 or 8), is also illustrated.

Claisen-Type Condensation of Vinylogous Acyl Triflates

[reaction: see text] The Claisen-type condensation reaction of cyclic vinylogous carboxylic acid triflates with lithium enolates and their analogues produces acyclic alkynes bearing a 1,3-diketone-type moiety. The present transformation is proposed to proceed via a 1,2-addition of the enolate to the vinylogous acyl triflate, followed by fragmentation of the aldolate intermediate.

A Tandem Carbanion Addition/Carbon−Carbon Bond Cleavage Yields Alkynyl Ketones

Carbanion addition to vinylogous acid triflates triggers carbon-carbon bond cleavage to form alkynyl ketones under mild conditions. Mechanistic factors affecting the cleavage event and its relationship to complementary fragmentations are discussed. A range of tethered keto-alkynes are obtained by a unified approach.

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.