Studies towards the Synthesis of (+)‐Dictyoxetane

The dolabellane-type diterpene dictyoxetane represents a significant challenge to synthetic organic chemistry. Methodology directed towards the total synthesis of naturally occurring (+)-dictyoxetane is reported. Catalytic asymmetric synthesis of the trans-hydrindane ring system is achieved through chemoselective deoxygenation of the Hajos-Parrish ketone. An alternative to the Garst-Spencer furan annulation is developed for the synthesis of a 2,5-dimethyl, tetrasubstituted furan, employing a tandem 5-exo-dig alcohol to alkyne cyclisation/aromatisation reaction as a key step. The (4+3) cycloaddition reaction of an oxyallyl cation with a tetrasubstituted furan is established on a cyclohexanone-derived model system, and a range of related (4+3) cycloadditions investigated on a homochiral, trans-hydrindane-fused furan, where regio- and diastereoselectivity is required for the natural product synthesis. In an alternative (4+2) Diels-Alder approach, a C₂ -symmetric vinyl sulfoxide-based chiral ketene equivalent is used to prepare oxanorbornenes with the same oxygen bridge stereochemistry found in the 2,7-dioxatricyclo[4.2.1.0^3,8 ]nonane ring system of the natural product.

Glutathione peroxidase mimics based on conformationally-restricted, peri-like, 4,5-disubstituted fluorene dichalcogenides

Glutathione peroxidase (GPx) regulates cellular peroxide levels through glutathione oxidation. GPx-mimics based on 4,5-disubstituted fluorene diselenides, their oxides, and ditellurides show catalytic activities consistent with conformational restriction about the dichalcogen bond.

Direct formation of 4,5-disubstituted carbazoles via regioselective dilithiation

Carbazoles are widely exploited for their interesting photophysical and electronic properties, however bay (4,5-) functionalization is challenging, and previously inaccessible through carbazole C-H activation. We report a simple methodology which introduces a range of versatile 4,5-functionality, enabling the wider investigation of ring annulation and close proximity effects on carbazole properties.

Gold-Catalyzed Intermolecular Alkyne Oxyarylation for C3 Functionalization of Benzothiophenes

C3-selective C-C bond formation on benzothiophenes is challenging, and few direct functionalization methods are available. A gold-catalyzed reaction of alkynes with benzothiophene S-oxides provides regioselective entry into C3-alkylated benzothiophenes with the C7-alkylated isomer as the minor product. This oxyarylation reaction works with alkyl and aryl alkynes and substituted and unsubstituted benzothiophenes. Mechanistic studies identify that sulfoxide inhibits the catalyst [DTBPAu(PhCN)]SbF₆, which also degrades and forms the unreactive complex [(DTBP)₂Au]SbF₆.

Design, synthesis and evaluation of tryptophan analogues as tool compounds to study IDO1 activity

The metabolism of l-tryptophan to N-formyl-l-kynurenine by indoleamine-2,3-dioxygenase 1 (IDO1) is thought to play a critical role in tumour-mediated immune suppression. Whilst there has been significant progress in elucidating the overall enzymatic mechanism of IDO1 and related enzymes, key aspects of the catalytic cycle remain poorly understood. Here we report the design, synthesis and biological evaluation of a series of tryptophan analogues which have the potential to intercept putative intermediates in the metabolism of 1 by IDO1. Functionally-relevant binding to IDO1 was demonstrated through enzymatic inhibition, however no IDO1-mediated metabolism of these compounds was observed. Subsequent T_m-shift analysis shows the most active compound, 17, exhibits a distinct profile from known competitive IDO1 inhibitors, with docking studies supporting the hypothesis that 17 may bind at the recently-discovered S_i site. These findings provide a start-point for development of further mechanistic probes and more potent tryptophan-based IDO1 inhibitors.

We report the rational design, novel syntheses and biophysical and in silico evaluation of tryptophan-inspired tool compounds to probe the illusive MOA of the clinically-relevant heme-dioxygenase protein, IDO1.

Alkynyl sulfoxides as α-sulfinyl carbene equivalents: gold-catalysed oxidative cyclopropanation

Gold catalysed oxidation of alkynyl sulfoxides provides access to a desirable α-sulfinyl metal carbene reactivity profile that is inaccessible using diazo chemistry.

Intercepting the Gold-Catalysed Meyer-Schuster Rearrangement by Controlled Protodemetallation: A Regioselective Hydration of Propargylic Alcohols

The regioselective gold‐catalysed hydration of propargylic alcohols to β‐hydroxy ketones can be achieved by diverting the gold‐catalysed Meyer–Schuster rearrangement through the addition of a protic additive with a pK_a of 7–9 such as p‐nitrophenol, boric acid or a boronic acid. This provides an interesting alternative to an aldol reaction when combined with the straightforward addition of an alkyne to an aldehyde or ketone. The gold‐catalysed reaction of an electron‐deficient, sterically hindered propargylic alcohol with a boronic acid led to the formation of an unusually stable cyclic boron enolate.

Regioselective functionalisation of dibenzothiophenes through gold-catalysed intermolecular alkyne oxyarylation

A protocol has been developed for direct Csp(3)-Csp(2) bond formation at the 4- and 6-positions of dibenzothiophenes using a gold(I) catalyst with terminal alkynes and dibenzothiophene-S-oxides. The sulfoxide acts as a traceless directing group to avoid the need to prefunctionalise at carbon. The iterative use of this protocol is possible and has been employed in the preparation of novel macrocyclic structures. In addition, a cascade process shows how oxyarylations can be combined with other processes resulting in complex, highly efficient transformations.

Carbamoyl radical-mediated synthesis and semipinacol rearrangement of β-lactam diols

In an approach to the biologically important 6-azabicyclo[3.2.1]octane ring system, the scope of the tandem 4-exo-trig carbamoyl radical cyclization-dithiocarbamate group transfer reaction to ring-fused β-lactams is evaluated. β-Lactams fused to five-, six-, and seven-membered rings are prepared in good to excellent yield, and with moderate to complete control at the newly formed dithiocarbamate stereocentre. No cyclization is observed with an additional methyl substituent on the terminus of the double bond. Elimination of the dithiocarbamate group gives α,β- or β,γ-unsaturated lactams depending on both the methodology employed (base-mediated or thermal) and the nature of the carbocycle fused to the β-lactam. Fused β-lactam diols, obtained from catalytic OsO4-mediated dihydroxylation of α,β-unsaturated β-lactams, undergo semipinacol rearrangement via the corresponding cyclic sulfite or phosphorane to give keto-bridged bicyclic amides by exclusive N-acyl group migration. A monocyclic β-lactam diol undergoes Appel reaction at a primary alcohol in preference to semipinacol rearrangement. Preliminary investigations into the chemo- and stereoselective manipulation of the two carbonyl groups present in a representative 7,8-dioxo-6-azabicyclo[3.2.1]octane rearrangement product are also reported.

A novel protecting group methodology for syntheses using nitroxides

The methoxyamine group represents an ideal protecting group for the nitroxide moiety. It can be easily and selectively introduced in high yield (typically >90%) to a range of functionalised nitroxides using FeSO4·7H2O and H2O2 in DMSO. Its removal is readily achieved under mild conditions in high yield (70-90%) using mCPBA in a Cope-type elimination process.

Diastereotopic group selectivity and chemoselectivity of alkylidene carbene reactions on 8-oxabicyclo[3.2.1]- oct-6-ene ring systems

α-Hydroxyalkylidene carbenes, generated from thermolysis of α,β-epoxy-N-aziridinylimines, undergo diastereotopic group selective 1,5 C–H insertion reactions on 2,4-dimethyl-8-oxabicyclo[3.2.1]oct-6-ene ring systems. Protection of a tertiary alcohol at C-3 of the bridged oxabicycle as a trimethylsilyl ether reverses the sense of diastereoselectivity. 1,5 C–H insertion into a methine adjacent to an OBn group, 1,5 O–R insertion into a tertiary alcohol (R = H) or silylether (R = TMS) at C-3 to form spirocyclic dihydrofurans, 1,2-rearrangement to an alkyne and fragmentation to a ketone are competing major pathways for 2-benzyloxy-substituted 8-oxabicyclo[3.2.1]oct-6-ene systems. Dihydrofuran formation is shown to be a result of substitution on the oxabicyclic ring system through comparison with other methods of alkylidene carbene formation.

In search of a new class of stable nitroxide: synthesis and reactivity of a peri-substituted N,N-bissulfonylhydroxylamine

Acyclic bissulfonylnitroxides have never been isolated, and degrade through fragmentation. In an approach to stabilising a bissulfonylnitroxide radical, the cyclic, peri-substituted N,N-bissulfonylhydroxylamine, 2-hydroxynaphtho[1,8-de][1,3,2]dithiazine 1,1,3,3-tetraoxide (1), has been prepared by formal nitrogen insertion into the sulfur-sulfur bond of a sulfinylsulfone, naphtho[1,8-cd][1,2]dithiole 1,1,2-trioxide. The heterocyclic ring of 1 is shown to adopt a sofa conformation by X-ray crystallography, with a pseudo-axial hydroxyl group. N,N-Bissulfonylhydroxylamine 1 displays high thermal, photochemical and hydrolytic stability compared to acyclic systems. EPR analysis reveals formation of the corresponding bissulfonylnitroxide 2 upon oxidation of 1 with the Ce(IV) salts CAN and CTAN. Although 2 does not undergo fragmentation, it cannot be isolated, since hydrogen atom abstraction to reform 1 occurs in situ. The stability and reactivity of 1 and 2 are compared with the known cyclic benzo-fused N,N-bissulfonylhydroxylamine, N-hydroxy-O-benzenedisulfonimide (6), for which the X-ray data, and EPR of the corresponding nitroxide 10, are also reported for the first time.

Stereoselective α,α'-annelation reactions of 1,3-dioxan-5-ones

Pyrrolidine enamines derived from three 1,3-dioxan-5-ones undergo α,α'-annelation reactions with methyl α-(bromomethyl)acrylate to produce bridged 2,4-dioxabicyclo[3.3.1]nonane ring systems with complete stereocontrol. Stereochemical outcomes have been rationalized based on steric and stereoelectronic interactions in intermediate boat-like conformations of the 1,3-dioxane ring and subsequent kinetic protonation to set an axial ester group on the cyclohexanone ring. Base-mediated ester epimerization provides the stereochemical array found in the highly oxygenated cyclohexane ring of phyllaemblic acid and glochicoccins B and D.

Thermal elimination of diethyldithiocarbamates and application in the synthesis of (+/-)-ferrugine

Dithiocarbamate-substituted lactams, prepared through group-transfer cyclization reactions of carbamoyl radicals, undergo a Chugaev-like thermal elimination of the dithiocarbamate group in refluxing diphenyl ether to form alpha,beta- and/or beta,gamma-unsaturated amides, depending on the structure of the starting material. This reaction sequence was used to prepare an unsaturated [3.2.2] bridged bicyclic amide, which was converted in a one-pot procedure to the 8-azabicyclo[3.2.1]octane ring system of the tropane alkaloid ferrugine by treatment with phenyllithium followed by aqueous sodium hydroxide.

Stereoselective synthesis of 2,5-disubstituted-1,4-oxathiane S-oxides

Beta-allyloxy and beta-propargyloxy tert-butyl sulfoxides undergo tandem sulfoxide eliminination-intramolecular sulfenic acid addition reactions to produce 1,4-oxathiane S-oxides.

Selective 1,5-Alkylidenecarbene Insertion Reactions on [3.2.1] Oxabicyclic Ethers: A New Approach toward the AB Ring System of Ingenol

Two methods to achieve the formal aldol reaction between acetone and two oxabicyclic [3.2.1] ketones are reported. The trimethylsilyl-protected beta-hydroxy ketones are converted by a Wittig reaction into vinyl chlorides as synthetic precursors to alkylidenecarbenes. Selective 1,5 C-H over 1,5 O-Si insertion has been applied to the synthesis of a model for the ABC ring system of ingenol.

A synthetic alternative to the type-II intramolecular 4 + 3 cycloaddition reaction

Oxyallyl cations generated in situ from dihaloketones have been found to undergo the Favorskii rearrangement in preference to an intramolecular Type-II 4 + 3 cycloaddition reaction in trifluoroethanol and hexafluoropropan-2-ol solvents, generating acrylate esters with high cis selectivity. A synthetic alternative to the intramolecular Type-II 4 + 3 cycloaddition has been developed on the basis of intramolecular enolate alkylation to close a 7-membered ring with an exocyclic double bond.

Diastereotopic group selective intramolecular cycloadditions of sulfenic acids to 1,4-dienesElectronic supplementary information (ESI) available: Ball and stick representations of the X-ray crystal structures of 12a and 13a. See http://www.rsc.org/suppdata/ob/b3/b314176d/

The ratio of diastereomeric cis-fused perhydrobenzothiophene S-oxides formed via the intramolecular addition of a sulfenic acid to a 1,4-diene is controlled by the nature of the protecting group on a chiral alcohol in the connecting chain.

Highly diastereoselective 1,3-dipolar cycloaddition reactions of trans-2-methylene-1,3-dithiolane 1,3-dioxide with 3-oxidopyridinium and 3-oxidopyrylium betaines: a route to the tropane skeleton

The C2-symmetric vinyl sulfoxide, trans-2-methylene-1,3-dithiolane 1,3-dioxide, was found to react with a range of 3-oxidopyridinium betaines (bearing different substituents on nitrogen) in high yield and with total diastereoselectivity. A 2.3:1 mixture of regioisomers was formed with all of the 3-oxidopyridinium betaines but the ratio was found to change over prolonged periods of time due to reversibility of the minor regioisomer. 3-Oxidopyridinium betaines bearing methyl substituents at either the 2- or 6-position were also tested in the cycloaddition process. Improved regioselectivity (8:1) and again high diastereoselectivity were observed with the betaine having an additional substituent at the 2-position, but with betaines having a substituent in the 6-position although high regioselectivity was observed (9.9:1), the major isomer was formed with low diastereoselectivity (5.5:4.4). The origin of the regio- and diastereo-selectivity with all the betaines is discussed. Finally, the C2-symmetric vinyl sulfoxide, trans-2-methylene-1,3-dithiolane 1,3-dioxide was reacted with an oxidopyrylium betaine in moderate yield. Good regioselectivity and moderate diastereoselectivity were observed.

Photomediated asymmetric synthesis of (-)-cuparene

Generation of a benzylic quaternary stereocentre via the photomediated cyclisation of a chiral alpha-(aminobutyl)styrene followed by a microwave-assisted Cope elimination has led to a total synthesis of the sesquiterpene (-)-cuparene.

A novel recyclable sulfur monoxide transfer reagent

[reaction: see text]. Trisulfide 2-oxide 11 has been prepared from disulfide 9 via reduction to the corresponding dithiol and subsequent trapping with thionyl chloride. Heating trisulfide oxide 11 in the presence of dienes results in transfer of sulfur monoxide to form cyclic unsaturated sulfoxides 13 in good to excellent yields, along with recovery of disulfide 9. A Pummerer reaction can be used to convert the cyclic sulfoxides into thiophenes.