Searching for radical intermediates and pathways implied in the biosynthesis of some polycyclic cembranoids. A new plausible mechanism for the origin of sinulanocembranolide A in the coral Sinularia gyrosa

The 3(2H)-furanone 11 underwent dimerisation to 13 (71%) via the captodative radical intermediate 12 on treatment with t-butyl hydroperoxide, but similar treatment of 5-episinuleptolide (5) failed to produce the natural product 7 via the presumed radical intermediate 8. In other attempts to generate 8, irradiation of a solution of 5 in benzene led only to the corresponding E-isomer 14, and treatment of 5 with Mn(OAc)₃ in HOAc produced the acetate of the isomeric alcohol 15. However, when a solution of 5-episinuleptolide (5) in THF was reacted with samarium diiodide, a sequence of transannular ketyl radical-induced coupling reactions ensued leading to the ring fused compound 20 in 62% yield. From these experiments in vitro it is difficult to make the case for the involvement of carbon-centred radical intermediates suggested earlier, in the biosynthesis of some polycyclic norcembranoids. It is proposed that the ring system in the recently described sinulanocembranolide A (1) is derived in vivo by way of a novel (3 + 2) type (possibly 1,3-dipolar) transannular cyclisation from the enedione intermediate 23 produced from 5-episinuleptolide (5).     

Computational insight into the thermal reactivity of N-methyl-3-cyanomethyl-2-vinylindole. Competition between two pericyclic reactions

The direct cycloaddition of a N-methyl-3-cyanomethyl-2-vinylindole (1) with a N-methylmaleimide (2) is not observed. The in situ formation of an indolo-2,3-quinodimethane intermediate (3) leads instead of the normal cycloadduct to an 1,2,3,4-tetrahydrocarbazole (5). To help our understanding of this reaction, we performed a DFT study. The formation of both, the direct cycloadduct (4) and the intermediate (3) are found to be not thermodynamically favorable. However, the small amount of 3 formed in the medium reacts with the dienophile in this way explaining the regio-selectivity of the reaction. The diastereo-selectivity is finally explained by a Curtin-Hammett-type energy profile.     

Transamidation reactions of 2-(2-sulfonylguanidino)acetamides

The reactivity of a series of sulfonylguanidinoacetamides 2A-E towards amines is reported. Guanidinoacetamides 2A-C, containing the arylsulfonylimino moiety, undergo a facile transamidation to give substituted carboxamides 4A-C, through the imidazolidinone intermediate 3. Acetamide 2D, having a methanesulfonylimino substituent, affords the imidazolidinone 3D and no transamidated carboxamides 4 are detected. In the case of guanidinoacetamide 2E, with a p-nitrobenzenesulfonylimino substituent, a Smiles rearrangement was observed.     

Formal total synthesis of aspergillide A

The formal total synthesis of aspergillide A 1 is described. The cross-metathesis of enone 6 with 6-hepten-2-ol derivative 5 provided E-olefin 15 corresponding to the C₄-C₁₄ backbone of 1. The CBS asymmetric reduction of 15 gave allyl alcohol 16, which was transformed into β-alkoxyacrylate 4 which had a formyl group. SmI₂-induced reductive cyclization of 4 gave a 2,6-syn-2,3-trans THP derivative 3 in good yield. After methoxymethylation of 3, the resulting compound 19 was submitted to desilylation and hydrolysis, to afford Fuwa’s key intermediate 2 for the total synthesis of 1.     

Domino routes to substituted benzoindolizines: tandem reorganization of 1,3-dipolar cycloadducts of nitrones with allenic esters/ketones and alternative cycloaddition-palladium catalyzed cyclization pathway

Reactions of C-(4-oxo-4H[1]benzopyran-3-yl)-N-phenyl nitrones (7) with allenic esters (8a-c) and allenic ketones (18a-d) furnish benzoindolizines (9a-k, 19a-d) in good yields. The formation of benzoindolizines is postulated to involve regioselective addition of 1,3-dipole to C2-C3 π bond of allenic esters/ketones followed by domino transformation of the cycloadducts, which involve an intramolecular aza Diels-Alder reaction in the intermediate C. DFT calculations of various parameters for diene and dienophile components in the proposed intermediate C have revealed that conformational constraints imposed by the alkyl groups (R=Me, Et) favor intramolecular aza-Diels-Alder cycloaddition. An alternative domino route to benzoindolizines (9a,d,g) involving sequential one-pot cycloaddition of azadienes (22a-c) with silyl-enol ether (23) followed by palladium(0)-catalyzed Heck coupling reaction has also been developed. Both these approaches represent novel domino routes for the synthesis of benzoindolizines.     

A practical approach for the preparation of monofunctional azulenyl squaraine dye

The synthesis of monofunctional azulenyl squaraine dye NIRQ₇₀₀ is described. The essential azulene intermediate 3, 1-(methoxycarbonyl)-2-methylazulene, was achieved via [8+2] cycloaddition between lactone 2, 2H-3-methoxycarbonyl-cyclohepta[b]furan-2-one, and the in situ generated vinyl ethers under high temperature and pressure conditions. Methylation on the cycloheptatriene ring of 2-methyl azulene 6 via Meisenheimer-type intermediate following Schrott's method formed the carboxylic acid intermediate 9, 3-(2-methyl-azulen-4-yl)-propionic acid. Condensation of 9 with squaric acid provided the title compound NIRQ₇₀₀ at moderate yields. The non-fluorescent squaraine dye NIRQ₇₀₀ absorbed in a 600-700 nm range and potentially can be used to quench a number of available NIR fluorochromes in order to extend the spectrum of biological quenching assays.     

Total synthesis of puerarin, an isoflavone C-glycoside

We completed the first total synthesis of puerarin (1), an isoflavone C-glycoside. The key intermediate, β-d-glucopyranosyl-2,6-dimethoxybenzene (9), was obtained by coupling of a lithiated aromatic reagent (3) with pyranolactone (2) in 56% yield. Condensation of (16) with p-methoxybenzaldehyde gave the chalcone (17). The protected chalcone (18) was cyclized to (19) in the presence of Tl(NO₃)₃. Demethylation of (19) was accomplished by refluxing with TMSI in CH₃CN to give puerarin (1).     

New synthesis of glycolipid immunostimulants RC-529 and CRX-524

An efficient and scalable synthesis of the potent vaccine adjuvant RC-529 (3) and TLR4 agonist CRX-524 (4) is described in eight steps from 1,3,4,6-tetra-O-acetyl-2-amino-2-benzyloxycarbonyl-2-deoxy-β-d-glucopyranose (10c) in ca. 25% overall yield. The synthesis features the strategic use of the N-Cbz group for β-glycosylation and the selective N,N,O-triacylation of common advanced intermediate 15 with (R)-3-tetradecanoyloxy or decanoyloxytetradecanoic acid (8, 9) late in the synthesis. A new method for preparing and enhancing the enantiopurity of (R)-3-hydroxytetradecanoic acid (6), a key component of 3 and 4 as well as bacterial lipid A, is also described.     

Photobehaviour of 2- and 3-heteroaryl substituted o-divinylbenzenes; formation of fused 2,3- and 3,2-heteroareno-benzobicyclo[3.2.1]octadienes and 3-heteroaryl benzobicyclo[2.1.1]hexenes

New β-3-thienyl (8) and β-3-furyl derivatives of o-divinylbenzene (9) have been synthesised and their photochemical behaviour compared with 2-thienyl (7) and 2-furyl derivatives (2). Whereas the β-(2-heteroaryl) substituted o-divinylbenzenes (7 or 2) give only bicyclo[3.2.1]octadiene structure (14 or 1) by 1,6-ring closure of the biradical intermediate, β-(3-heteroaryl) substituted o-divinylbenzenes (8 or 9) give bicyclo[3.2.1]octadiene structure (23 or 24) and bicyclo[2.1.1]hexene structure (25 or 26) by 1,6- and 1,4-ring closure, respectively. This photochemical approach provides a simple method to 2,3- and 3,2-fused thiophene and furan polycyclic compounds.     

An unexpected ring contraction of two nitroaryl pro-drugs: conversion of N-(nitroaryl)-3-chloropiperidine derivatives into N-(nitroaryl)-2-chloromethylpyrrolidines

Treatment of the N-nitroaryl-3-hydroxypiperidine derivatives 12 and 13 with thionyl chloride afforded the corresponding N-aryl-2-chloromethylpyrrolidines 5 and 15 via a ring-contraction process involving an intermediate aziridinium ion.     

Novel formation of 1,2-dithiolane-3-thione from β-dithiolactone. Isolation of dithiolato-palladium and -platinum complexes

Sulfurization of β-dithiolactone (4) gave corresponding 1,2-dithiolane-3-thione (2a) via an ionic intermediate. Oxidation of β-dithiolactone 4 by m-CPBA afforded corresponding S-oxide (11), while dioxide (12) was obtained when 3 equiv of m-CPBA was used. Dithiolane-3-thione 2a reacted with ethylenebis(triphenylphosphine)platinum or tetrakis(triphenylphosphine)palladium to afford the corresponding dithiolato-platinum (20) and dithiolato-palladium (21) complexes in good yields.     

Application of the aza-Wittig reaction to the synthesis of pyrazinothienotriazolopyrimidinones: a new tetracyclic ring system

A simple one-pot and efficient method is described for the synthesis of pyrazinothienopyrimidines 6 by domino processes involving aza-Wittig/intermolecular nucleophilic addition/intramolecular cyclization. A tandem aza-Wittig reaction of phosphazenes 7, derived from 6, with heterocumulenes (isocyanates, carbon disulfide or carbon dioxide) generates the pyrazinothienotriazolopyrimidinones 9, 11 and 12, respectively. Pyrazino[2′,3′:4,5]thieno[3,2-d]-1,2,4-triazolo[1,5-a]pyrimidin-4(3H)-ones 15 and bis(pyrazinothienotriazolopyrimidinones) 17 were synthesized by the intermolecular aza-Wittig reaction of phosphazenes 7 with acyl chlorides or α,ω-dichlorides followed by heterocyclization via imidoyl chloride intermediate 16. Further S-alkylation of 11 and reaction of 6 with phosgeniminium chloride produce 2-alkylthio- and 2-N,N-dimethylaminopyrazinothienotriazolopyrimidinones 13 and 19, respectively.     

Regioselective synthesis of novel 4-aryl-2-ethylthio-7-methyl pyrazolo[1,5-a]-[1,3,5]-triazines

Pyrazolo[1,5-a]-[1,3,5]-triazines 6a-d were obtained by an efficient one-step reaction from S,S-diethyl aroyliminodithiocarbonates 4a-d and 5-amino-3-methylpyrazole 5 or by an alternative two-step reaction from 5 and aroyl isothiocyanates 8a-d to give initially the thiourea derivatives 9a-d, which after S-ethylation and cyclization afforded compounds 6a-d. The intermediate 7a isolated from reaction between 4a and 5 permitted us to establish the orientation.     

Asymmetric syntheses of 6-deoxyfagomin, d-deoxyrhamnojirimycin, and d-rhamnono-1,5-lactam

N-Allyl protected 3-O-benzyloxglutarimide 11 was synthesized as a useful variant of the chiral building block 10. This modification allowed a high-yielding deprotection of the allyl group from the lactam intermediate 14. Starting from this building block, the asymmetric syntheses of aza-sugars 6-deoxyfagomine (2), d-rhamnono-1,5-lactam (6), as well as d-deoxyrhamnojirimycin (5) have been achieved in high regio- and/or diastereo-controlled manner.     

Carbanucleosides: synthesis of both enantiomers of 2-(6-chloro-purin-9-yl)-3,5-bishydroxymethyl cyclopentanol from d-glucose

The key intermediate 1,2:5,6-di-O-isopropylidene-3-deoxy-3β-allyl-α-d-glucofuranose (8) could be conveniently prepared through radical induced allyl substitution at C-3 of appropriate 1,2:5,6-di-O-isopropylidene-α-d-glucofuranose derivatives (7a,b) and used to synthesize enantiomeric bishydroxymethyl aminocyclopentanols 13 and 19 by the application of a 1,3-dipolar nitrone cycloaddition reaction involving the C-5 or C-1 aldehyde functionality. The products were subsequently transformed into carbanucleoside enantiomers 15 and 21. The diastereomeric isoxazolidinocyclopentane derivative 20 was similarly converted to carbanucleoside 22.     

Methyl 3-bromomethyl-3-butenoate as an isopentane building block for the stereoselective preparation of (S)-4-methyl-3,6-dihydro-2H-pyran-2-carbaldehyde and (+)-faranal

(S)-4-Methyl-3,6-dihydro-2H-pyran-2-carbaldehyde (3), the common intermediate in the syntheses of the C17-C27 subunit of laulimalide (4) and (+)-faranal (5), the trail pheromone of the pharaoh ant, Monomorium pharaonis, were obtained via transformation of methyl 3-bromomethyl-3-butenoate (1) into allylstannane 2 and subsequent allylation of (benzyloxy)acetaldehyde (6) in accordance with the Keck procedure as the key steps.     

Preparation of new pyrido[3,4-b]thienopyrroles and pyrido[4,3-e]thienopyridazines

Two new types of pyrido-fused tris-heterocycles (1a,b and 2a,b) have been prepared from 3-aminopyridine in five/six steps. A synthetic strategy for the preparation of the novel pyrido[3,4-b]thieno[2,3- and 3,2-d]pyrroles (1a,b) and pyrido[4,3-e]thieno[2,3- and 3,2-c]pyridazines (2a,b) has been studied. The Suzuki cross coupling of the appropriate 2- and 3-thienoboronic acids (3,4) and 4-bromo-3-pyridylpivaloylamide (9) afforded the biaryl coupling products (10,11) in high yields (85%). Diazotization of the hydrolysed (2-thienyl)-coupling product (12) and azide substitution gave the 3-azido-4-(2-thienyl)pyridine intermediate (72%, 14). 3-Azido-4-(3-thienyl)pyridine (15) was prepared by exchanging the previous order of reactions. The desired β-carboline thiophene analogues (1a,b) were obtained via the nitrene by thermal decomposition of the azido precursors (14,15). By optimising conditions for intramolecular diazocoupling, the corresponding pyridazine products (72-83%, 2a,b) were afforded.     

Synthetic study of hetisine-type aconite alkaloids. Part 2: Preparation of hexacyclic compound lacking the C-ring of the hetisan skeleton

A hexacyclic compound 1, having almost the full hetisine-type aconite alkaloid framework lacking only the C-ring with an exo-methylene group, was synthesized from the intermediate 3 reported in the preceding paper. The synthesis involved the following key reactions the crucial conversion of 3 to 4, a stereoselective hydrocyanation reaction to obtain 5 from 4, and construction of the azabicyclic ring system (5→1).     

Facile conversion of pyridine propargylic alcohols to enones: stereochemistry of protonation of allenol

The conversion of 4-pyridyl propargylic alcohols 1 to the (E)-propenones 3 and propynones 2 occurs under mild reaction conditions, pyridinium chloride in methanol at room temperature. (Z)-4-Pyridyl propenones 11 are detected as initial products when large substituents such as trimethylsilyl, tert-butyl and phenyl are attached at C-3 of the propynols and these (Z)-enones 11 are isomerised to the (E)-isomers 3 under the reaction conditions. In the presence of deuterated solvent, both hydrogens at the double bond of enone 3d are deuterated. An allenol is proposed as intermediate whose preferential protonation occurs at the less hindered side giving the (Z)-enone. The propargylic alcohols, pyridin-2-yl 12 and quinolin-4-yl 5, are converted to (E)-enones 13 and 7, respectively.     

Synthesis of a regioisomeric analogue of the 3C-protease inhibitor thysanone via a Hauser annulation strategy

Hauser annulation of 3-cyano-5,7-dimethoxy-(3H)-isobenzofuran-1-one 4 with ethyl acrylate as a method to access activated naphthoquinone 3, a key intermediate for the synthesis of thysanone 1, proved unreliable. In contrast to this, Hauser annulation of regioisomeric 3-cyano-4,6-dimethoxy-(3H)-isobenzofuran-1-one 13 with ethyl acrylate proceeded readily affording ethyl 5,7-dimethoxy-1,4-naphthoquinone 12, after oxidation of the initial dihydroxynaphthalene 16. Allylation of naphthoquinone 12 followed by reductive methylation and Wacker oxidation afforded ketone 11 that underwent CBS reduction to (2′S)-alcohol 19 followed by cyclisation to lactone 20. Reduction of the lactone followed by oxidative demethylation afforded (1S,3S)-6,8-dimethoxy-1-hydroxy-3-methylpyrano[2,3-c]-1,4-naphthoquinone 22, a regioisomeric analogue of the 3C-protease inhibitor thysanone 1.