Search for a simpler synthetic model system for intramolecular 1,3-dipolar cycloaddition to the 5,6-double bond of a pyrimidine nucleoside

In search for a simpler model system for the study of intramolecular thermal reactions between the base and 5'-functionalized sugar moiety in nucleosides, 1-(3-azidopropyl)uracil (2), 1-(4-azidobutyl) pyrimidines (12 and 13) and 1-(5-azidopentyl)-uracil (14) was synthesized through the corresponding ω-benzoyloxy-(6,7 and 8) and ω-hydroxyalkyl-pyrimidines (9,10 and 11). Heating 2 gave 1,N⁶-trimethylene-6-aminouracil (4), while heating 12 and 13 gave N¹-C₆ cleaved addition products. 15 and 16, respectively. 15 was regiospecifically transformed to 1,2,3-triazole derivatives, 17,18 and 19. Heating 1-(4-azidobutyl)-5-bromouracil (20) yielded 3,9-tetramethylene-8-azaxanthine (22). 9 with NBA gave 1,0⁶-tetramethylene-5-bromo-6-hydroxy-5,6-dihydrouracil (24) and the 5-brominated analog of 9 (25). The 4-functionalized butyl side chain proved to serve as a substitute for the 5'-functionalized sugar moiety in pyrimidine ribonucleosides.     

Biomimetic synthesis of a preclavulone a model

A synthetic route to the preclavulone-A (3) analogs 11 and 14 via allene oxide and 2-oxidopentadienyl cation intermediates is described which is based on the proposed biosynthesis of 3.     

The unexpected conversion of a thiophene ring into a pyrrole ring via a putative nitrene intermediate

Triethylphosphite induced reductive cyclisation of 1-(2-nitrophenyl)-3,5-di(2-thienyl)pyrazole 6 afforded the expected 1,3-di(2-thienyl)pyrazolo[1,2-a]benzotriazole 7 (14%) together with an unexpected product which was shown to be a 2-(2-thienyl)pyrazolo[1,5-a]pyrrolo[2,1-c]quinoxaline 8 derivative (27%). A mechanism for the extrusion of sulfur during the transformation of heterocycle 6 into product 8 is proposed.     

Synthesis of asymmetrized 2-benzyl-1,3-diaminopropane by a chemoenzymatic route: a tool for combinatorially developing peptidomimetics

Both enantiomers of monoacetamide 5, together with `dipeptides' 30a,b and monocarbamates 24, (R)-43 and (S)-43, all derived from 2-benzyl-1,3-diaminopropane 4, were synthesized by a chemoenzymatic route starting from the known monoacetate 12. The behaviour of 4 and of the bis(acylated) derivatives 8–11 with respect to hydrolytic enzymes is also presented, together with an extensive study on the configurational stability of monoacylated derivatives of 4.     

Halichonic acid,a new rearranged bisabolene-type sesquiterpene from a marine sponge Halichondria sp.

A new compound, halichonic acid (1), was isolated from a marine sponge Halichondria sp., together with (6R,7S)-7-amino-7,8-dihydro-α-bisabolene (2). The structure of 1 was elucidated by spectroscopic analysis and ECD spectrum calculation to be a rearranged bisabolene-type sesquiterpene having a 3-azabicyclo[3.3.1]nonene moiety. Compound 2 was cytotoxic against HeLa cells with an IC₅₀ value of 50?μM, whereas 1 did not show cytotoxicity even at 50?μM. It is possible that 1 is biosynthesized from farnesyl pyrophosphate and glycine, with rearrangement.     

The acid catalysed rearrangement of a diterpenoid epoxide

A study has been made of the principal rearrangement products resulting from formic acid treatment of 19-hydroxy-ent-beyerene epoxide (3). In concentrated solutions 3 has been found to undergo a deep seated rearrangement to the allylic alcohol (14). A mechanism for the formation of 14 is proposed involving a novel 1,4-hydride shift in the bicyclo[3:2:1]octane C/D ring system following cleavage of the C₁₅O bond. Supporting evidence has been obtained from a study of the specifically labelled epoxide (4), the deuterium in 14 appearing exclusively at C₁₂. Four products (7 and 11–13) emanating from the known beyerane → kaurane interconversion have been identified.     

Straightforward synthesis of a derivative of purpurosamine C from d-galactose

Purpurosamine C (1) is a component of the aminoglycoside antibiotic gentamicin C_1a. A derivative of 1 was synthesized from d-galactose via its 2-acetoxy-3,4,6-tri-O-acetyl glycal (3). Compound 3 undergoes glycosylation with 2-propanol in the presence of SnCl₄, with two succesive allylic rearrangements of the double bond to give isopropyl 6-O-acetyl-3,4-dideoxy-α-d-glycero-hex-3-enopyranosid-2-ulose (7). Compound 7 was hydrogenated, and O-deacetylated to afford 8. The free OH group of 8 was tosylated and substituted by azide, and the carbonyl function of the resulting ulose 10 reacted with hydroxylamine to give the E,Z-oximes (11,12). Highly diastereoselective reduction of the oxime acetate (13) by borane, which also reduced the azide function, led to the purpurosamine C derivative 14 (∼40% yield from 3).     

Heteroaromatic alkaloids,nakijinamines, from a sponge Suberites sp.

The structures of seven new secondary metabolites isolated from an Okinawan marine sponge Suberites sp., nakijinamines A (1), B (2), and F–I (3–6) and 6-bromoconicamin (7), have been elucidated on the basis of spectroscopic analysis, chemical conversion, and conformational analysis. These analyses disclosed that 1–6 were heteroaromatic alkaloids having the hybrid structures of an aaptamine-type alkaloid and an indole alkaloid, while 7 was a bromoindole alkaloid. Nakijinamine I (6) is the first example of an aaptamine-type alkaloid possessing a 1,4-dioxane ring. Antimicrobial activities of 1–7 were evaluated.     

Asymmetric hydrogenation reactions mediated by a new class of bicyclic bisphosphinites

The bicyclic alcohol (−)-4 was prepared from (−)-bicyclo[3.2.0]hept-2-en-6-one (−)-1 in 50% yield. The diol (−)-4 was coupled to selected chlorophosphines 6–12 to produce a series of bisphosphinites 13–19 in 89–95% yield. From these bisphosphinites were prepared the rhodium complexes 20–26 which were characterised by ³¹P NMR and used in situ for the asymmetric hydrogenation of α-enamides 27–29. Complexes 21, 23–25 proved to be the superior catalysts for the production of (R)-N-acetylphenylalanine (91, 84, 90 and 87.5% ee) from 27 and (S)-N-acetylalanine methyl ester (70, 72, 68 and 71% ee) from 28.     

A synthetic approach to C-nor-D-homosteroids based on a cascade of radical cyclisations from a vinylcyclopropane-substituted acyl radical precursor

Treatment of the arylvinylcyclopropane-substituted seleno ester 5 with Bu₃SnH–AIBN, under high dilution in benzene at 80 °C, led to a 1:1 mixture of C10 methyl epimers of the C-nor-D-homosteroid ring system 24/25. The homosteroid was formed from 5 via a cascade of sequential acyl 13-endo trig radical macrocyclisation, benzyl radical 5-exo trig transannulation and alkyl radical transannulation reactions (Scheme 1). The macrocyclic dienone 23 was also isolated as an intermediate in the radical cascade between 5 and 24/25, and the dioxolanes 29 were interesting by-products. The cascade of radical cyclisations leading to the homosteroid 24/25 from the acyl radical precursor 5 is compared and contrasted with similar radical cascades from arylvinylcyclopropane-substituted alkyl radical precursors, i.e, 30→31 and 32b→38.     

Photochemistry of 5.6-epoxy-1,3-dienes : Influence of a 7-hydroxy substituent

Triplet sensitization of the 7-hydroxy-5,6-epoxy-1,3-diene 10 causes cleavage of the C(5),0 epoxide bond followed by the formation of the three isomers 11–13 or induces scission of the C(6), C(7) bond followed by the formation of the aldehyde 14 and the bicyclo[3.2.0]heptanols 15 and 16. However, irradiation of the corresponding acetate 18 gives only C(5),0 epoxide bond scission (18 → f) followed by the cyclization process f → 19, the 1,2-Me shifts f → 20, 21 and the γ-H abstraction f → g leading to the cyclopropane formation g → 22.     

Phloroglucinol derivatives and a chromone glucoside from the leaves of Myrtus communis

Seven new phloroglucinol derivatives, myrtucommunins A–D (1–4), 6-methylisomyrtucommulone B (5), 4-methylmyrtucommulone B (6), and 2-isobutyryl-4-methylphloroglucinol 1-O-β-d-glucopyranoside (7), and one new chromone derivative, undulatoside A 6′-O-gallate (8), were isolated from the leaves of Myrtus communis (Myrtaceae). Myrtucommunins A–D (1–4) were conjugates of polymethylated acylphloroglucinol and flavonol rhamnoside. The absolute configurations of the rhamnosyl moieties for 1–4 were confirmed to be l in each case by HPLC analyses, while those of the aglycones were assigned by comparisons of the experimental and TDDFT calculated ECD spectra. 6-Methylisomyrtucommulone B (5) and 4-methylmyrtucommulone B (6) were assigned as 6/6/6 tricyclic acylphloroglucinol derivatives with a racemic nature. Antimicrobial activities of 1–8 and related known compounds were evaluated.     

Synthesis of a new series of pyridine and fused pyridine derivatives

The reaction of 4-methyl-2-phenyl-1,2-dihydro-6-oxo-5-pyridine- carbonitrile (1) with arylidene malononitrile afforded isoquinoline derivatives 2a,b. 6-Chloro-4-methyl-2-phenyl-5-pyridinecarbonitile (3) obtained by chlorination of compound 1 with phosphoryl chloride was converted into 6-amino-4-methyl-2-phenyl-5-pyridinecarbonitrile (4) and 6-hydrazido-4-methyl-2-phenyl-5-pyridinecarbonitrile (5) in good yield, through reactions with ammonium acetate and hydrazine hydrate, respectively. Treatment of 4 with ethyl acetoacetate, acetic anhydride, formic acid, urea and thiourea gave the corresponding pyrido [2,3-d] pyrimidine derivatives 7-10a,b. A new series of 6-substituted-4-methyl-2-phenyl-5-pyridine carbonitriles 11-13 has been synthesized via reaction of 4 with phenyl isothiocyanate, benzenesulphonyl chloride and acetic anhydride. Treatment of 4 with malononitrile gave 1,8-naphthyridine derivative 14. The reactivity of hydrazide 5 towards acetic acid, phenylisothiocyanate and methylacrylate to give pyrazolo-[3,4-b]-pyridine derivatives 15-17 was studied. Treatment of 5 with acetic anhydride, phthalic anhydride and carbon disulphide gave pyridine derivatives 18,19 and 1,2,4-triazolo-[3,4-a]-pyridine derivative 20.     

The synthesis of a natural product family: from debromoisolaurinterol to the aplysins

Total syntheses of (±)-aplysin 1, (±)-debromoaplysin 2, (±)-isoaplysin 3, (±)-aplysinol 4, (±)-debromoaplysinol 5, (±)-aplysinal 6, (±)-isolaurinterol 7 and (±)-debromoisolaurinterol 8 are described. Key features are a diastereoselective, sulfur mediated radical cyclisation of diene 12 to give 35; a new radical to polar crossover sequence mediated by Bu₃Sn that transforms diene 12 into (±)-debromoisolaurinterol 8; and a series of biomimetic cyclisation and oxidation reactions.     

Isoagatholactone,a diterpene of a new structural type from the sponge Spongia officinalis

A new diterpene, isoagatholactone, isolated from the sponge Spongia officinalis, is the first natural compound with the carbon skeleton of isoagathic acid (2), the acid-catalyzed cyclization product of agathic acid (1). Structure 4 was assigned to isoagatholactone on spectral grounds and chemical correlation with grindelic acid (5).     

Two new aromatic compounds and a new d-arabinitol ester from the mushroom Hericium erinaceum

Two new aromatic compounds (1, 2), new d-arabinitol ester (3), and two known compounds (4, 5) were isolated from mushroom Hericium erinaceum. The structures of 1–5 were elucidated on the basis of spectral data. Compounds 1, 2, 4, and 5 exhibited α-glucosidasae inhibitory activity.     

Photochemical and thermal rearrangements of a benzoylnaphthobarrelene-like system

8-Benzoyl-9-deuterionaphtho[de-2.3.4]bicyclo[3.2.2]nona-2,6,8-triene (12a)rearranges in a photochemical di-π-methane-type process to the l-benzoylatho[de-[2.3.4]tricyclo[4.3.0.0^2,9]nona-3,7-dienes 14a-c.The dihydro derivate 13a and the hydroxypheoylmethyl analogs 21a and 22a undergo similarly regioselective rearrangements to 15a+c, 23a-c, and 24a, respectively. At 298 K the primary photoreaction directly occus from the S₁(n,π^*) and T₂(n,π^*) states, and it proceeds from T₁(π,π^*) and from S₂(π,π^*) either directly or via T₂. At lower temperature on direct irradiation. S₁→T intersystem crossing and triplet reaction compete with reaction from the singlet. The rearrangement 12a→14a-c proceeds along three reaction paths evolving from the two primary photochemical processes of naphthyl-vinyl and vinyl-vinyl bonding in β-position to the CO (12→25+29). Two ground-state triplet diradical intermediates such as 25 and 27 have been shown to intervene consecutively—for the first time in di-π-methane photochemistry. Each has been characterized by ESR and IR, and the second one additionally by fluorescence and fluorescence excitation, and by laser flash photolysis.The failure of products 14a-c to interconvert photochemically is ascribed to efficient energy dissipation through thermally reversible pbotocleavage of the 3-membered ring.Compounds 12 and 14 thermally interconvert in the dark which constitutes the first example of a ground-state counterpart of a di-π-methane photorearrangement. The thermal reaction includes a path with highly regioselective (and possibly concerted) product formation competing with a stepwise process causing positional scrambling. The sequence 12→14 (photochemically; Φ = 1.0 at 366 nm and 298 K) and an electrophile-catalyzed reversal 14→12 in the dark is a model of a chemical light energy storage cycle which can be conducted without loss of reactants.     

Synthese totale de steroides a cycle c aromatique

The ring-C aromatic analogue 4 of 4-androstene 3,17-dione and its 11 hydroxylated (methoxylated) derivatives 20a(b) were prepared by a practical synthetic route. The key steps of the sequence are achieved in superacid: the cyclisation of diarylethane 5a to phenanthrenone 17, and the hydroxylation by hydrogen peroxide of ketone 4 to phenol 20a.     

Chirality sensing of saccharides using a boronic acid-appended chiral ferrocene derivative

A diboronic acid-appended chiral ferrocene derivative (R)-9 was designed and synthesized. This chiral ferrocene scaffold was obtained by resolution of the diastereomer with a monosaccharide derivative. One can therefore expect that (R)-9 would show d/l selectivity for specific monosaccharides. The complex formation of (R)-9 with various saccharides using the two boronic acid functions was conveniently monitored by a change in the circular dichroism (CD) spectra. The CD spectral change (Δ[θ]) induced by added monosaccharides was chiroselective: in particular, d/l-alloses and d/l-galactoses induced the 8.0- and 7.0-fold difference in the magnitude of the CD spectral change. The association constants for d- and l-saccharides (K_D and K_L, respectively) were determined: among them, (R)-9 showed a significant discrimination ability for mannose (K_L/K_D=2.6) and arabinose (K_L/K_D=1/2.4). The origin of d/l selectivity was discussed on the basis of computational studies on (R)-9·saccharide complexes.     

(R)-2,3-O-Cyclohexylideneglyceraldehyde: a useful template for a simple entry into carbafuranose stereoisomers

(R)-2,3-O-Cyclohexylideneglyceraldehyde 1 provides a simple route for the preparation of carbafuranoses. This has been exemplified by the preparation of 10c and 10d, the derivatives of carba d-xylofuranose and carba-l-arabinofuranose respectively, starting from homoallylic alcohol 2a derived from 1. The key step in this protocol was the intramolecular allylation of 9 promoted by several metals under wet conditions that resulted in the construction of the carbafuranose skeleton of 10. The potential of several metals regarding the efficacy and stereoselectivity of this crucial intramolecular allylation reaction has been studied. The moderate stereoselectivity in all of the successful intramolecular allylations of 9 yielding both d-10c and l-10d as the major products contributed significantly in attaining stereo-divergence in this route. The utility of this route was due to the easy availability of 1, and the operational simplicity as well as scalability of all of the reactions involved.