Synthetic studies on nogalamycin congeners [2]1,2 chiral synthesis of the cdef-ring system of nogalamycin

The chiral synthesis of the (+)-naphthoquinone (4), the CDEF-ring system of nogalamycin congeners, has been accomplished following the synthetic scheme developed for the model (-)-DEF-ring system (3) in the preceding paper. This synthesis features (1) stereoselective construction of the C5'-asymmetric center by introducing the naphthalene moiety into the (-)-methyl ketone (5), the glycoside part, (2) regio-selective oxidation of the 1,4,5,8-tetramethoxynaphthalene moiety with cerium(III) ammonium nitrate , and (3) efficient formation of bicyclic acetal system.     

Synthetic studies on nogalamycin congeners [4]1,2 syntheses amd antitumor activity of various nogalamycin congeners

Various structural types of nogalamycin congeners and their partial structures, which had been previously synthesized in the course of our synthetic studies on the total syntheses or were originally produced by employing the explored synthetic scheme, were subjected to in vitro cytotoxicity and in vivo antitumor activity assay against P388 munne leukemia. These studies obviously disclosed novel aspects of the structure-activity relationships of nogalamycin congeners.     

First total syntheses of (+)-7-deoxynogarol and (+)-7-con-o-methylnogarol

The first total syntheses of (+)-7-deoxynogarol (2) and (+)-7-con-O-methylnogarol (3), the notable members of nogalamycin congeners, were achieved employing the Diels-Alder reaction of the highly functionalized dienes (11 and 20) with the (+)-naphthoquinone (6), the CDEF-ring system of nogalamycin congeners, as a key synthetic step.     

Highly stereoselective synthesis of new α-amino-β-hydroxy six-membered heterocyclic phosphonic acids, serine analogues

The synthesis of 2-hydroxy-4-heterocyclic phosphonic acids was achieved in a six-step sequence from the appropriate ketones. Thus, 2-hydroxyheterocyclic ketone acetals were prepared and then esterified by N-Boc-l-phenylalanine, used as a chiral auxiliary. The resulting heterocyclic acetal esters gave by a one-pot reaction bicyclic ketimines. These imines underwent nucleophilic addition with phosphite to provide efficiently and stereoselectively, under kinetic control, bicyclic aminophosphonates. Cleavage of the phenylalanine moiety by oxidation followed by acidic hydrolysis of the resulting heterocyclohexylphosphonates provided the new (4-amino-3-hydroxypiperidin-4-yl)-, (4-amino-3-hydroxytetrahydro-2H-pyran-4-yl)- and (1-amino-2-hydroxycyclohexyl)phosphonic acids.     

Studies on the syntheses of the aplysiatoxins: Synthesis of a selectively-protected form of the C27 – C30 (dihydroxybutanoate) moiety of oscillatoxin A

A protected form of (R)-3,4-dihydroxybutanoic acid bearing a benzyl protecting group at the C₄ hydroxyl and a dimethylthexylsilyl protecting group at the C₃ hydroxyl was synthesized via a selective Ag(l)-mediated monobenzylation of (R)-methyl 3,4-dihydroxybutanoate. An alternative synthetic route from a chiral allylic ether was successful but problematic. The acid could be cleanly coupled to a model for the C₃ – C₁₁ moiety of the aplysiatoxins.     

Chiral ligand-exchange high-performance liquid chromatography with copper (II)-L-phenylalanine complexes for separation of 3,4-dimethoxy-α-methylphenylalanine racemes

L-3, 4-dimethoxy-α-methylphenylalanine (L-DMMD) is an important intermediate for the synthesis of 3-hydroxy-α-methyl-L-tyrosine (L-methyldopa). This paper describes an efficient, accurate, and low-priced method of high-performance liquid chromatography (HPLC) using chiral mobile phase and conventional C₁₈ column to separate L-DMMD from its enantiomers. The effects of ligands, copper salts, organic modifiers, pHs of mobile phase, and temperatures on the retention factors (k') and selectivity (α) were evaluated to achieve optimal separation performance. Then, thermal analysis of the optimal separation conditions was investigated as well. It was confirmed that the optimal mobile phase was composed of 20 % (v/v) methanol, 8 mM L-phenylalanine (L-Phe), and 4 mM cupric sulfate in water of pH 3.2, and the column temperature was set at 20 °C. Baseline separation of two enantiomers could be obtained through the conventional C₁₈ column with a resolution (R) of 3.18 in less than 18 min. Thermodynamic data (??H and ??S) obtained by Van't Hoff plots revealed the chiral separation was an enthalpy-controlled process. To the best of our knowledge, this is the first report regarding the enantioseparation of DMMD by chiral ligand-exchange HPLC.     

Yellow Circularly Polarized Luminescence from C1-Symmetrical Copper(I) Complexes

The synthesis of chiral C₁-symmetrical copper(I) complexes supported by chiral carbene ligands is described. These complexes are yellow emitters with modest quantum yields. Circularly polarized luminescence (CPL) spectra show a polarized emission band with dissymmetry factors |g_lum|=1.2×10⁻³. These complexes are the first reported examples of molecular copper(I) complexes exhibiting circularly polarized luminescence. In contrast with most CPL-emitting molecules, which possess either helical or axial chirality, the results presented show that simple chiral architectures are suitable for CPL emission and unlock new synthetic possibilities.     

Efficient synthesis and biological activity of enantiomeric pairs of thiolactomycin and its 3-demethyl derivative

The title total synthesis was achieved by employing deconjugative asymmetric α-sulfenylation of the chiral 3-(α,β,γ,δ-unsaturated acyl)oxazolidin-2-one with a 3,3-dimethoxypropyl methanethiosulfonate as a key step. From the biological activity assay carried out using the title compounds, it appeared evident that in vitro antibacterial and mammalian type I FAS inhibitory activity can be cleanly separated by changing not only the substituent at the C₃-position but also the absolute configuration at the C₅-position, and that unnatural (S)-(−)-3-demethylthiolactomycin and its congeners might be usable as selective mammalian type I FAS inhibitors.     

An efficient synthesis of the C27–C45 fragment of lagunamide A,a cyclodepsipeptide with potent cytotoxic and antimalarial properties

An efficient and stereoselective synthesis of the entire C27–C45 moiety of lagunamide A has been achieved from 1-[(4S)-4-benzyl-2-thioxothiazolidin-3-yl]propan-1-one in six steps with 22% overall yield. The key step in the synthesis is an asymmetric acetal aldol reaction featuring the enantioselective addition of a chiral thiazolidinethione-derived titanium enolate to an acetal to establish the stereochemistry at C39.     

Yellow Circularly Polarized Luminescence from C1‐Symmetrical Copper(I) Complexes

The synthesis of chiral C₁‐symmetrical copper(I) complexes supported by chiral carbene ligands is described. These complexes are yellow emitters with modest quantum yields. Circularly polarized luminescence (CPL) spectra show a polarized emission band with dissymmetry factors |g_lum|=1.2×10^?3. These complexes are the first reported examples of molecular copper(I) complexes exhibiting circularly polarized luminescence. In contrast with most CPL‐emitting molecules, which possess either helical or axial chirality, the results presented show that simple chiral architectures are suitable for CPL emission and unlock new synthetic possibilities.     

(2-Fluoro­phenyl­imino)­tri(1-pyrrolyl)­phospho­rane

The P atom of the title compound, C₁₈H₁₆FN₄P, has a slightly distorted tetrahedral geometry. The P—N bond lengths range from 1.671 (3) to 1.680 (3) Å, while the P=N bond is 1.517 (3) Å. The pyrrolyl groups are arranged around the P atom in a chiral propeller-like geometry.     

Diastereoselective Total Syntheses of (±)‐Caseabalansin A and (±)‐18‐Epicaseabalansin A via Intramolecular Robinson‐type Annulation

Highly diastereoselective total syntheses of (±)‐caseabalansin A ( 1 ) and (±)‐18‐epicaseabalansin A ( 2 ) are described in this paper. We revealed that the intramolecular Robinson‐type annulation of an alkynone was effective in the stereocontrolled construction of the bicyclic skeleton of 1 and 2 . Further transformation of the resulting enone, including diastereoselective reduction by LiAlH(OtBu)₃, hydroxy‐group‐directed hydrogenation, cyclization to form the cyclic acetal moiety, and introduction of a side chain by a C(sp³)?C(sp³) Stille coupling reaction, resulted in the total syntheses of (±)‐ 1 and (±)‐ 2 .     

Die Struktur des Antibioticums Roridin H. Verrucarine und Roridine, 20. Mitt. [1]

Structure 1 has been established for roridin H (C₂₉H₃₆O₈), an antibiotic isolated from Myrothecium verrucaria and formerly namend verrucarin H [3]. Base catalysed hydrolysis of 1 gave the known terpene alcohol verrucarol ( 8 ; C₁₅H₂₂O₄) and myrothecinic acid ( 3 ; C₁₄H₁₈O₆), a previously undescribed dicarboxylic acid. The structure of 3 was determined by spectral methods and by chemical cleavage of the unusual acetal group of dimethyl hexahydromyrothecinoate ( 6 ) by hydrolysis with HCl in the presence of 2, 4-dinitrophenylhydrazine, producing the 2, 4-dinitrophenylhydrazone 13 of 3-methylglutaric acid semialdehyde 10 and methyl 6, 7-dihydroxy-octanoate ( 11 ). 13 was transformed in to dimethyl 3-methylglutarate ( 15 ). Cleavage of 11 with HIO₄ gave acetaldehyde and adipic acid semialdehyde ( 17 ) which was oxidized to adipic acid ( 19 ). The 220 MHz NMR spectra of roridin H ( 1 ) and dimethyl hexahydromyrothecinoate ( 6 ) allowed unequivocal assignment of the signals to the majority of the protons in the antibiotic.     

Synthesis of the four stereoisomers of several 3-(1-aminoethyl)pyrrolidines. Important intermediates in the preparation of quinolone antibacterials

The use of S-α-methylbenzyl as a chiral auxiliary at N₁ allowed separation of diastereomeric 2-pyrrolidinones substituted with an ester, ketoester, ketone and oxime at the C₄ position. Reduction of each diastereomer of 4-[1-(hydroxyimino)ethyl]-1-(1-phenylethyl)-2-pyrrolidinone, 10s and 10r , provided a pair of epimeric amines, [ 11sr and 11ss ] and [ 11rs and 11rr ], that were separated by chromatography. The four stereoisomers of 4-(1-aminoethyl)-1-(1-phenylethyl)-2-pyrrolidinone, 11 , were elaborated into several stereochemically pure 3-(1-aminoethyl)pyrrolidines, 1 and 15-23 . These compounds are useful intermediates (C₇ side chains) for quinolone antibacterials.     

Some Diels–Alder adducts of 6-vinyl-1-oxa-4-thiaspiro[4.5]dec-6-ene

6-Vinyl-1-oxa-4-thia­spiro­[4.5]­dec-6-ene has been reacted with dienophiles, such as N-phenyl­male­imide (NPM), N-methyl­triazoline-2,5-dione (MTAD) and di­methyl­acetyl­ene di­carboxyl­ate (DMAD), to assess the 1,3-diastereofacial selection caused by the acetal function. In each case, a mixture of two diastereoisomers was produced. The crystal structures of the products of the addition of NPM and MTAD syn to the acetal oxy­gen, 2-phenyl-2,3,3a,4,5,5a,6,7,8,9,9a,9b-dodeca­hydro-1H-benz­[e]­iso­indole-6-spiro-2′-[1′,3′]­oxa­thiol­ane-1,3-dione, C₂₀H₂₁NO₃S, (IIa), and 2-methyl-5,7,8,9,10,10a-hexa­hydro-1H-1,2,4-triazolo­[1,2-a]­cinnoline-7-spiro-2′-[1′,3′]­oxa­thiol­ane-1,3-dione, C₁₃H₁₇N₃O₃S, (IIIa), respectively, and the product of the addition of DMAD syn to the acetal sulfur, di­methyl 1,2,3,4,4a,7-hexa­hydro­naphthalene-1-spiro-2′-[1′,3′]­oxa­thiol­ane-5,6-di­carboxyl­ate, C₁₆H₂₀O₅S, (IVb), have been determined. All three structures are composed of independent mol­ecules separated by normal van der Waals distances. The 1-oxa-4-thia heterocyclic ring has an envelope conformation in the three structures and the S—Csp³ bond distances differ significantly from each other, as observed in comparable structures; the remaining molecular dimensions are as expected.     

A simple and efficient synthesis of enantiomeric (3aRS,4RS,6aSR)-4-hydroxy-3,3a,4,6a-tetrahydro-1H-cyclopenta[c]furan-1-ones

Diastereomeric amides produced via the cleavage of easily available (±)-7,7-dichloro-4-exo-trimethylsilylbicyclo[3.2.0]hept-2-en-6-one by treatment with (+)-α-methylbenzylamine were transformed into bicyclic lactam-aminals, which can easily be separated by column chromatography on SiO₂. The latter products lead to enantiomeric (3a,6a)-4-hydroxy-3,3a,4,6a-tetrahydro-1H-cyclopenta[c]furan-1-ones after the removal of the chiral auxiliary and epoxidation.     

Synthetic studies on nogalamycin congeners [3]1,2 total syntheses of (+)-nogarene, (+)-7-deoxynogarol,and (+)-7-con-o-methylnogarol

According to the retrosynthetic perspective, the title total syntheses were accomplished by employing the regioselective Diels-Alder reactions of the (+)-naphthoquinone (5), the CDEF-ring system of nogalamycin congeners, with various structural types of dienes (8, 16, and 26). The highly functionalized dienes (16 and 26) incorporating all the functionalities present in the A-rings of (+)-7-deoxynogarol (3) and (+)-7-con-O-methylnogarol (2), were prepared efficiently by way of the 1 ,4-cyclohexadiene and 2-cyclohexanone derivatives (6 and 21), respectively. Reaction mechanism of the key Diels-Alder reaction was also discussed in terms of its stereoselectivity.     

An Efficient Synthesis of a Potential (−)‐Reserpine Intermediate from (−)‐Shikimic Acid of the Chiral Pool

A highly stereoselective route to the polysubstituted chiral octahydrobenzofuran 12 , a potential synthon for the E‐ring core in the (?)‐reserpine synthesis, is described. The α‐bromo acetal 11 was obtained from inexpensive (?)‐shikimic acid ( 3 ) through a series of highly stereoselective chemical reactions (Scheme). Et₃B/Bu₃SnH‐Mediated intramolecular radical cyclization of 11 led to compound 12 with the required configuration.     

Chiral Acylsilanes in Organic Synthesis. Part 3. Silicon-directed stereoselective preparation and Ireland ester-enolate rearrangement of O-acyl-substituted α-silylated allyl alcohols

Stereocontrolled addition of alk-1-enylmetal reagents to the chiral (alkoxymethyl)-substituted acylsilanes (±)- 6 gave rise to α-silylated allyl alcohols, which were converted to the corresponding acetates or propionates 11–16 (Scheme 2). Deprotonation and silylation with Me₃SiCl afforded – in an Ireland ester-enolate-accelerated Claisen rearrangement – stereoselectively αδ-silylated γδ-unsaturated carboxylic acids 18–24 (Scheme 4). The Me₃Si groups in α-position to the COOH group of these compounds were removed chemoselectively in presence of the chiral silyl group in δ-position by treatment with Bu₄NF · 3 H₂O or Et₃N · 3 HF (→ 27–32 ; Scheme 5). The reaction sequence allows a novel stereocontrolled access to chiral C-frameworks possessing a vinylsilane moiety with its full reaction potential.     

Synthetic studies on bafilomycin A1: stereoselective synthesis of the enantiopure C1-C11 fragment

The synthesis of the enantiopure C₁-C₁₁ fragment of bafilomycin A₁ has been achieved with a 4% overall yield over 18 steps from (R)-(+)-citronellol. Key steps involve Sharpless asymmetric epoxidation, Miyashita reaction of a γ,δ-epoxymethacrylate with trimethylaluminum in the presence of water, bis-OTMS selective Swern oxidations, Corey-Fuchs alkyne formation, Negishi's carbometalation, and stereoselective formation of the C₂-C₃ trisubstituted bond of the conjugated diene by a Wittig-type olefination of the α,β-unsaturated C₃-C₁₁ aldehyde with the ylide derived from the readily available phosphonium salt [Cl⁻, Ph₃P⁺CH(OMe)COOMe].