Total synthesis of the 5-epimers of naturally occurring (−)-hyacinthacine A5 and unnatural (+)-hyacinthacine A4

(1R,2S,3R,5S,7aR)-1,2-Dihydroxy-3-hydroxymethyl-5-methylpyrrolizidine 10 [(+)-5-epihyacinthacine A₅] and (1R,2S,3R,5S,7aS)-1,2-dihydroxy-3-hydroxymethyl-5-methylpyrrolizidine 17 [ent-5-epihyacinthacine A₄] have been synthesized by either Horner–Wadsworth–Emmons (HWE) or Wittig methodology using aldehydes 6 and 13, prepared from (2R,3S,4R,5R)-3,4-dibenzyloxy-N-benzyloxycarbonyl-2′-O-tert-butyldiphenylsilyl-2,5-bis(hydroxymethyl)pyrrolidine 5 (partially protected DALDP) and (2R,3S,4R,5S)-3,4-dibenzyloxy-N-benzyloxycarbonyl-2,5-bis(hydroxymethyl)-2′-O-pivaloylpyrrolidine 12 (partially protected DGADP), respectively, and the appropriated ylide, followed by cyclization through an internal reductive amination process of the corresponding intermediate pyrrolidinic ketones 7 and 14 and subsequent deprotection.     

Trennung und absolute Konfiguration der C(8)-epimeren (all-E)-Neochrome (Trollichrome) und -Dinochrome

Separation and Absolute Configuration of the C(8)-Epimeric (app-E)-Neochromes (Trollichromes) and -Dinochromes The C(8′)-epimers of (all-E)-neochrome were separated by HPLC and carefully characterized. The faster eluted isomer, m.p. 197.8–198.3°, is shown to have structure 3 ((3S,5R,6R,3′S,5′R,8′R)-5′,8′-epoxy-6,7-dodehydro-5,6,5′,8′-tetrahydro-β,β-carotene-3,5,3′-triol). To the other isomer, m.p. 195-195.5°, we assign structure 6 , ((3S,5R,6R,3′S,5′R,8′R)-5′,8′-epoxy-6,7-didehydro-5,6,5′,8′-tetrahydro-β,β-carotene-3,5,3′-triol). The already known epimeric dinochromes (= 3-O-acetylneochromes) can now be formulated as 4 and 5 , (‘epimer 1’ and its trimethylsilyl ether) and 7 and 8 , (‘epimer 2’ and its trimethylsilyl ether), respectively.     

Reisolation of Carotenoid 3,6-Epoxides from Red Paprika (Capsicum annuum)

Cucurbitaxanthin A (= (3S,5R,6R,3′R)-3,6-epoxy-5,6-dihydro-β,β- carotene-5,3′-diol; 5 ), cucurbitaxanthin B (= (3S,5R,6R,3′S,5′R,6′S)-3,6,5′, 6′-diepoxy-5,6,5′,6′-tetrahydro-β,β-carotene-5,3′-diol; 6 ), the epimeric cucurbitachromes 1 and 2 (= (3S,5R,6R,3′S,5′R,8′S)- and (3S,5R,6R,3′S,5′R,8′R)-3,6,5′, 8′-diepoxy-5,6,5′,6′-tetrahydro-β,β-carotene-5,3′-diol, resp.; 9/10 ), cycloviolaxanthin (= (3S,5R,6R,3′S,5′R,6′R)-3,6,3′, 6′-diepoxy-5,6,5′,6′-tetrahydro-β,κs-carotene-5,5′-diol; 8 ), and capsanthin 3,6-epoxide (= (3S,5R,6R,3′S,5′R)-3,6-epoxy-5,6-dihydro ?5,3′-dihydroxy-β,κ-caroten-6′-one; 7 ) were isolated from red spice paprika (Capsicum annuum var. longum) and characterized by their ¹H- and ¹³C-NMR, mass, and CD spectra.     

Synthese von diastereo- und enantio-selektiv deuterierten β,ε-, β,β-, β,γ- und γ,γ-Carotinen

Synthesis of Diastereo- and Enantioselectively Deuterated β,ε-, β,β-, β,γ- and γ,γ-Carotenes We describe the synthesis of (1′R, 6′S)-[16′, 16′, 16′-²H₃]-β, εcarotene, (1R, 1′R)-[16, 16, 16, 16′, 16′, 16′-²H₆]-β, β-carotene, (1′R, 6′S)-[16′, 16′, 16′-²H₃]-γ, γ-carotene and (1R, 1′R, 6S, 6′S)-[16, 16, 16, 16′, 16′, 16′-²H₆]-γ, γ-carotene by a multistep degradation of (4R, 5S, 10S)-[18, 18, 18-²H₃]-didehydroabietane to optically active deuterated β-, ε- and γ-C₁₁-endgroups and subsequent building up according to schemes \documentclass{article}\pagestyle{empty}\begin{document}${\rm C}_{11} \to {\rm C}_{14}^{C_{\mathop {26}\limits_ \to }} \to {\rm C}_{40} $\end{document} and C₁₁ → C₁₄; C₁₄+C₁₂+C₁₄→C₄₀. NMR.- and chiroptical data allow the identification of the geminal methyl groups in all these compounds. The optical activity of all-(E)-[²H₆]-β,β-carotene, which is solely due to the isotopically different substituent not directly attached to the chiral centres, is demonstrated by a significant CD.-effect at low temperature. Therefore, if an enzymatic cyclization of [17, 17, 17, 17′, 17′, 17′-²H₆]lycopine can be achieved, the steric course of the cyclization step would be derivable from NMR.- and CD.-spectra with very small samples of the isolated cyclic carotenes. A general scheme for the possible course of the cyclization steps is presented.     

Breaking the Mirror: pH‐Controlled Chirality Generation from a meso Ligand to a Racemic Ligand

To study the conversion from a meso form to a racemic form of tetrahydrofurantetracarboxylic acid (H₄L), seven novel coordination polymers were synthesized by the hydrothermal reaction of Zn(NO₃)₂ ? 6 H₂O with (2S,3S,4R,5R)‐H₄L in the presence of 1,10‐phenanthroline (phen), 2,2′‐bipyridine (2,2′‐bpy), or 4,4′‐bipyridine (4,4′‐bpy): [Zn₂{(2S,3S,4R,5R)‐L}(phen)₂(H₂O)] ? 2 H₂O ( 1 ), [Zn₄{(2S,3R,4R,5R)‐L}{(2S,3S,4S,5R)‐L}(phen)₂(H₂O)₂] ( 2 ), [Zn₂{(2S,3S,4R,5R)‐L}(H₂O)₂] ? H₂O ( 3 ), [Zn₄{(2S,3R,4R,5R)‐L}{(2S,3S,4S,5R)‐L} (2,2′‐bpy)₂(H₂O)₂] ? 2 H₂O ( 4 ), [Zn₂ {(2S,3S,4R,5R)‐L}(2,2′‐bpy)(H₂O)] ( 5 ), [Zn₄{(2S,3R,4R,5R)‐L}{(2S,3S,4S,5R)‐L} (4,4′‐bpy)₂(H₂O)₂] ( 6 ), and [Zn₂ {(2S,3S,4R,5R)‐L}(4,4′‐bpy)(H₂O)] ? 2 H₂O ( 7 ). These complexes were obtained by control of the pH values of reaction mixtures, with an initial of pH 2.0 for 1 , 2.5 for 2 , 4 , and 6 , and 4.5 for 3 , 5 , and 7 , respectively. The expected configuration conversion has been successfully realized during the formation of 2 , 4 , and 6 , and the enantiomers of L, (2S,3R,4R,5R)‐L and (2S,3S,4S,5R)‐L, are trapped in them, whereas L ligands in the other four complexes retain the original meso form, which indicates that such a conversion is possibly pH controlled. Acid‐catalyzed enol–keto tautomerism has been introduced to explain the mechanism of this conversion. Complex 1 features a simple 1D metal–L chain that is extended into a 3D supramolecular structure by π–π packing interactions between phen ligands and hydrogen bonds. Complex 2 has 2D racemic layers that consist of centrosymmetric bimetallic units, and a final 3D supramolecular framework is formed by the interlinking of these layers through π–π packing interactions of phen. Complex 3 is a 3D metal–organic framework (MOF) involving meso‐L ligands, which can be regarded as (4,6)‐connected nets with vertex symbol (4⁵.6)(4⁷.6⁸). Complexes 4 and 5 contain 2D racemic layers and (6,3)‐honeycomb layers, respectively, both of which are combined into 3D supramolecular structures through π–π packing interactions of 2,2′‐bpy. The structure of complex 6 is a 2D network formed by 4,4′‐bpy bridging 1D tubes, which consist of metal atoms and enantiomers of L. These layers are connected through hydrogen bonds to give the final 3D porous supramolecular framework of 6 . Complex 7 is a 3D MOF with novel (3,4,5)‐connected (6³)(4².6⁴)(4².6⁶.8²) topology. The thermal stability of these compounds was also investigated.     

(5R,6S,5′R,6′S)-5,6,5′,6′-Diepoxy-β,β-cartin: Sythese,Spectroskopische, chiroptische und chromatographische Eigenschaften

(5R,6S,5′R,6′S)-5,6,5′,6′-Diepoxy-β,β-Carotene: Synthesis, Spectroscopical and Chiroptical Properties, and HPLC-Behaviour Using the scheme C₁₃ + C₂→C₁₅ + C₁₀→C₄₀, whereby C₁₃ = (5R,6S)-5,6-epoxy-β-ionone [8], the title compound 11a , (R = H), has been prepared and characterized. It exhibits nearly identical CD spectra as violaxanthin ( 11b , R = OH).     

Polynuclear Iron and Rhodium Complexes of 7-Oxa[2.2.1]hericene (2,3,5,6-tetramethylidenebicyclo[2.2.1]heptan-7-one)

μ-Carbonyl(Rh? Rh)di(η⁵-indenyl)[(2R,3S)-C,2,3,C-η-(2,3,4,5-tetramethylidenebicyclo[2.2.1]heptan-7-one)]]-dirhodium(I)(Rh? Rh) (7) and cis-μ-[(2R,3S,5R,6S))-C,2,3,C-η:C,5,6,C-η-(2,3,5,6-tetramethylidenebicyclo[2.2.1]heptan-7-one)]bis[μ-carbonyldi(η⁵-indenyl)dirhodium(I)(Rh? Rh)] ( 8 ) have been prepared. Complex 7 reacts with Fe₂(CO)₉ in hexane/MeOH and gives cis-μ-[(2R,3S,5R,6S] ( 9 ), trans-μ-[(2R,3S,5S,6R)-C,2,3,C-η: C,5,6, C-η-(2,3,5,6-tetramethylidenebicyclo[2.2.1]heptan-7-one)-μ-carbonyldi(η⁵-indenyl)dirhodium(I)(Rh? Rh)-(tricarbonyliron) ( 10 ), and, μ-carbonyl(Rh? Rh)[(2R,3S)-C,2,3,C-η-(2,3-dimethyl-5,6-dimethylidenebicyclo-[2.2.1]hept-2-en-7-one)]di(η⁵-indenyl)dirhodium(I)(Rh? Rh) ( 11 ). Treatment of 7-oxa[2.2.1]hericene ( 4 ) with Fe₂(CO)₉ or (cyclooctene)₂Fe(CO)₃ gave a 1:2 mixture of cis-μ-[(2R,3S,5R,6S)-] ( 12 ) and trans-μ-[(2R,3S,5S,6R)-C,2,3,C-η:C,5,6,C-η-(2,3,5,6-tetramethylidenebicyclo[2.2.1]heptan-7-one)]bis(tricarbonyliron)( 13 ).     

Die diasteromeren Aurochrome: Synthese,Analytik und Chiroptische Eigenschaften

The Diastereomeric Aurochromes: Their Synthesis, Analysis and Chiroptical Properties (all-E)-Aurochrome (5,8:5′,8′-diepoxy-5,8,5′,8′-tetrahydro-β,β-carotene; 1 ) has two pairs of constitutionally identical chiral centres and, therefore, is expected to exist in four pairs of enantiomers and two meso-forms. Using starting materials with well-defined configuration, we performed the syntheses of the following pure aurochromes: (5R,8R,5′R,8′R)-aurochrome ( 2 ) and its racemate, Meso-(5R,8R,5′S,8′S)-aurochrome ( 3 ), (5 R,8 S,5′ R,8′ S)-aurochrome ( 4 ) and its racemate, meso-(5R,8S,5′S,8′R)-aurochrome ( 5 ), (5R,8R,5′R,8′S)-aurochrome ( 6 ) and its racemate. The (5RS,8RS,5′SR,8′RS)-aurochrome ( 7 ) was detected chromatographically, using a HPLC system that allows clean separation of the four racemic- (or optically active) and the two meso-aurochromes. The optically active autochromes 2 and 4 exhibit non-conservative CD spectra with strong Cotton effects of opposite but not mirror-like tracings. Solutions of aurochromes in CHCl₃, in the presence of HCl, undergo epimerization at C(8). Those epimers with CH₃ trans to C(9) slightly predominate under equilibrium conditions. Deprotonation of the phosphonate (±)- 14 with strong base causes isomerization at the terminal oxirane into a dihydrofuran. This reaction allowed convenient syntheses of the diastereoisomeric aurochromes (±)- 2, 3 , (±)- 4, 5 , (±)- 6 , and (±)- 7 and of (5RS, 8RS)- and (5RS, 8SR)-12′-apo-aurochrome-12′-als ( 21 and 22 , respectively).     

ASYMMETRIC SYNTHESIS OF PROLINE USING COBALT(III)-TETRAAMINE COMPLEXES

Abstract

The decarboxylation reaction of δ -cis-β-[Co(L₁)(pdH)]²⁺ complex yielded δ -cis-β-[Co(L₁) (R-pro)]²⁺, while the δ -cis-β-[Co(L₂) (S-pro)]²⁺ was obtained from the reaction of δ -cis-β-[Co(L₂) (pdH)]²⁺, where L₁ is (3R)3-methyl-1, 6-bis[(2S)-pyrrolidin-2-yl]-2, 5-diazahexane, L₂ is (3S) 3-methyl-1, 6-bis-[(2S)-pyrrolidin-2-yl]-2, 5-diazahexane, and pdH is the pyrrolidine-2, 2-dicarboxylate ion. The asymmetrically synthesized prolines were isolated via the decomposition of the decarboxylated complexes. The proline isolated from δ -cis-β-[Co(L₁) (R-pro)]²⁺ showed a specific rotation of +12.0, representing a 24% excess of R-proline over S-proline, while the proline isolated from δ -cis-β-[Co(L₂) (S-pro)]²⁺ showed a specific rotation of -10.0, indicating a 20% excess of S-proline over R-proline.     

Stereochemical revision of communiols D and H through synthesis

Based on the previously revised stereochemistries for communiols A-C, the ent-8-epi- and ent-6-epi-stereoisomers of the original structures proposed for communiols D and H, respectively, were synthesized as highly probable candidates for their genuine structures by using the Sharpless asymmetric dihydroxylation as the source of chirality. Complete accord in spectral properties between each synthetic candidate and the corresponding natural material as well as the fact that communiols A-D and H were all isolated from the same fungal source, led us to the conclusion that the stereochemistries of communiols D and H should also be revised to their (3S,5S,7R,8S,11R)- and (5S,7R,8S)-forms, respectively.     

Über die Carotinoide Flavoxanthin und Chrysanthemaxanthin: 1H-NMR.-, 13C-NMR.- und Massen-Spektren und absolute Konfiguration. (Kritische Überprüfung veröffentlichter chemischer und physikalischer Daten)

On the Carotenoids Flavoxanthin and Chrysanthemaxanthin: ¹H-NMR., ¹³C-NMR. and Mass Spectra, Absolute Configuration, Survey of Published Data Very pure flavoxanthin ( 1 ) and chrysanthemaxanthin ( 2 ) have been reisolated from flower heads of Taraxacum officinale. Both compounds were characterized by chromatographic, chemical and chiroptical data and extensive NMR. measurements. Examination of many published data on 1 and 2 led to the conclusion that flavoxanthin has often been mistaken for chrysanthemaxanthin and vice versa. Chemical degradation of 1 and 2 to (?)-loliolide ( 5 ) coupled with NMR. evidence (ASIS, spin-decoupling, NOE) allowed the assignment of the absolute configurations of 1 and 2 . Thus (+)-flavoxanthin is (3S,5R,8R, 3′R,6′R)-5, 8-epoxy-5, 8-dihydro-β, ε-carotene-3, 3′-diol and (+)-chrysanthemaxanthin its C(8)-epimer.     

Epoxidierung von Cucurbitaxanthin A: Herstellung von Cucurbitaxanthin B und seines 5′,6′-Epimeren

Epoxidation of Cucurbitaxanthin A: Preparation of Cucurbitaxanthin B and of Its 5′,6′-Epimer Cucurbitaxanthin A (= (3S,5R,6R,3′S)-3,6-epoxy-5,6-dihydro-β,β-carotene-5,3′-diol; 1 ) isolated from red pepper (Capsicum annuum var. longum nigrum) was trimethylsiylated and then epoxidized with monoperphthalic acid. After deprotection and chromatographic separation, cucurbitaxanthin B (= (3S,5R,6R, 3′S,5′R,6′S)-3,6:5′,6′-diepoxy-5,6,5′,6′-tetrahydro-β,β-carotene-5,3′-diol; 2 ) and 5′,6′-diepicucurbitaxanthin B (= (3S,5R,6R, 3′S,5′S,6′R)-3,6:5′,6′-diepoxy-5,6,5′,6′-tetrahydro-β,β-carotene-5,3′-diol; 5 ) were obtained and carefully characterized. They show mirror-like CD spectra and, therefore, emphasize the importance of the torsion angle of C(6)–C(7) on the electronic interaction between the polyene chain and the chiral end group.     

Mechanistic and Synthetic Studies on the Formation of 1,2,4-Trioxanes Related to Arteannuin. Photooxygenation of a bicyclic dihydropyran

The photooxygenation of (4R,4aS,7R)-4,4a,5,6,7,8-hexahydro-4,7-dimethyl-3H-2-benzopyran ( 16 ) was performed in (i) MeOH, (ii) acetaldehyde, and (iii) acetone at ?78°. The products obtained respectively were (i) (2R)-2-[(1S,4R)-4-methyl-2-oxocyclohexyl]propyl formate ( 17 ; 72% yield), (ii) 17 (54.5%), (1R,4R,4aS,7R)-3,4,4a,5,6,7-hexahydro-4,7-dimethyl-1H-2-benzopyran-2-yl hydroperoxide ( 19 ; 16.7%), a 12:1 ratio of (3R,4aR,7R,7aS,10R,11aR)-7,7a,8,9,10,11-hexahydro-3,7,10-trimethyl-6H-[2]benzopyrano[1,8a-e]-1,2,4-trioxane ( 20 ) and its C(3)-epimer 21 (17%), together with evidence for the 1,2-dioxetane ( 22 ) originating from the addition of dioxygen to the re-re face of the double bond of 16 , and iii) unidentified products and traces of 22 . Addition of trimethylsilyl trifluoromethanesulfonate (Me₃SiOTf) to the acetone solution of 16 after photooxygenation afforded (4aR,7R,7aS,10R,11aR)-7,7a,8,9,10,11-hexahydro-3,3,7,10-tetramethyl-6H-[2]benzopyrano[1,8a-e]-1,2,4,-trioxane ( 23 , 40%). The photooxygenation of 16 in CH₂Cl₂ at ?78° followed by addition of acetone and Me₃SiOTf afforded 17 (11%), 23 (59%), and (4aR,7R,7aS,10R,11aR)-7,7a,8,9,10,11-hexahydro-3,3,7,10-tetramethyl-6H-[2]benzopyrano[8a,1-e]-1,2,4-trioxane ( 24 ; 5%. Repetition of the last experiment, but replacing acetone by cyclopentanone, gave 17 (16%), (4′aR,7′R,7′aS,10′R,11′aR)-7′,7′a,8′,9′,10′,11′-hexahydro-7′,10′-dimethylspiro[cyclopentane-1,3′-6′H-[2]benzopyrano[1,8a-e]-1,2,4-trixane] ( 25 ; 61%), and (4′aR,7′R,7′aS,10′R,11′aR)-7′,7′a,8′,9′,10′,11′-hexahydro-7′,10′-dimethylspiro[cyclopentane-1,3′-6′H-[2]benzopyrano[8a,1-e]-1,2,4-trixane] ( 26 , 4%). The X-ray analysis of 23 was performed, which together with the NMR data, established the structure of the trioxanes 20, 21, 24, 25 , and 26 . Mechanistic and synthesis aspects of these reactions were discussed in relation to the construction of the 1,2,4-trioxane ring in arteannuin and similar molecules.     

Synthesis of (5R)- and (5S)-5-Methyl-5, 6-dihydrouridine Derivatives

The hydrogenation of 2′, 3′-O-isopropylidene-5-methyluridine (1) in water over 5% Rh/Al₂O₃ gave (5 R)- and (5 S)-5-methyl-5, 6-dihydrouridine (2) , separated as 5′-O-(p-tolylsulfonyl)- (3) and 5′-O-benzoyl- (5) derivatives by preparative TLC. on silica gel and ether/hexane developments. The diastereoisomeric differentiation at the C(5) chiral centre depends upon the reaction media and the nature of the protecting group attached to the ribosyl moiety. The synthesis of iodo derivatives (5 R)- and (5 S)- 4 is also described. The diastereoisomers 4 were converted into (5 R)- and (5 S)-2′, 3′,-O-isopropylidene-5-methyl-2, 5′-anhydro-5, 6-dihydrouridine (7) .     

Polyhydroxylated pyrrolizidines. Part 8. Enantiospecific synthesis of looking-glass analogues of hyacinthacine A5 from DADP

(1R,2S,3S,5R,7aR)-1,2-Dihydroxy-3-hydroxymethyl-5-methylpyrrolizidine[(−)-3-epihyacinthacine A₅, 1a] and (1S,2R,3R,5S 7aS)-1,2-dihydroxy-3-hydroxymethylpyrrolizidine[(+)-3-epihyacinthacine A₅, 1b] have been synthesized either by Wittig's or Horner-Wadsworth-Emmond's (HWE's) methodology using aldehydes 4 and 9, both prepared from (2S,3S,4R,5R)-3,4-dibenzyloxy-2′-O-tert-butyldiphenylsilyl-2,5-bis(hydroxymethyl)pyrrolidine (2, partially protected DADP), and the appropriate ylides, followed by cyclization through an internal reductive amination process of the resulting α,β-unsaturated ketones 5 and 10, respectively, and total deprotection.     

Investigation of the reactions of fluorine containing 3-hydrazino-5H-1, 2, 4-triazino[5, 6-b]indoles with ethyl acetoacetate. Synthesis of some novel tetracyclic ring systems

Novel tetracyclic ring systems viz. 3-methyl-1-oxo-12H-1, 2, 4-triazepino[3′,4′:3, 4][1, 2, 4]triazino[5, 6-b]indole ( 4a ) and 3-methyl-5-oxo-12H-1, 2, 4-triazepino[4′,3′:2, 3][1, 2, 4]triazino[5, 6-b]indole ( 5a ), having angular and linear structures respectively, were synthesized by the cyclization of 3-oxobutanoic acid [5H-1, 2, 4-triazino-[5, 6-b]indole-3-yl]hydrazone ( 3a ). However, cyclization of 3b (R = CH_a, R¹ = R² = H) afforded the angular product 4b exclusively. Moreover, cyclization of 3c (R = R³ = H, R¹ = F) yielded 7-fluoro-1-0xo-10H-1, 3-imidazo[2′,3′:3, 4][1, 2, 4]triazino[5, 6-b]indole ( 6c ) and 7-fluoro-3-oxo-10H-1, 3-imidazo[3′,2′:2, 3][1, 2, 4]triazino-[5, 6-b]indole ( 7c ) instead of the expected triazepinone derivatives. Compound 3d (R = R¹ = H, R² = CF₃) also gave an imidazole derivative but only one angular product was obtained. In all these reactions, formation of the angular product involving cyclization at N-4 is favoured. Characterization of these products have been done by elemental analyses, ir, pmr, ¹⁹F nmr and mass spectral studies.     

Polyhydroxylated pyrrolizidines. Part I: Short and highly stereocontrolled syntheses of hyacinthacines

Two short and highly stereocontrolled syntheses for 7a-epi-hyacinthacine A₂ (7-deoxyalexine) 3 and 5,7a-diepi-hyacinthacine A₃ 4 are, respectively, reported herein. An appropriately protected polyhydroxypyrrolidine, (2R,3R,4R,5S)-3,4-dibenzyloxy-N-benzyloxycarbonyl-2′-O-tert-butyldiphenylsilyl-2,5-bis(hydroxymethyl)pyrrolidine 5, readily available from d-fructose, was chosen as the chiral starting material.     

Absolute Konfiguration von Antheraxanthin, «cis-Antheraxantliirt» und der diastereomeren Mutatoxanthine

Absolute Configuration of Antheraxanthin, ‘cis-Aritheraxanthin’ and of the Stereoisomeric Mutatdxanthins The assignement of structure 2 to antheraxanthin (all-E)-(3 S, 5 R, 6 S, 3′ R)-5,6-epoxy-5,6-dihydro-β,β-carotene-3,3′-diol and of 1 to ‘cis-antheraxanthin’ (9Z)-(3 S, 5 R, 6 S, 3′ R)-5,6-epoxy-5,6-dihydro-β,β-carotene-3,3′-diol is based on chemical correlation with (3 R, 3′ R)-zeaxanthin and extensive ¹H-NMR. measurements at 400 MHz. ‘Semisynthetic antheraxanthin’ ( = ‘antheraxanthin B’) has structure 6 . For the first time the so-called ‘mutatoxanthin’, a known rearrangement product of either 1 or 2 , has been separated into pure and crystalline C(8)-epimers (epimer A of m.p. 213° and epimer B of m.p. 159°). Their structures were assigned by spectroscopical and chiroptical correlations with flavoxanthin and chrysanthemaxanthin. Epimer A is (3 S, 5 R, 8 S, 3′ R)-5,8-epoxy-5,8-dihydro-β,β-carotene-3,3′-diol ( 4 ; = (8 S)mutatoxanthin) and epimer B is (3 S, 5 R, 8 R, 3′ R)-5,8-epoxy-5,8-dihydro-β,β-carotene-3,3′-diol ( 3 ; = (8 R)-mutatoxanthin). The carotenoids 1 – 4 have a widespread occurrence in plants. We also describe their separation by HPLC. techniques. CD. spectra measured at room temperature and at ? 180° are presented for 1 – 4 and 6 . Antheraxanthin ( 2 ) and (9Z)-antheraxanthin ( 1 ) exhibit a typical conservative CD. The CD. Spectra also allow an easy differentiation of 6 from its epimer 2 . The isomeric (9Z)-antheraxanthin ( 1 ) shows the expected inversion of the CD. curve in the UV. range. The CD. spectra of the epimeric mutatoxanthins 3 and 4 (β end group) are dissimilar to those of flavoxanthin/chrysanthemaxanthin (ε end group). They allow an easy differentiation of the C (8)-epimers.     

Über Pterinchemie. 85. Mitteilung. Polyacetylierte 5,6,7,8-Tetrahydro-D- und -L-neopterine. Ein besonderer Fall von N(5)-Alkylierung des 5,6,7,8-Tetrahydroneopterin-Gerüstes

Polyacetylated 5,6,7,8-Tetrahydro-D - and L -neopterins. A Special Case of N(5)-Alkylation of 5,6,7,8-Tetrahydroneopterins Improved conditions are reported for the preparation of the earlier described (6R)- and (6S)-1′-O,2′-O,3′-O,2-N,5-pentaacetyl-5,6,7,8-tetrahydro-L -neopterins, one of which could be obtained as pure crystals. Its structure, determined by X-ray-diffraction analysis, corresponds to the (6R)-enantiomer. The method has also been used to make the corresponding D -diastereoisomers. Further acetylation of (6RS)-1′-O,2′-O,3′-O,2-N-tetraacetyl-5,6,7,8-tetrahydro-D -neopterin under drastic conditions yields a mixture of several polyacetylated D -neopterin derivatives and a polyacetylated ethyl-tetrahydro-D -neopterin which was isolated in crystalline form and established by X-ray-diffraction analysis to be (6R)-1′-O,2′-O,3′-O,4-O,2-N,2-N,8-heptaacetyl-5-ethyl-5,6,7,8-tetrahydro-D -neopterin.     

DFT 1H–1H coupling constants in the conformational analysis and stereoisomeric differentiation of 6‐heptenyl‐2H‐pyran‐2‐ones: configurational reassignment of synargentolide A

Density functional theory (DFT) ¹H–¹H NMR coupling constant calculations, including solvation parameters with the polarizable continuum model B3LYP/DGDZVP basis set together with the experimental values measured by spectral simulation, were used to predict the configuration of hydroxylated 6‐heptenyl‐5,6‐dihydro‐2H‐pyran‐2‐ones 1 , 2 , 4 , and 7 , allowing epimer differentiation. Modeling of these flexible compounds requires the inclusion of solvation models that account for stabilizing interactions derived from intramolecular and intermolecular hydrogen bonds, in contrast with peracetylated derivatives ( 3 , 5 , and 6 ) in which the solvation consideration can be omitted. Using this DFT NMR integrated approach as well as spectral simulation, the configurational reassignment of synargentolide A ( 8 ) was accomplished by calculations in the gas phase among four possible diastereoisomers ( 8–11 ). Calculated ³J_H,H values established its configuration as 6R‐[4′S,5′S,6′S‐(triacetyloxy)‐2E‐heptenyl]‐5,6‐dihydro‐2H‐pyran‐2‐one ( 8 ), in contrast with the incorrect 6R,4′R,5′R,6′R‐diastereoisomer previously proposed by synthesis ( 12 ). Application of this approach increases the probability for successful enantiospecific total syntheses of flexible compounds with multiple chiral centers. Copyright © 2014 John Wiley & Sons, Ltd.