Nucleosides and Nucleotides. Part 13. Synthesis and spectral properties of 1- (3′-O-Phosphoryl-2′-deoxy-β -D-ribofuranosyl)-2(1H)-pyridone-(3′-5′)-1-(2′-deoxy-β-D-ribofuranosyl)-2 (1H)-pyridone

The dinucleoside phosphate Π_dpΠ_d ( 4 ) was synthesized from the monomers 1-(5′-O-monomethoxytrityl - 2′ - deoxy - β - D - ribofuranosyl) - 2 (1 H) - pyridone ((MeOTr) Π_d, 2 ) and 1-(5′-O-phosphoryl-3′-O-acetyl-2′-deoxy-β-D -ribofuranosyl)-(1H)-pyridone (pΠ_d(Ac), 3 ). Its 6.4% hyperchromicity and an analysis of the ¹H-NMR. spectra indicate that the conformation and the base-base interactions in 4 are similar to those in natural pyrimidine dinucleoside phosphates.     

Synthese neuer Heptafulvene,Röntgenstrukturanalyse von ‘8,8-(1′,4′-Dioxotetramethylen)heptafulven’(2-(Cyclohepta-2,4,6-trien-1-yliden)cyclopentan-1,3-dion)

Synthesis of New Heptafulvenes; X-Ray Analysis of ‘8,8-(1′,4′-Dioxotetramethylene)heptafulvene’ (2-(Cyclohepta-2,4,6-trien-1-ylidene)cyclopentane-1,3-dione) Experimental procedures for the synthesis of heptafulvene ( 3a ), 8,8-tetramethylene heptafulvene ( 3c ) and ‘8,8-(1′,4′-dioxotetramethylene) heptafulvene’ (2-(cyclohepta-2,4,6-trien-1-ylidene)-cyclopentane-1,3-dion; 3d ) are described. The most important sequences include a low-temperature reaction of tropylium salts with lithium or Grignard carbenoids (Scheme 1) to give 3a and 3b as well as hydride abstraction from substituted cycloheptatrienes followed by deprotonation to give 3c and 3d . Limitations of these sequences are discussed. Two other heptafulvenes 3h and 3i are available by silylation of heptafulvenolates according to well-known procedures. NMR-Spectroscopic evidence as well as an X-ray analysis of 3d are presented. Compound 3d is a relatively polar heptafulvene with a planarised seven-membered ring as well as a partly delocalized π system.     

2′, 3′-Dideoxy-3′-fluorotubercidin and Related Dideoxyribonucleosides:. Synthesis via a 2′-deoxy-3′-oxoribofuranoside intermediate and conformation studies

An efficient synthesis of the unknown 2′-deoxy-D-threo-tubercidin ( 1b ) and 2′, 3′-dideoxy-3′-fluorotubercidin ( 2 ) as well as of the related nucleosides 9a, b and 10b is described. Reaction of 4-chloro-7-(2-deoxy-β-D-erythro-pentofuranosyl)-7H-pyrrolo[2,3-d]pyrimidine ( 5 ) with (tert-butyl)diphenylsilyl chloride yielded 6 which gave the 3′-keto nucleoside 7 upon oxidation at C(3′). Stereoselective NaBH₄ reduction (→ 8 ) followed by deprotection with Bu₄NF(→ 9a )and nucleophilic displacement at C(6) afforded 1b as well as 7-deaza-2′-deoxy-D-threo-inosine ( 9b ). Mesylation of 4-chloro-7-{2-deoxy-5-O-[(tert-butyl)diphenylsilyl]-β-D-threo-pentofuranosyl}-7H-pyrrolo[2,3-d]-pyrimidine ( 8 ), treatment with Bu₄NF (→ 12a ) and 4-halogene displacement gave 2′, 3′-didehydro-2′, 3′-dideoxy-tubercidin ( 3 ) as well as 2′, 3′-didehydro-2′, 3′-dideoxy-7-deazainosne ( 12c ). On the other hand, 2′, 3′-dideoxy-3′-fluorotubercidin ( 2 ) resulted from 8 by treatment with diethylamino sulfurtrifluoride (→ 10a ), subsequent 5′-de-protection with Bu₄NF (→ 10b ), and Cl/NH₂ displacement. ¹H-NOE difference spectroscopy in combination with force-field calculations on the sugar-modified tubercidin derivatives 1b , 2 , and 3 revealed a transition of the sugar puckering from the ^3′T_2′ conformation for 1b via a planar furanose ring for 3 to the usual ^2′T_3′ conformation for 2.     

Eine Suche nach 3′-Epilutein ( = (3R,3′S,6′R)-β,ε-Carotin-3,3′-diol) und 3′, O-Didehydrolutein (=(3R, 6′R)-3-Hydroxy-β,ε-carotin-3′-on) in Eigelb,in Blüten von Caltha palustris und in Herbstblättern

Search for the Presence in Egg Yolk, in Flowers of Caltha palustris and in Autumn Leaves of 3′-Epilutein ( =(3R,3′S,6′R)-β,ε-Carotene-3,3′-diol) and 3′,O-Didehydrolutein ( =(3R,6′R)-3-Hydroxy-β,ε-carotene-3′-one) 3′.O-Didehydrolutein ( =(3R, 6′R)-3-hydroxy-β,ε-carotene-3′-one; 2) has been detected in egg yolk and in flowers of Caltha palustris. This is the first record for its occurrence in a plant. The compound shows a remarkable lability towards base; therefore, it may have been overlooked til now, because it is destroyed under the usual conditions of saponification of the carotenoid-esters. One of the many products formed from 2 with 1% KOH in methanol has been purified and identified as the diketone 3 ( =(3R)-3-hydroxy-4′, 12′-retro-β,β-carotene-3′,12′-dione). The identification of this transformation product from lutein might throw a new light on the metabolism of this important carotenoid in green plants. 3′-Epilutein ( =(3R,3′S,6′R)-β,ε-carotene-3,3′-diol; 1) was not detected in egg yolk, but is present besides lutein in flowers of C. palustris, thus confirming an earlier report of the occurrence of an isomeric (possibly epimeric) lutein (‘calthaxanthin’) in that plant [21]. We were not able to detect even traces of 1 or 2 in the carotenoid fraction from autumn leaves of Prunus avium (cherry), Parrotia persica, Acer montanum (maple) and yellow needles of Larix europaea (larch). α-Cryptoxanthin (4) , a very rare carotenoid, was isolated in considerable quantity for the first time from flowers of C. palustris.     

Variants of Solid-State and Solution Structures of (η3-Allyl)- {2-[2′-(diphenylphosphino)phenyl]-4,5-dihydrooxazole-P,N}palladium(II) hexafluorophosphates and tetraphenylborates

Crystal and solution structures of the enantiomerically pure and the racemic pairs of (η³-allyl) {2-[2′-(diphenylphosphino)phenyl]-4,5-dihydro-4-phenyloxazole}palladium(II) hexafluorophosphates ( 1 , and rac- 1 , resp.) and tetraphenylborates ( 2 , and rac- 2 , resp.) as well as (η³-allyl){2-[2′-(diphenylphosphino)phenyl]-4,5-dihydro-4-isopropyloxazole}palladium(II) tetraphenylborate ( 3 ) were characterized by X-ray crystallography and ¹H-NMR spectroscopy. In the solid state, rac- 1 and rac- 2 proved to be disordered with both diastereoisomeric complexes in the crystal. The complexes 2 and 3 exist only in the ‘exo’ form. The X-ray structures show that the [Pd^II(η³-allyl)] moiety may adopt different configurations between a nearly symmetrical three-electron Pd^II(π³-allyl) system and an asymmetrical allyl group with a η¹- and a η²-bonding to the metal center. The [Pd^II(η³-allyl)] system of rac- 1 and of ‘endo’ rac- 2 is closer to the former, and that of 2 , ‘exo’-rac- 2 , and 3 closer to the later geometry. The ¹H-NMR spectra of the hexafluorophosphates 1 and rac- 1 show two sets of signals of the allylic protons in an ‘exo’/‘endo’ ratio of 2:3. The tetraphenylborates 2, rac- 2 , and 3 give only one set of broad signals of the allylic protons.     

Asymmetric Synthesis of (S)-5,5,5,5′,5′,5′,5′-Hexafluoroleucine

(S)-5,5,5,5′,5′,5′-Hexafluoroleucine ((S)- 13 ) of 81 % ee is prepared from hexafluoroacetone ( l ) and ethyl bromopyruvate (= ethyl 2-oxopropanoate) in 7 steps with an overall yield of 18% (Schemes 1 and 2). Key step in this sequence is the highly enantioselective reduction of the carbonyl group in α-keto ester 4 either by bakers' yeast (91 % ee) or by ‘catecholborane’ 6 utilizing an oxazaborolidine catalyst, yielding hydroxy ester (R)- 5 with 99% ee. The absolute configuration was determined by X-ray analysis of the HCl adduct (S,R)- 9b of (2S)-N-[(R)- l-phenylethyl]-5,5,5,5′,5′,5′-hexafluoroleucine ethyl ester.     

1,2-Epoxycarotinoide. 6. Mitteilung. Isolierung von 1′,2′-Epoxy-1′,2′-dihydro-ε,ψ-carotin aus der ‘Delta-Mutanten’-Tomate

1,2-Epoxycarotenoids Isolation of 1′,2′-Epoxy-1′,2′-dihydro-ε,ψ-carotene from a ‘Delta Mutant’ Tomato From ‘Delta Mutant’ tomatoes, optically active 1′,2′-epoxy-1′,2′-dihydro-ε,ψ-carotene ( 7 ) was isolated. According to the CD data, the configuration is 6R and presumably 2′S.     

Analogs of Cinchona Alkaloids Incorporating a 9,9′-Spirobifluorene Moiety

The Cinchona alkaloid analogs (+)- and (?)- 5 with a quinuclidine-2-methanol residue attached to C(2) of a 9,9′-spirobifluorene moiety were prepared as a racemic mixture by reacting lithiated 2-bromo-9,9′-spirobifluorene 7 with (2-ethoxycarbonyl)quinuclidine (±)- 6 to give ketone (±)- 8 , followed by diastereoselective reduction with diisobutylaluminum hydride (DIBAL-H). The absolute configuration at C(9) and C(8), i.e., at the methanol bridge and the adjacent quinuclidine C-atom, in the two enantiomers of 5 is identical to the configuration at the corresponding centers in (?)-quinine ( 1 ) and (+)-quinidine ( 2 ), respectively. For the optical resolution of (±)- 5 , a chiral stationary phase for HPLC was prepared by covalently bonding quinine via a thiol spacer to a silica-gel surface. The enantiomer separation was accomplished at an α value of 1.61 with (±)- 5 being eluted last, in agreement with ¹H-NMR studies in CDCl₃ which showed that (+)- 5 underwent a more stable host-guest association with quinine than (?)- 5 . ¹H{¹H} Nuclear Overhauser effect (NOE) difference spectroscopical analysis of the host-guest associations with quinine in CDCl₃, combined with computer-model examinations, allowed the assignment of the absolute configurations as (+)-(8R,9S)- 5 and (?)-(8S,9R)- 5 . A detailed conformational analysis displayed excellent agreement between the results of computational methods (Monte Carlo multiple minimum simulations, analyses of the total energy as a function of the flexible dihedral angles in the molecule) and ¹H{¹H}-NOE difference spectroscopical data. It was found that (?)- 5 and (+)- 5 differ significantly in their conformational preference from their natural counterparts quinine ( 1 ) and quinidine ( 2 ). Whereas the natural alkaloids prefer the ‘open’ conformation, with the quinuclidine N-atom pointing away from the quinoline ring, analog (±)- 5 adopts preferentially (by ca. 4 kcal mol^?1) a ‘closed’ conformation, in which the quinuclidine N-atom points into the cleft of the 9,9′-spirobifluorene moiety. Since the basic quinuclidine N-atom in the ‘closed’ conformation is sterically shielded from forming strong H-bonds, the new Cinchona alkaloid analogs form less stable host-guest associations via H-bonding than quinine or quinidine.     

Nucleic-Acid Analogues with Constraint Conformational Flexibility in the Sugar-Phosphate Backbone (‘Bicyclo-DNA’). Part 1. Preparation of (3S,5′R)-2′-Deoxy-3′,5′-ethano-αβ-D-ribonucleosides (‘Bicyclonucleosides’)

We describe the synthesis of 2′-deoxy-3′,5′-ethano-D -ribonucleosides 1 – 8 (= (5′,8′-dihydroxy-2′-oxabicyclo-[3.3.0]oct-3′-yl)purines or -pyrimidines) of the nucleobases adenine, thymine, cytosine, and guanine. They differ from natural 2′-deoxyribonucleosides only by an additional ethylene bridge between the centers C(3′) and C(5′). The configuration at these centers (3S,5′R) was chosen as to match the geometry of a repeating nucleoside unit in duplex DNA as close as possible. These nucleosides were designed to confer, as constituents of an oligonucleotide chain, a higher degree of preorganization of a single strand for duplex formation with respect to natural DNA, thus leading to an entropic advantage for the pairing process. The synthesis of these ‘bicyclonucleosides’ was achieved by construction of an enantiomerically pure carbohydrate precursor 18 / 19 (Schemes 1), which was then converted to the corresponding nucleosides by known methods in nucleoside synthesis (Schemes 2 and 3). In all cases, both anomeric forms of the nucleosides were obtained in pure crystalline form, the relative configuration of which was established by ¹H-NMR-NOE spectroscopy. A conformational analysis of the nucleosides with β-configuration at the anomeric center by means of X-ray and ¹H-NMR (including NOE) spectroscopy show the furanose part of the molecules to adopt uniformly a 1′exo-conformation with the base substituents preferentially in the anti-range in the pyrimidine nucleosides (anti/syn ca. 2:1) distribution in the purine nucleosides (in solution).     

[2+3] Cycloadditions of ‘Thiocarbonyl Ylides’ (= (Alkylidenesulfonio)methanides) and diazoalkanes with N,N′-(thiocarbonyl)diimidazole (= 1,1′-(carbonothioyl)bis[1H-imidazole])

Heating of a mixture of N,N′-(thiocarbonyl)diimidazole (= 1,1′-(carbonothioyl)bis[1H-imidazole]; 1 ) and 2,5-dihydro-1,3,4-thiadiazole 2a or 2b gave the 1,3-dithiolanes 4a and 4b , respectively, via a regiospecific 1,3-dipolar cycloaddition of the corresponding ‘thiocarbonyl methanides’ 3a , b onto the C?S group of 1 (Schemes 1 and 2). The adamantane derivative 4b was not stable in the presence of 1H-imidazole and during chromatographic workup. The isolated 1,3-dithiole 5 is the product of a base-catalyzed elimination of 1H-imidazole from the initial cycloadduct 4b . The formation of the S,N-acetal 6 can be rationalized by a protonation of the ‘thiocarbonyl ylide’ 3b followed by a nucleophilic addition of 1H-imidazole. With the diazo compounds 8a–e (Scheme 3) 1 underwent a regiospecific 1,3-dipolar cycloaddition to give the corresponding 2,5-dihydro-1,3,4-thiadiazole derivatives 9 , which spontaneously eliminated 1H-imidazole to yield (1H-imidazol-1-yl)-1,3,4-thiadiazoles 10 . The structures of 10a and 10d were established by X-ray crystallography. In the case of diazodiphenylmethane ( 8f ), the initial cycloadduct 9f decomposed via a ‘twofold extrusion’ of N₂ and S to give 1,1′-(2,2-diphenylethenylidene)bis[1H-imidazole] ( 11 ; Scheme 3).     

5‐Ethyl‐2′‐deoxy‐4′‐thiouridine (R)‐S‐oxide monohydrate

The pyrimidine ring of the title compound, C₁₁H₁₆N₂O₅S·H₂O, is planar to within 0.026 (1) Å and makes an angle of 77.73 (8)° with the mean plane of the thiosugar ring. In terms of standard nucleoside nomenclature, this ring has a C1′‐exo,C2′‐endo conformation. The O5′—C5′—C4′—C3′ torsion angle is ?167.4 (2)° and the glycosidic S4′—C1′—N1—C2 torsion angle is ?101.8 (2)° (anti).     

Spirocyclization of 1-(o-aminophenyl)cycloalkanols and 1-(2′-amino-3′-pyridinyl)cycloalkanols

A one-step synthesis of spiro[cycloalkane-1,4′-2H-3′,1-benzoxazin]-2′-ones and spiro[cycloalkane-1,4′-1H-pyrido[2′,3′-d][1,3′]oxazin]-2′-ones, obtained in good yield from the corresponding 1-(o-aminophenyl) and 1-(2′-amino-3′-pyridinyl)cycloalkanols is described using ethyl carbonate in presence of n-butyllithium.     

Nucleoside und Nucleotide. Teil 10. Synthese von Thymidylyl-(3′-5′) -thymidylyl-(3′-5′)-1-(2′-desoxy-β-D-ribofuranosyl)-2(1 H)-pyridon

Nucleosides and Nucleotides. Part 10. Synthesis of Thymidylyl-(3′-5′)-thymidylyl-(3′-5′)-1-(2′-deoxy-β-D - ribofuranosyl)-2(1 H)-pyridone The synthesis of 5′-O-monomethoxytritylthymidylyl-(3′-5′)-thymidylyl-(3′-5′)-1-(2′-deoxy-β-D -ribofuranosyl)-2(1H)-pyridone ((MeOTr)T_dpT_dp∏_d, 5 ) and of thymidylyl-(3′-5′)-thymidylyl-(3′-5′)-1-(2′-deoxy-β-D -ribofuranosyl)-2(1 H)-pyridone (T_dpT_dp∏_d, 11 ) by condensing (MeOTr) T_dpT_d ( 3 ) and p∏_d(Ac) ( 4 ) in the presence of DCC in abs. pyridine is described. Condensation of (MeOTr) T_dpT_dp ( 6 ) with Π_d(Ac) ( 7 ) did not yield the desired product 5 because compound 6 formed the 3′-pyrophosphate. The removal of the acetyl- and p-methoxytrityl protecting group was effected by treatment with conc. ammonia solution at room temperature, and acetic acid/pyridine 7 : 3 at 100°, respectively. Enzymatic degradation of the trinucleoside diphosphate 11 with phosphodiesterase I and II yielded T_d, pT_d and p∏_d, T_dp and Π_d, respectively, in correct ratios.     

Nucleoside und Nucleotide. Teil 16. Verhalten von 1-(2′-Desoxy-β-D-ribofuranosyl)-2(1 H)-pyrimidinon-5′-triphosphat, 1-(2′-Desoxy-β-D-ribofuranosyl)-2(1 H)pyridinon-5′-triphosphat und 4-Amino-1-(2′-Desoxy-β-D-ribofuranosyl)-2(1 H)-pyridinon-5′-triphosphat gegenüber DNA-Polymerase

Nucleosides and Nucleotides. Part 16. The Behaviour of 1-(2′-Deoxy-β-D -ribofuranosyl)-2(1H)-pyrimidinone-5′-triphosphate, 1-(2′-Deoxy-β-D -ribofuranosyl-2(1H))-pyridinone-5′-triphosphate and 4-Amino-1-(2′-desoxy-β-D -ribofuranosyl)-2(1H)-pyridinone-5′-triphosphate towards DNA Polymerase The behaviour of nucleotide base analogs in the DNA synthesis in vitro was studied. The investigated nucleoside-5′-triphosphates 1-(2′-deoxy-β-D -ribofuranosyl)-2(1 H)-pyrimidinone-5′-triphosphate (pppM_d), 1-(2′-deoxy-β-D -ribofuranosyl)-2(1 H)-pyridinone-5′-triphosphate (pppII_d) and 4-amino-1-(2′-deoxy-β-D -ribofuranosyl)-2(1 H)-pyridinone-5′-triphosphate (pppZ_d) can be considered to be analogs of 2′-deoxy-cytidine-5′-triphosphate. However, their ability to undergo base pairing to the complementary guanine is decreased. When pppM_d, pppII_d or pppZ_d are substituted for pppC_d in the enzymatic synthesis of DNA by DNA polymerase no incorporation of these analogs is observed. They exhibit only a weak inhibition of the DNA synthesis. The mode of the inhibition is uncompetitive which shows that these nucleotide analogs cannot serve as substrates for the DNA polymerase.     

Ein neuer Zugang zu 2′-O-Alkylribonucleosiden und Eigenschaften deren Oligonucleotide

A New Access to 2′-O-Alkylated Ribonucleosides and Properties of 2′-O-Alkylated Oligoribonucleotides A general access to 2′-O-alkylated ribonucleosides using the key intermediate 5 is presented. The incorporation of 2′-O-‘ethyleneglycol’- and 2′-O-‘glycerol’-substituted (i.e., 2′-O-(2-hydroxyethyl)- and 2′-O-(2,3-dihydroxypropyl)-substituted) ribonucleosides into oligonucleotides affords a new generation of oligonucleotides with high affinity for RNA, high specificity, and increased nuclease resistance.     

Nucleotides Part LI. Synthesis and biological activities of (2′-5′)adenylate trimer conjugates with 2′-terminal 3′-O(ω-hydroxyalkyl) and 3′-O-(ω-carboxyalkyl) spacers

An efficient strategy for the synthesis of (2′-5′)adenylate trimer conjugates with 2′-terminal 3′-O-(ω-hydroxyalkyl) and 3′-O-(ω-carboxyalkyl) spacers is reported. Npeoc-protected adenosine building blocks 37--40 for phosphoramidite chemistry carrying a 3′-O-[11-(levulinoyloxy)undecyl], 3′-O-{2-[2-(levulinoyloxy)ethoxy]ethyl}, 3′-O-[5-(2-cyanoethoxycarbonyl)pentyl], and 3′-O-{5-[(9H-fluoren-9-ylmethoxy)carbonyl]pentyl} moiety, respectively, were prepared (npeoc = 2-(4-nitrophenyl)ethoxycarbonyl). Condensation with the cordycepin (3′-deoxyadenosine) dimer 1 led to the corresponding trimers 42, 43, 47 , and 48. Whereas the levulinoyl (lev) and 9H-fluoren-9-ylmethyl (fm) blocking groups could be cleaved off selectively from the trimers 42, 43 , and 48 yielding the intermediates 44, 45 , and 49 for the synthesis of the 3′-O-(ω-hydroxyalkyl)trimers 53, 54 and the cholesterol conjugates 59--61 , the 2-cyanoethyl (ce) protecting group of 47 , however, could not be removed in a similar manner from the carboxy function. Trimer 47 served as precursor for the preparation of the trimer 55 with a terminal 3′-O-(5-carboxypentyl)adenosine moiety. The metabolically stable 3′-O-alkyl-(2′--5′)A derivatives were tested regarding inhibition of HIV-1 syncytia formation and HIV-1 RT activity. Only the conjugate 59 showed significant effects, whereas the trimers 53--55 and the conjugates 60 and 61 were less potent inhibitors, even at 100-fold larger concentrations.     

Nucleotides. Part XXXV. Synthesis of 3′-deoxyadenylyl-(2′–5′)-3′-deoxyadenyIyl-(2′–ω)-9-(ω-hydroxyalkyl)adenines

Via the phosphotriester approach, new structural analogs of (2′–5′)oligoadenyiates, namely 3′-deoxyadenylyl-(2′–5′)-3′-dcoxyadenylyl-(2′–ω)-9-(ω-hydroxyalkyl)adenines 18 – 21 , have been synthesized (see Scheme) which should preserve biological activity and show higher stability towards phosphodiesterases. The newly synthesized oligonucleotides 18 – 21 have been characterized by ¹H-NMR spectra, TLC, and HPLC analysis.     

(all-E)-12′-Apozeaxanthinol,Persicaxanthine und Persicachrome

( all-E)-12′-Apozeanthinol, Persicaxanthine, and Persicachromes Reexamination of the so-called ‘persicaxanthins’ and ‘persicachromes’, the fluorescent and polar C₂₅-apocarotenols from the flesh of cling peaches, led to the identification of the following components: (3R)-12′-apo-β-carotene-3,12′-diol ( 3 ), (3S,5R,8R, all-E)- and (3S,5R,8S,all-E)-5,8-epoxy-5,8-dihydro-12′-apo-β-carotene-3,12′-diols (4 and 5, resp.), (3S,5R,6S,all-E)-5,6-epoxy-5,6-dihydro-l2′-apo-β-carotene-3,12′-diol =persicaxanthin; ( 6 ), (3S,5R,6S,9Z,13′Z)-5,6-dihydro-12′apo-β-carotene-3,12′-diol ( 7 ; probable structure), (3S,5R,6S,15Z)-5,6-epoxy-5,6-dihydro-12′-apo-β-carotene-3,12′-diol ( 8 ), and (3S,5R,6S,13Z)-5,6-epoxy-5,6-dihydro-12′-apo-β-carotene-3,12′-diol ( 9 ). The (Z)-isomers 7 – 9 are very labile and, after HPLC separation, isomerized predominantly to the (all-E)-isomer 6 .     

5‐Methyl‐2′‐deoxy‐4′‐thio‐α‐uridine (R)‐­S‐oxide

The pyrimidine ring of the title compound, C₁₀H₁₄N₂O₅S, is planar to within 0.024 (1) Å and makes an angle of 75.46 (10)° with the mean plane of the thio­sugar ring. In terms of standard nucleoside nomenclature, this ring has the C3′‐endo conformation. The O5′—C5′—C4′—C3′ torsion angle is 166.5 (3)° and the glycosidic torsion angle S4′—C1′—N1—C2 is ?52.1 (2)° (syn).     

Absolute Konfiguration von Loroxanthin (=(3R, 3′R, 6′R)-β, ϵ-Carotin-3, 19, 3′-triol)

Absolute Configuration of Loroxanthin (=(3R, 3′R, 6′R)-β, ?-Carotene-3, 19, 3′-triol) ‘Loroxanthin’, isolated from Chlorella vulgaris, was separated by HPLC. methods in two major isomers, a mono-cis-loroxanthin and the all-trans-form. Solutions of the pure isomers easily set up again a mixture of the cis/trans-isomers. Extensive ¹H-NMR. spectral measurements at 400 MHz allowed to establish the 3′, 6′-trans-configuration at the ?-end group in both isomers and the (9E)-configuration in the mono-cis-isomer. The absolute configurations at C(3) and C(6′) were deduced from CD. correlations with synthetic (9Z, 3R, 6′R)-β, ?-carotene-3, 19-diol ( 5 ) and (9E, 3R, 6′R)-β, ?-carotene-3, 19-diol ( 6 ), respectively. Thus, all-trans-loroxanthin ( 3 ) is (9Z, 3R, 3′R, 6′R)-β, ?-carotene-3, 19, 3′-triol and its predominant mono-cis-isomer is (9E, 3R, 3′R, 6′R)-β, ?-carotene-3, 19, 3′-triol ( 4 ). Cooccurrence in the same organism and identical chirality at all centers suggest that loroxanthin is biosynthesized from lutein ( 2 ).