bluuebunny/crossref_metadata_2025_split huggingface dataset:
Metabolic products of microorganisms, 271. Momofulvenone A and B, new benzo[b]fluorene quinones from Streptomyces
Authors: Volkmann Corinna , Rössner Ellen , Metzler Monika , Zähner Hans , Zeeck Axel 
Published: July 2007

AbstractFrom the cultures of Streptomyces diastatochromogenes (strain Tü 2895) two novel benzo[b]fluorene quinones, named momofulvenone A (1) and B (2), were isolated. Difficulties in their spectroscopic characterization due to salt effects were overcome by fermentation in the presence of ammonium acetate. The structures were established by a detailed spectroscopic analysis. The polyketide origin of 1 and 2 was verified by feeding [1,2‐13C2]acetate to the growing cultures of Tü 2895. The momofulvenones represent the nitrogen‐free parent compound of the kinamycin family of antibiotics.
Preparation and reactions of silyl nitronates derived from 2,2,2‐trifluoronitroethane. Diastereoselective synthesis of trifluoromethyl‐substituted aminoethanols and ‐propanols
Authors: Marti Roger E. , Heinzer Josef , Seebach Dieter 
Published: July 2007

AbstractSilyl nitronates of 2,2,2‐trifluoronitroethane and of 1,1,1‐trifluoro‐2‐nitropropane [SiMe3, SiMe2(tBu), SiMe2CMe2‐CHME2] were prepared and their reactions studied. – The TBDMS nitronate of the nitroethane is present at the (E) and (Z) isomer (2.3:1 at room temperature; EA = 9.3 kcal mol−1 for their isomerization). BU4NF‐catalyzed silyl nitro aldol additions to aldehydes show reasonable selectivities for the formation of the syn diastereoisomers of 5–13 (a. 4:1; with the non‐fluorinated analogs anti isomers prevailing!). The anti epimers (5–9, 12, 13, 17–20) in the trifluoromethyl series can be prepared by diastereoselective protonation of the corresponding O‐silyl lithium nitronates (ca. 9:1, Scheme 3). Careful reduction of the NO2 groups (H2/Raney nickel) gives trifluoroamino alcohol derivatives (22–30, Scheme 4). –The trimethylsilyl nitronates are also used for 1,3‐dipolar cycloadditions (to 1‐hexene, styrene, methyl vinyl ketone, and acrylates) to give N‐(silyloxy)isoxazolidines (2–40; Scheme 5). The configuration of the major products (2,3‐cis, 3,5‐trans) was derived from NMR measurements, X‐ray structure determinations, and thermal equilibration studies. The major course of reactions was thus derived to be an exo approach of the dipolarophiles to the (Z)‐silyl nitronates (Scheme 8).– Some conversions of the N‐(silyloxy)isoxazolidines (treatment with acid, base, and Raney nickel) are also described (products 42–51).
Synthesis and conformational studies of calixarene‐analogous trihydroxy[3.3.3]metacyclophanes and their O‐alkylated derivatives
Authors: Yamato Takehiko , Doamekpor Louis Korbla , Koizumi Ken‐Ichiro , Kishi Kumi , Haraguchi Mitsuteru , Tashiro Masashi 
Published: July 2007

AbstractTri‐tert‐butyltrihydroxy[3.3.3]metacyclophane 13 was prepared in 6 steps from p‐tert‐butylanisole in 25% overall yield by using the TosMIC method, followed by Wolff‐Kishner reduction and demethylation with boron tribromide. AlCl3/MeNO2‐catalyzed trans‐tert‐butylation of 13 in benzene gave the desired metacyclophane 14 in 85% yield, along with tert‐butylbenzene (15). Weak intramolecular hydrogen bonding was observed in the triols 13 and 14.–6,15,24‐Tri‐tert‐butyl‐9,18,27‐trihydroxy[3.3.3]MCP 13 was tri‐O‐alkylated with alkyl halides (RX: R = Et, Pr, and Bu) in the presence of Cs2CO3 to yield partial‐cone confomer 12b‐d in quantitative yield. Ring inversion by oxygen‐through‐the‐annulus rotation is allowed for triethoxy derivative 12b (coalescence temperature ca. 90°C) but inhibited for the tripropoxy‐12c and the tributoxy derivative 12d. The reaction of the triol 13 with ethyl bromoacetate in the presence of Cs2CO3 yielded cone and partial‐cone 12e, respectively. In contrast, a significant amount of cone conformer 12f resulted when NaH was used as base. Only when the template metal can hold the ester or amide group(s) and the oxide group(s) on the same side of the [3.3.3]MCP is the conformation immobilized to the cone.–Titration of cone amide cone‐12f with KSCN monitored by 1H NMR clearly demonstrates that a 1:1 complex is formed which is stable on the NMR time scale, and the original C3 symmetry is retained after the complete metal cation complexation.–The stability of multimembered carbon skeletons permits the interconversion of functional groups without special regard to ring‐opening side reactions as was observed for homotrioxacalix[3]arenes with ethereal linkages. For example, the introduction of nitro groups was accomplished on the upper rim by direct threefold ipso‐nitration of tert‐butyltrimethoxy derivative 12a.
Synthesis and characterization of water‐soluble and photolabile 10‐arylisoalloxazines: Tools for studying the mechanism of action of flavin‐type antimalarials
Authors: Kirsch Peer , Schönleben‐Janas Annette , Schirmer R. Heiner 
Published: July 2007

AbstractIsoalloxazine derivatives such as 1a‐d are promising anti‐malarial agents which act as inhibitors of the antioxidant enzyme glutathione reductase and possibly of other proteins. The molecular mechanism of the pharmacological effects has not been studied in detail because compounds 1a‐d are poorly soluble in aqueous solutions of physiological pH. In the present study we introduce two new types of isoalloxazine derivatives with improved solubility properties: The 10‐aryl‐3‐carboxymethylisoalloxazines 2a‐d, and the isomeric 3‐methyl‐10‐(N‐methylpyridiniumyl)isoalloxazine salts 3 and 4. In addition, for the purpose of photoaffinity labeling experiments, the 10‐aryl‐8‐azido‐3‐methylisoalloxazine 5 was designed. The syntheses and characterizations of these new flavins as well as an alternative synthetic approach to the known antimalarials 1a‐d are described.
Synthesis of (+)‐ and (−)‐hydroxysedamine and (+)‐N‐methylpseudoconhydrine from (S)‐glutamic acid via homochiral acyliminium ion
Authors: Herdeis Claus , Held Walter A. , Kirfel Armin , Schwabenländer Franz 
Published: July 2007

AbstractEfficient syntheses of (+)‐N‐methylpseudoconhydrine (5) and (−)‐hydroxysedamine (10) are described starting from the readily available (2RS,5S)‐Methyl 5‐(tert‐butyldimethyl‐silyloxy)‐2‐ethoxypiperidine‐1‐carboxylate (1) and proceeding via homochiral acyliminium ion as an intermediate. Compounds 5 and 7b were obtained in 32 and 34%, resp., overall yield from 1. After deprotection of the OH function of 3a, b the diastereomeric mixture of 6a, b was separated and converted to pseudoconhydrine 7b and epi‐pseudoconhydrine 7a, respectively. Reduction of the urethane moiety of 3a, b followed by treatment with hydrochloric acid provided enantio‐ and diastereopure 5. Reaction of 1‐phenyl‐1‐(trimethylsiloxy)ethene with 1 afforded 8a and 8b in a ratio of 1:9. Deprotection of 8a, b and separation of the diastereomers give after diastereoselective reduction of the ketone function and conversion of the urethane moiety into the methyl group natural (−)‐hydroxysedamine (10). Starting from 8b, deprotection and inversion of the OH function furnished 12 as a single homochiral diastereomer which was converted to ent‐10.
C‐glycoside syntheses, IV. Henry condensation of nitroethane and nitropropane with 4,6‐O‐benzylidene‐D‐glucopyranose
Authors: Wang Xiaojing , Gross Paul H. 
Published: July 2007

AbstractThe condensation of 4,6‐O‐benzylidene‐D‐glucopyranose (1) with nitroethane gave the diastereomeric α‐nitroethyl„α”︁ refers to C‐1 of the alkyl substituent; see 2 in Scheme 1.
β‐C‐glucosides 2 (minor), and 6. The latter was only obtained pure by an acetylation‐deacetylation procedure and was reduced by a mixture of elemental Fe0/Ni0 in aqueous THF under CO2 to α‐aminoethyl β‐C‐glucoside 7, with retention of the 4,6‐O‐benzylidene blocking group. Acetylation of 2 produced an open‐chain compound 3. An oxime derivative 5 was separated as a byproduct from the acetylation. The condensation of 1‐nitropropane with 1, followed by acetylation, similarly gave diastereomers 8 and 9. Deacetylation of 8 gave 10.
Total Synthesis of (±)‐20‐epiapotacamine and its conversion into (±)‐apotacamine by the Polonovski‐Potier reaction
Authors: Lounasmaa Mauri , Belle David Din , Tolvanen Arto 
Published: July 2007

Abstract(±)‐20‐Epiapotacamine (8) was synthesized in two steps from aldehyde 6 and was converted into the indole alkaloid (±)‐apotacamine (2) by the Polonovski‐Potier reaction.
Pheromone synthesis CLXIX. Improved synthesis of the racemate of the hemiacetal pheromone of the spined citrus bug Biprorulus bibax
Authors: Amaike Masayasu , Mori Kenji 
Published: July 2007

AbstractThe racemate of (3R,4S,1′E)‐3,4‐bis(1′‐butenyl)tetrahydro‐2‐furanol (1) was synthesized by employing Ireland's ester enolate Claisen rearrangement of (±)‐5 as the key step. The synthesis was efficient enough to provide 23.7 g of (±)‐1 in 41% overall yield based on (±)‐7 (8 steps) without having recourse to such a toxic solvent as HMPA in the key step.
Cyclization of ethyl (E)/(Z)‐[2‐(1‐cyclohexen‐1‐yl)cyclohexylidene]cyanoacetate to 10‐amino‐1,2,3,4,5,6,7,8‐octahydrophenanthrene‐9‐carboxylic acid — a route to 1,2,3,4,5,6,7,8‐octahydrophenanthrene derivatives
Authors: Wilamowski Jaroslaw , Krasodomski Wojciech , Nowak Barbara , Sepioł Janusz J. 
Published: July 2007

AbstractCyclization of ethyl (E)/(Z)‐[2‐(1‐cyclohexen‐1‐yl)cyclohexylidene]cyanoacetate (2) in cold concentrated sulfuric acid affords 10‐amino‐1,2,3,4,5,6,7,8‐octahydrophenanthrene‐9‐carboxylic acid (4) and 3,5,6,7,8,8a‐hexahydro‐3‐oxospiro‐[1H‐2‐benzopyran‐1,1′‐cyclohexane]‐4‐carbonitrile (5). The o‐amino acid 4 is employed in the synthesis of several 1,2,3,4,5,6,7,8‐octahydrophenanthrene derivatives: the carboxylic acid 6, o‐hydroxy carboxylic acid 7, and the amine 8. The lactone 5 undergoes decarboxylation on heating yielding a mixture of unsaturated nitriles 9 and 10 which are cyclized to the amine 8.
Hydrogen bonding of benzenesulfonamides in molecular recognition processes
Authors: König Burkhard , Möller Oliver , Jones Peter G. , Ahrens Birte 
Published: July 2007

AbstractBenzenesulfonamides represent a hydrogen donor‐acceptor moiety that is changed to a bisacceptor by deprotonation. The dimerization constant of N‐tosyl‐n‐butylamine (7) and N‐tosyl‐L‐alanine (9‐H) in CDCl3 and the binding constant of the tetrabutylammonium salt 7·NBu4 to urea in DMSO were determined by 1H‐NMR titration methods. The X‐ray structure analysis of 9‐H is reported.
Synthesis of β,γ‐unsaturated phenyl esters by aluminium chloride initiated reactions of allylsilanes with aryl chloroformates
Authors: Mayr Herbert , Gabriel Andreas O. , Schumacher Ralf 
Published: July 2007

AbstractPhenyl chloroformate (1) reacts with the allylsilanes 2a–h in the presence of aluminium chloride to give the β,γ‐unsaturated phenyl esters 3a–h in 50–90% yield. While these reactions proceed in the usual manner with allylic inversion, (1,1‐dimethylallyl)trimethylsilane (2i) reacts with allyl retention to yield the 2,2‐dimethyl‐3‐butenoate 3i. Attempts to use 1 for the phenoxycarbonylation of arenes were not successful.
A new approach to C‐branched monosaccharides by the stereoselective hydrodesulfurization of methyl 4,6‐O‐benzylidene‐3‐[bis(methylthio)‐methylene]‐3‐deoxy‐α‐D‐erythro‐hexopyranosid‐2‐ulose
Authors: Peseke Klaus , Thiele Gabriela , Michalik Manfred 
Published: July 2007

AbstractThe stereoselective reduction of methyl 4,6‐O‐benzylidene‐3‐[bis(methylthio)methylene]‐3‐deoxy‐α‐D‐erythro‐hexopyranosid‐2‐ulose 1 with sodium tetrahydridoborate and tributyltin hydride, respectively, to (Z)‐configured 4 is described. The increased reactivity of 4 compared to 1 is reflected in several substitution reactions resulting in the displacement of the methylthio group by different N nucleophiles. The substitution products 6–8 were stereoselectively obtained as (Z) isomers. Pyrano[3,4‐c]pyrazole derivatives 3 and 10 were prepared by treatment of 1 and 4, respectively, with hydrazine hydrate.
A facile synthesis of 5H‐1‐thia‐3,5,6,8‐tetraazaacenaphthylenes
Authors: Tumkevičius Sigitas 
Published: July 2007

AbstractEthyl 5‐substituted 7‐(methylthio)‐5H‐1‐thia‐3,5,6,8‐ tetra‐azaacenaphthylene‐2‐carboxylates (5a‐f) representing a new heterocyclic system were prepared by acid‐catalyzed cyclocondensation of ethyl 5‐amino‐4‐(substituted amino)‐2‐(methylthio)thieno[2,3‐d]pyrimidine‐6‐carboxylates (3a‐f) with triethyl orthoformate. Compounds 3 were obtained by aminolysis of 4,6‐dichloro‐2‐(methylthio)pyrimidine‐5‐carbonitrile (1) affording 2 followed by base‐promoted cyclization with sodium ethyl thioglycolate.
Cyclodehydrogenation of 2′‐hydroxychalcones with hypervalent iodine reagent: A new synthesis of flavones
Authors: Litkei György , Gulácsi Katalin , Antus Sandor , Blaskó Gábor 
Published: July 2007

AbstractA new synthesis of chrysin (1) and luteolin (4) was accomplished by the cyclodehydrogenation of the appropriately substituted 2′‐hydroxychalcones 21 and 22 in the presence of iodosobenzene diacetate/potassium hydroxide in methanol. The scope and limitation of this transformation is discussed.
Azepines by photochemical ring enlargement of 9‐azidopodophyllotoxin‐ and 9‐azido‐9′ ‐demethylepipodophyllotoxin derivatives
Authors: Laatsch Hartmut , Erns Bernd Peter , Hoffmann Dieter 
Published: July 2007

AbstractThe azepines 5a‐5c were obtained by photochemical nitrene rearrangement of the azides 1e/1f and 4b/4c in cyclohexene, but not in other solvents. They are ring expansion products of podophyllotoxin (1a) and resemble steganacin (9), but show only low biological activity. The triazenes 7a/8a and the aziridine 8b are also less active than 1a.
Azo bridges from azines, XVI. 1,2,3‐oxadiazolidines by intramolecular [3 + 2] cycloaddition
Authors: Hünig Siegfried , Schmitt Michael 
Published: July 2007

AbstractOlefinic bonds and an azoxy group held in a rigid parallel position in systems of type I undergo smooth thermal [3 + 2] cycloadditions which can be strongly catalyzed by acids. Double bonds of a bicycloheptene (norbornene) moiety are ca. 10 times more reactive than those of a bicyclooctene system. The newly formed cage compounds of type K are the first stable derivatives of the so far unknown 1,2,3‐oxadiazolidine.
Synthesis of novel HMG‐CoA reductase inhibitors, II. Heterocyclic analogs of mevinolin
Authors: Novák Lajos , Hornyánszky Gábor , Rohály János , Kolonits Pál , Szántay Csaba 
Published: July 2007

AbstractThe indole (1) and chroman analogs (2) of mevinolin and their 6S diastereomers 15c and 25d were synthesized in several steps starting with methyl (R)‐3‐[(tert‐butyldimethylsilyl)oxy]‐4‐formylbutyrate (12) and 2‐(indol‐3‐yl)ethanol (3) or 6‐methoxychroman‐4‐one (16), respectively. Compounds 1 and 2 inhibited moderately the HMG‐CoA reductase. The diastereomers 15c and 25d were shown to be inactive.
Total synthesis of angucyclines, 7. Hydroaromatic angucyclines of the SF‐2315 type
Authors: Krohn Karsten , Khanbabaee Karamali , Jones Peter G. 
Published: July 2007

AbstractOsmylation of the Diels‐Alder adduct 3 afforded the cis‐diol 4, which was dehydrated by acid treatment to the phenol 6. Reaction with tetrafluoroboric acid converted the diol 4 by intramolecular acyl shift into the acetate 7. The relative stereochemistry of 7 was determined by single‐crystal X‐ray analysis. Photooxygenation of 4 afforded the C‐1 carbonyl compound 8 and treatment with sulfuric acid the silanol 9. Functionalization of C‐6 was achieved by treatment of 4 with methanolic KOH to yield the cis‐methyl ether 11.
Geminal substituent effects, 11. The anomeric effect in orthoesters — the concept of geminal pairwise interactions for the interpretation of standard enthalpies of formation
Authors: Rakus Klaus , Verevkin Sergey P. , Peng Wei‐Hong , Beckhaus Hans‐Dieter , Rüchardt Christoph 
Published: July 2007

AbstractNew enthalpies of formation ΔH(g) are provided for the series of orthoesters 2a‐2f, 3 and 4 from their enthalpies of combustion ΔH and their enthalpies of vaporization ΔH or enthalpies of sublimation ΔH. From these data and from literature data the following new thermochemical increments were calculated [kJ · mol−1]: CH[O3] – 106.4, C[O3, C] ‐103.7, and C[O3, Ph] ‐124.5. In addition the increment C[O4] – 167.1 was calculated from literature data for ΔH(g) of the orthocarbonates 9a and 9b. The anomeric stabilization derived from these results and from previous results for acetals and ketals are reported in Table 3. – A new concept of additive structural increments for the quantitative analysis of the group increments of hydrocarbons, ethers, acetals, and ketals is proposed. It is shown that the increments for CH3, CH2, CH, and C groups with their different neighboring atoms (see Table 4) are determined by the number of hydrogen atoms in the group and by the number of pairwise geminal interactions between all atoms attached to the central carbon atom. – Using this procedure for orthoesters and orthocarbonates, deviations from additivity for the geminal interaction between oxygen atoms, i.e. the anomeric effect, are observed.
New 2,5‐dihydroimidazole‐1‐oxyls with functional side groups (N, O, S donors)
Authors: Hintermaier Frank , Volodarsky Leonid B. , Polborn Kurt , Beck Wolfgang 
Published: July 2007

AbstractA series of functionalized 2,5‐dihydro‐1‐hydroxy‐2‐R1‐2‐R2‐4‐R3‐imidazoles were prepared by cyclocondensation of α‐oxo hydroxylamines with ketones R1R2C = O in the presence of ammonia (R1 = R2 = 2‐pyridyl, 4,5‐diazafluoren‐9‐ylene, R3 = Me; R1 = CH2OCOMe, CH2OCOPh, 2‐thienyl, CH2OH, CO2K, R2 = R3 = Me; R1 = Py, R2 = Me, R3 = Ph; R1 = R2 = CH2OCOPh, CH2OH, R3 = Me) and oxidized to the corresponding nitroxide radicals. The structures of 2′,5′‐dihydro‐1′‐hydroxy‐4′,5′,5′‐trimethylspiro[4,5‐diazafluoren‐9,2′‐imidazole] (2) and of 2‐(acetoxymethyl)‐2,5‐dihydro‐2,4,5,5‐tetramethylimidazole‐1‐oxyl (14) were determined by X‐ray diffraction. In the crystal 2 forms one‐dimensional chains through hydrogen bonding between N atoms of the hydroxylamine and imidazoline groups.
Bifunctional Catalysis of Ester Aminolysis – A Computational and Experimental Study
Authors: Wang Li‐Hsing , Zipse Hendrik 
Published: July 2007

AbstractVarious pathways for the reaction of methylamine with methyl acetate catalyzed by 2(1H)‐pyridone have been investigated at the Becke3LYP/6–31G**/HF/6–31G** level of theory. The most favorable pathway consists of a one‐step reaction, in which all bond‐forming and ‐breaking processes occur in concert. These modeling studies also show the pyridone to function as an acid, which led to the investigation of several substituted pyridones. 4‐Cyano‐, 3‐cyano‐6‐methyl‐, and 3‐cyano‐2(1H)‐pyridone have indeed been shown to provide better catalysis in the ester aminolysis reaction by ab initio modeling at the Becke3LYP/6–31G**/HF/3–21G level. In order to verify the theoretical results, the catalytic activities of various substituted 2‐pyridones in the reaction of n‐butylamine with p‐nitrophenyl acetate in chlorobenzene solution have been determined. The observed catalytic rate constants are indeed in agreement with the theoretical predictions. Other catalyst characteristics such as solubility and state of aggregation are, however, also of paramount importance in this reaction.
Synthesis of HR 916 K: An Efficient Route to the Pure Diastereomers of the 1‐(Pivaloyloxy)ethyl Esters of Cephalosporins
Authors: Defoßa Elisabeth , Fischer Gerd , Gerlach Uwe , Hörlein Rolf , Isert Dieter , Krass Norbert , Lattrell Rudolf , Stache Ulrich , Wollmann Theo 
Published: July 2007

AbstractHR 916 K (5), the 1‐(S)‐(pivaloyloxy)ethyl prodrug ester of the cephalosporin cefdaloxime, exhibits a significantly higher oral bioavailability than the 1‐(R) diastereomer HR 916 J. An efficient synthesis of HR 916 K was developed. The separation of the diastereomers was achieved by precipitation of the 1‐(R)‐hydrochloride 9 followed by crystallization of the 1‐(S)‐amine 10 (de > 96%). The 1‐(R) diastereomer 9 was recycled by acidic saponification or enzymatic cleavage to AMCA (7). The amine 10 was acylated with mercaptobenzothiazole thioesters or mixed anhydrides, prepared from carboxylic acids 13 and 14, in almost quantitative yield. Deprotection of the oxime and formation of the tosylate proceeded in one step. Using thioester 18, we obtained HR 916 K (5) from AMCA (7) in 42% yield.
Asymmetric Alkylation of Chiral 2‐Azapentadienyl Metal Compounds: Diastereoselective Synthesis of Alkyl‐Substituted N‐Allylimines (2‐Aza‐1,4‐pentadienes)
Authors: Stakemeier Hubertus , Würthwein Ernst‐Ulrich 
Published: July 2007

AbstractN‐Allylimines (2‐aza‐1,4‐pentadienes) 3 bearing a chiral auxiliary group at C‐1 were prepared by a three‐step sequence involving condensation of pivaloyl chloride with allylamine, chlorination and reaction with a chiral amine (SMP or SMEMP) as an auxiliary group. Deprotonation of 3 by treatment with butyllithium in tert‐butyl methyl ether as solvent at ‐20°C afforded the chiral 2‐azapentadienyllithium compounds 4. Transmetalation of 4a with MgBr2 · Et2O gave the corresponding magnesium bromide 5. Alkylation with primary alkyl halides is predominantly kinetically controlled and leads to the 3‐substituted N‐allylimines 6. Secondary and tertiary alkyl halides, however, preferentially give 5‐substituted 2‐aza‐1,3‐pentadienes 7. The diastereoselectivity of the alkylation reaction may be increased by transmetalation from the lithium compound to the corresponding magnesium bromide or by exchange of the SMP group for the stronger chelating SMEMP group. The halide ion of the alkylating agent strongly influences the configuration of the newly formed stereogenic center. Thus, the chlorides lead to the opposite stereochemistry compared to the bromides. The effect of different solvents on the regio‐ and diastereoselectivity of the alkylation reactions was studied. Semiempirical calculations (PM3, MNDO) were used to design a stereochemical model for the metalated intermediates in order to rationalize the stereochemistry observed in the experiments.
Synthetic Applications of Conjugated Nitrones, 5 A Novel Entry to the 13‐Azasteroid System
Authors: Lopez‐Calle Eloisa , Höfler Johannes , Eberbach Wolfgang 
Published: July 2007

AbstractA synthetic approach to 13‐azasteroid and C‐nor‐13‐azasteroid derivatives (type 17/18) as well as to the 4,13‐diaza analogues 27/28 is described. For the construction of the tetracyclic compounds precursors with preformed rings A, B and D containing a conjugated nitrone system were used (16/26). These were prepared from the annulated cyclohexenones 10/20a, b in six steps according to slightly modified standard procedures. The failure in obtaining the terminal silyl alkynes 15b by Michael addition of the enolate of 14b to nitrostyrene is explained on the basis of a strong Si–O complexation. After replacement of the trimethylsilyl substituent by the sterically more demanding triisopropylsilyl group the formation of 15c was accomplished as expected. Upon thermally induced 1,7‐cyclization of the nitrones 16/26 a multistep rearrangement of the primary cycloallenic intermediates takes place resulting in the formation of the corresponding 6,6,6,5‐ and 6,6,5,5‐ring steroids (17/27 and 18/28) in 55–86% yield.
Derivatives of Natural Tetrapyrroles for Photodynamic Therapy, 4. Oxidative Degradation Studies: Direct Analysis and Determination of Ether and Ester Linkages in Porphyrin Dimers and Oligomers of Hematoporphyrin Derivative (HPD),
Authors: Risch Nikolaus , Hesse Ulrike , Josephs Andrea , Gauler Rainer 
Published: July 2007

AbstractOxidative degradation studies of dimeric porphyrins provide valuable information about the individual linkages between the porphyrin units. MS and NMR data of the degradation products of hematoporphyrin derivative furnish unambiguous evidence of ester and ether linkages in the di‐ and oligomeric compounds of HPD.