bluuebunny/crossref_metadata_2025_split huggingface dataset:
Kurze Totalsynthesen von (±)‐Sativen und (±)‐cis‐Sativendiol
Authors: Sigrist Rofl , Rey Max , Dreiding André S. 
Published: December 2004

Short Total Syntheses of (±)‐Sativene and (±)‐cis‐SativenediolOur approach to (±)‐sativene (7) and (±)‐cis‐sdtivenediol (9) involves: (a) reaction of 3‐methylbutanoyl chloride with Et3N/cyclopentadiene to give the endo‐isopropyl‐ketone 1 (here improved to 71%), (b) NBS bromination of 1 to a 5:1 mixture (87%) of the bromo‐ketones 2 and 3, (c) NFD‐reaction sequence initiated by the attack of 1,2‐butadienyl titanate (complex of 15, obtained from 2‐butine) on 2/3 to afford 52% of the brexenone derivative 4 (along with 8% of its epimer 16), (d) addition of dibromomethane to 4 forming 63% of the diene‐alcohol 5 (along with 13% of the diene‐carbaldehyde 38), and (e) carbenoid ring‐expansion with MeLi applied to 5 resulting in 41% the diene‐ketone 6 (along with 15% of a 1:3 mixture of the diene‐ketones 32 and 33). Wolff‐Kishner reduction of 6 led to 81% of (±)‐sativene (7), when enough O2 was present, but to 97% of the diene 8 in the strict absence of O2. (±)‐cis‐Sativenediol (9) was obrained (86%) by OsO4 hydroxylation of 8. The brexenone derivatives 4 and 16 (6:1, 50%) were also produced when the NFD‐reaction sequence was applied to the isomeric bromo‐ketone mixture 13/13 (1:3). The latter was obtained by NBS bromination of 10, which in turn was available by base epimerization of 1, followed by destructive removal of unreacted 1 by repeated gas‐flow thermolysis. An analogous (less convenient) route to (±)‐sativene (7) passed through a series of dihydro compounds (the ene series) it started with the methylidene‐ketone 36, which was the product (97%) of a partial hydrogenation of 4. Addition of dibromomethane to 36 led t 62% of the methylidene‐alcohol 39 (along with a little tetracyclic ether 40). Carbenoid ring expansion of 39 with MeLi afforded ca. 42% of the methylidene‐ketone 41 (along with 7% of the methylidene‐ketone 43 or, under slightly different condition, along with 9% of the methylidene‐ketone 42 and 10% of the methylidene‐carabaldehyde 44). The methylidene‐alcohol 39 and the methylidene‐ketone 43 were also obtained by partial hydrogenation of 5 and 33, respectively. Wolff‐Kisher reduction converted 41 into (±)‐sativene (7 99%); the same conditons applied to 42 afforded only ca. 8% 7 (along with three other hydrocarbons, one of them (ca. 21%) probably being (±)‐copacamphene (45)). In the diene series, the two succeeding reactions (4→5 and 5→6) competed with the same side reaction, a rearrangement leading to the brendene‐aldehyde 38. In the ene series, the corresponding dihydro‐by‐product 44 was found in the reacton 39→41, but not during 36→39. These side reactons could largely be suppressed by keeping the reaction temperature low. An explanation is proposed.
Axially Dissymmetric Bis(triaryl)phosphines in the Biphenyl series: Synthesis of (6,6′‐dimethylbiphenyl‐2,2′‐diyl)bis(diphenylphosphine) (‘BIPHEMP’) and analogues, and their use in Rh(I)‐catalyzed asymmetric isomerizations of N,N‐diethylnerylamine
Authors: Schmid Rudolf , Cereghetti Marco , Heiser Bernd , Schönholzer Peter , Hansen Hans‐Jürgen 
Published: December 2004

AbstractThe axially dissymmetric diphosphines (−)‐(R)‐ and (+)‐(S)‐(6‐6′‐dimethylbiphenyl‐2,2′‐diyl)bis(diphenyl‐phosphine) ((−)‐(R)‐10 and (+)‐(S)‐10; ‘BIPHEMP’) have been synthesized, starting from (R)‐ and (S)‐6,6′‐dimethylbiphenyl‐2,2′‐diamine ((R)‐ and(S)‐16), respectively, via Sandmeyer reaction, liathiation, and phosphinylation. Moreover, racemic 4,4′‐ dimethyl‐ and 4,4′‐bis(dimethylamino)‐substituted analogues 11 and 12 respectively, and the 6,6′‐bridged analogues 1,11‐bis(diphenylphosphino)‐5,7‐dihydrodibenz[c,e]oxepin (13) were synthesized and resolved into optically pure (R)‐ and(S)‐enantiomers via complexation with di‐μ‐chlorob is {(R)‐2‐[1‐(dimethylamino)ethyl]pheny‐CN}dipalladium(II) ((R)‐18). The molecular structures of the diphosphines (S)‐10 and (R)‐13 and of two derived cationic Rh(I) complexes,[Rh((S)‐ 10)(nbd)]BF4 and [Rh((R)‐ 13)(nbd)]BF4 were determined by x‐ray analyses. Absolute configurations were established for (+)‐(S)‐ 10 by X‐ray analyses of both the free diphosphine and of the derived Rh(I) complex, and for (−)‐(R)‐ 13 by X‐ray analysis of the derived Rh(I) complex. Configurational assignments for the substituted BIPHEMP analogues 11 12 were achieved by means of 1H‐NMR comparisons. The BIPHEMP ligand 10 and analogues 11, 12 and 13 are the first examples of optically active bis(triaylphosphines) containing the axially dissymmetric biphenyl moiety. All these new diphosphines proved to be excellent asymmetry‐inducing ligands in Rh(I)‐catalyzed isomerizations of N,N‐diethylnerylamine affording citronellat enamine of 98‐99% ee.
Isolation from the Mediterranean Stolonifern Coral Sarcodictyon roseum of Sarcodictyin C, D, E, and F, novel diterpenodic alcohols esterified by (E)‐ or (Z)‐N(1)‐methylurocanic acid. Failure of the carbon‐skeleton type as a classification criterion
Authors: D'Anbrosio Michele , Guerriero Antonio , Pietra Franceseco 
Published: December 2004

AbstractIt is shown here that the stoloniferan coral Sarcodictyon roseum of east Pyrenean waters contains four novel diterpenoids, sarcodictyin C ((−)‐3), D ((−)‐4), E ((+)‐5), F ((+)‐6), which are related to sarcodictyin A ( = (−)‐(4R,4aR,7R,10S,11S,12aR,1Z,5E,8Z‐7,10‐epoxy‐3,4,4a,7,10,11,12,12a‐octahydro‐7‐hydroxy‐6‐(methyoxycarbonyl)‐1,10‐dimethyl‐4‐(1‐methylethyl)‐benzocyclodecen‐11‐yl (E)‐N1‐methylyrocanate; ((−)−1), previously isolated from the same coral. Sarcodictyin C ((−)‐3) and D ((−)‐4) and the 3α‐hydroxy and 3α‐acetoxy derivatives of (−)‐1), sarcodictyin E ((+)‐5) is the (Z)‐urocanate isomer of (−)‐3), and sarcodictyin F ((+)‐6) is the 1α‐hydroxy‐2‐ene isomer of (−)‐3. In all cases, the nine‐membered ring is locked, and the molecule stabilized, by the urocanic appendage; when this is removed in MeOH/KOH, the C(11)–O− function is free to attack at C(5), and retro‐condensations then lead to the ring‐contracted butenolides 11 (from (−)‐3) or 10 (from(−)‐1) with extrusion of the hydroxyfuran nucleus (Scheme 3). Under the same conditions, with (−)‐3, the C(3)‐O− group competitively attacks at C(5), the hydroxyfuran nucleus is expelled, and aldehyde 14 is formed. Peculiarly, in the reaction of (−)‐3 with MeOD/KOD, the ring‐contracted butenolide 17 contains D at the 4′‐ax position. The sarcodictyins are unique in these chemical properties, not shared by the cladiellanes which have the same C‐skeleton.
Synthese neuer Aryl‐(3,‐dichloro‐4‐pyridazinly)‐ketone und ihre Reaktion mit N,N‐Dinucleophilen
Authors: Eichhorn Thomas A. , Piesch Steffen , Ried Walter 
Published: December 2004

Synthesis of New Aryl (3,6‐Dichloro‐4‐pyridazinyl) Ketones and their Reaction with N,N‐DinucleophilesThe synthesis of the new aryl (3,6‐dichloro‐4‐pyridazinyl) ketones 3a–e vïa Friedel‐Crafts acylation of the aromatic compounds 2a–e with 3,6‐dichloro‐4‐pyridazincarbonyl chloride (1) is described. The ketones 3a–e cyclized with N,N‐dinucleophilic reagents to the 3‐aryl‐5‐chloro‐l H ‐pyrazolo[3,4‐c]pyridazines 4a–d, and 3a–c are converted into the 3‐aryl‐5‐chloro‐l H ‐pyrazolo[3,4‐c]pyridazin ‐ l ‐ethanols 5a–c and to the hitherto unknown ring system of the 5‐aryl‐3‐chloro‐7,8‐dihydro‐9 H ‐pyridazino[3,4‐e][1,4]diazepines (6a‐c).
8‐Aza‐7‐deaza‐2′‐deoxyguanosine: Phosphoramidite synthesis and properties of octanucleotides
Authors: Seela Frank , Driller Hansjürgen 
Published: December 2004

AbstractBase‐modified octanucleotides derived from d(G1–G2–A–A–T–T–C–C–) (11) but containing 8‐aza‐7‐deaza‐2′‐deoxyguanosine (2) instead of 2′‐deoxyguanosine (1) have been prepared by solid‐phase synthesis employing P(III) chemistry. Isobutyrylation of 2, followed by 4, 4′‐dimethoxytritylation and subsequent phosphitylation yielded the methyl or the cyanoethyl phosphoramidites 6a or 6b, respectively. They were used as building blocks in automated DNA synthesis. The resulting octanucleotides 12–14 containing 2 showed increased Tm values compared to the parent oligomer 11. The oligomers 11–14 were employed as sequence‐specific probes in endo‐deoxyribonuclease Eco RI oligonucleotide recognition. Whereas displacement of dG‐2 (enzymic cleavage site of 11) abolished phosphodiester hydrolysis, replacement of dG‐1 enhanced the cleavage rate compared to 11.
Präparative chromatographische Enantiomerentrennung von synthetisch nützlichen cyclischen Acetalen
Authors: Seebach Dieter , Müller Stefan G. , Gysel Urs , Zimmermann Jürg 
Published: December 2004

Preparative Chromatographic Resolution of Synthetically Useful Cyclic AcetalsRacemic cyclic acetals derived from aldehydes and glycine, glycolic acid, thioglycolic acid, formylacetic acid, and acetoacetic acid (oxazolidinones 4–13, dioxolanones 14, 15, oxathiolanone 16, dioxinones 17–23) are resolved by preparative high‐pressure liquid chromatography on silica gel coated with the polymer from N‐acryloylphenylalanine ethyl ester (Chiraspher®). The separation factors α range from1,1 to 2,4. Use of a Prepbar®‐chromatography system allows injection of several grams at a time. The enantiomeric acetals thus obtained are fully characterized. First application to amino‐acid synthesis are mentioned.
From Dichlorocyclopropanes to Furans and Cyclopentadienes via Vinylcarbenes
Authors: Müller Paul , Pautex Nicole 
Published: December 2004

AbstractBase‐induced elimination of dichlorocarbene adducts 2 to 9‐alkoxyphen threnes 1 leads to furans 6, presumably via cyclopropenes 3 which undergo rearrangement to vinylcarbenes 4 and CH insertion. By the same sequence, the 9‐substituted alkylphenanthrene adduct 10 and 14 afford cyclopentadienes 11 and 15. Carbene adducts of simple enol ethers, however, react differently and give preferentially 2‐chloroalken‐2‐ones.
Intramolecular Dynamics of Tetranuclear Carbonyliridium Cluster Compounds. Part II. Disubstituted complexes with one chelating ligand. Crystal structure of [Ir4(CO)7(μ‐CO)3(1,5‐cyclooctadiene)]
Authors: Strawczynski Andrés , Ros Renzo , Roulet Raymond , Grepioni Fabrizia , Braga Dario 
Published: December 2004

AbstractThe two complexes [Ir4(CO)10(diarsine)] (1) and [Ir4(CO)10(1,5‐cyclooctadiene)] (2) bear a bidentate ligand chelating one metal atom of the basal face of the Ir4 tetrahedron. However, they differ in fluxional behaviour as observed by 2D‐exchange and variable‐temperature 13C‐NMR. The CO‐site exchange with lowest activation energy proceeds via an unbridged intermediate in 2, whereas that in 1 occurs via a concerted edge‐bridging of CO's to an alternative face of the metal core. This difference is apparently related to different ground‐state geometries: the basal CO's are symmetrically bridging in 2, whereas two CO's are semi‐bridging in 1. The molecular structure of 2 was determined by single crystal X‐ray diffractometry. The crystals are orthorhombic, space group Pbca, a = 11.651(4), b = 13.118(3), c = 28.64(1)Å. The idealized molecular symmetry is Cs. The diolefin chelates a basal Ir‐atom replacing an axial and a radial CO group on the tetrahedral metal‐atom framework.
Versuche zur Synthese von Calicen aus trisubstituierten Cyclopropanen und Cyclopentenon
Authors: Weber Andreas , Galli Roberto , Sabbioni Gabriele , Stämpfli Urs , Walther Stefan , Neuenschwander Markus 
Published: December 2004

Attempted Synthesis of Calicene from Trisubstitued Cyclopropanes and CyclopentenoneThe Li carbenoids 4, prepared by treatment of substituted 1,1‐dihalocyclopropanes with BuLi, are reacted with cyclopent‐2‐enone under thermodynamic and kinetic control (Scheme 1). In general, the latter procedure gives better yields of cyclopropylcyclopentenols 5a–e, but the reaction seems to be controlled mainly by the steric and electronic properties of the substituent Y. So, with 4b and 4e, the main reaction is the attack of the carbenoid at C(1) of cyclopent‐2‐enone, while 4a (Y = PhS) predominantly deprotonates the ketone (Scheme 4). Whereas 5d and 5e can easily be converted to the dihydrocalicenes 6d and 6e (Scheme 6), the attempted elimination of H2O from 5a–c leads to the rearranged products 13–2 due to the opening of the cyclopropane ring (Scheme 5). Finally, the generation of the parent compound 2 from the silylated precursor 6d is attempted: treatment with MeO− gives the addition products 18A/18B, while the reaction with Br2 provides 19 by a bromination/dehydrobromination sequence (Scheme 7).
Asymmetric Diels‐Alder reactions: X‐Ray crystal‐structure analysis of [N‐((E)‐But‐2‐enoyl)bornane‐10,2‐sultam]tetrachlorotitanium
Authors: Oppolzer Wolfgang , Rodriguez Inès , Blagg Julian , Bernardinelli Gérald 
Published: December 2004

AbstractAn X‐ray diffraction analysis of the crystalline complex 2b (MLn  TiCl4) shows a chelation of the carbonyl O‐atom and the upper SO2 O‐atom by TiCl4, a slightly pyramidal N‐atom, and a π ‐face‐stereoselective shielding of the C(α)‐Si face by the CH2(3) group. The Ti‐atom is in a pseudo‐octahedral environment.
C45 und C50‐Carotinoide. 6. Mitteilung. Synthese eines optisch aktiven cyclischen C20‐Bausteins und von Decaprenoxanthin (= (2R, 6R, 2′R, 6′R)‐2,2′‐Bis(4‐hydroxy‐3‐methylbut‐2‐enyl)‐ϵ, ϵ‐carotin)
Authors: Gerspacher Marc , Pfander Hanspeter 
Published: December 2004

C45‐ and C50‐Carotenoids: Synthesis of an Optically Active Cyclic C20‐Building Block and of Decaprenoxanthin ( = (2R, 6R, 2′R, 6′R)‐2,2′‐Bis(4‐hydroxy‐3‐methylbut‐2‐enyl)‐ϵ, ϵ‐carotene)The synthesis of the optically active cyclic C20‐building block (R, R)‐15 and of the optically active C50‐carotenoid (2R, 6R, 2′R, 6′R)‐decaprenoxanthin (1) starting from (‐)‐β‐pinene ((S)‐2) is reported.
Photo‐oxygenation of Styrenic Estrogens: Product characterization and kinetics of the dye‐sensitized photo‐oxygenation of 9,11‐didehydroestrone derivatives
Authors: Planas Antoni , Lupon Pilar , Cascallo Montse , Bonet Juan‐Julio 
Published: December 2004

AbstractThe dye‐sensitized photo‐oxygenation of 3‐hydroxyestra‐1,3,5(10), 9(11)‐tetraen‐17‐one (8) gives a complex mixture from which only the 1,4‐endo‐peroxide 10 can be isolated in low yield. In contrast, the 3‐methoxy derivative 9 yields the C‐seco‐aldehyde 11 as a major product, suggesting that 1,2‐dioxetane is a primary photo‐oxygenation intermediate. As the electron‐donating character of the substituent at C(3) decreases in the sequence 12 (R = PhCO), 13 (R = Ac), and 14 (R = Ts), the rate constant in substrate disappearance becomes drastically smaller as compared with 8 and 9, and no photoproducts are detected. The results are rationalized by means of electronic and conformational factors.
Ring‐opened analogues of Ambrox®: Synthesis and Structure‐Odour relationships
Authors: Winter Beat 
Published: December 2004

AbstractEight bicyclic ether derivatives 11–18, related to the tricyclic odorant Ambrox® (1) and its analogues 2–6 by formal heteroring‐opening, have been prepared; their odoriferous activity/in activity is correlated with the steric accessibility of the ether O‐atom, calculated by computer‐aided molecular modeling.
New Irone‐Related Constituents from the essential oil of Iris germanica L.
Authors: Maurer Bruno , Hauser Arnold , Froidevaux Jean‐Claude 
Published: December 2004

AbstractTwenty irone‐related compounds 1–20 containing from 10 to 16 C‐atoms have been identified for the first time in commercial Iris oil of Moroccan origin (Iris germanica L.). Most of the structures and the absolute configurations of natural 6, 16, and 17 are corroborated by partial synthesis. The organoleptic properties of some of the new constituents are discussed.
Aplysinopsin‐type alkaloids from Dendrophyllia sp., a scleractinian coral of the family dendrophylliidae of the philippines, facile photochemical (Z/E) photoisomerization and thermal reversal
Authors: Guella Graziano , Mancini Ines , Zibrowius Helmut , Pietra Francesco 
Published: December 2004

AbstractThe dendrophylliid Dendrophyllia sp. of Palawan contains the indole alkaloids 2′‐demethylaplysinopsin (4) and 2′‐demethyl‐3′‐N‐methylaplysinopsin (6) and their 6‐bromo analogues in H (Z/E) ratio larger than 95:5; these mixtures undergo facile photoisomerization to give mixtures richer in the (E) stereoisomer which undergo thermal isomerization to give back the original mixtures.
Der Zugang zum Antitumor‐Antibiotikum CC‐1065 mittels Hetero‐Cope‐Umlagerung von Vinyl‐N‐phenylhydroxamaten
Authors: Martin Pierre 
Published: December 2004

The Access to the Three Subunits of the Antitumor Antibiotic CC‐1065 by Hetero‐Cope Rearrangement of Vinyl N‐Phenylhydroxamates.The use of the hetero‐Cope rearrangement of vinyl N phenylhydroxamates to indoles for the preparation of the 1,2‐dihydro‐3H, 6H‐benzo[1,2‐b:4,3‐b′]dipyrrole skeleton, the structural subunits characteristic of the antitu‐mor antibiotic CC‐1065 as well as the phosphodiesterase inhibitors PDE‐I and PDE‐II is described.
Aromatization of tetrahydrocyclopropa[a]naphthalenes: An alternative synthesis of 1H‐Cyclopropa[a]naphthalene
Authors: Müller Paul , Bernardinelli GéRald , Thi Huong Can Godoy‐Nguyen 
Published: December 2004

Abstract1H‐Cyclopropa[a]naphthalene (1a) is accessible via reduction of the dichloro compound 1c with LiAlH4/AlCl3. Several derivatives of tetrahydrocyclopropa[a]naphthalene were synthesized. However, contrary to their 1,1‐dihalogeno analogues, they afforded no cycloproparenes upon attempted aromatization.
Synthese von Plectranthonen, diterpenoiden Phenanthren‐1,4‐chinonen
Authors: Kaliakoudas Dimitrios , Eugster Conrad Hans , Rüedi Peter 
Published: December 2004

Synthesis of Plectranthons, Diterpenoid Phenanthrene‐1,4‐dionesThe following phenanthrene‐1,4‐diones have been synthesized by using the photocyclization of the corresponding highly substituted stilbenes as the key step: 3‐hydroxy‐5,7,8‐trimethyl‐2‐(prop‐2‐enyl)phenanthrene‐1,4‐dione (1), (RS)‐, (R)‐, and (S)‐2‐[3‐hydroxy‐5,7,8‐trimethyl‐1,4‐dioxophenanthren‐2‐yl]‐1‐methylethyl acetate (2, 31, and 32, resp.), 3‐hydroxy‐7,8‐dimethyl‐2‐(prop‐2‐enyl)phenanthrene‐1,4‐dione (3), 3‐hydroxy‐7,8,10‐tri‐methyl‐2‐(prop‐2‐enyl)phenanthrene‐1,4‐dione (4), 5,7,8‐trimethyl‐2‐(prop‐2‐enyl)phenanthrene‐1,4‐dione (17), and 3‐hydroxy‐2‐methylphenanthrene‐1,4‐dione (42). The quinones 1 and 3 proved to be identical with the recently isolated plectranthons A and C. Compounds 2, 31, and 32 exhibited the same UV/VIS, IR, 1H‐NMR and mass spectra as natural plectranthon B, but had different melting points. This might be due either to crystal modifications or to diastereoisomerism caused by the helical structure of the phenanthrene‐1,4‐dione skeleton. The spectral data of synthetic 4 were not compatible with those of natural plectranthon D for which structure 4 had been proposed based mainly on 1H‐NMR arguments concerning the chemical shifts of HC(9) and HC(10) in 1–3. Extensive 1H‐NMR investigations have now revealed that the currently stated assignments of the HC(9)/ HC(10) AB system have to be reversed for highly substituted phenanthrene‐1,4‐diones: in the model compounds 2‐methylphenanthrene‐1,4‐dione (41) and 2, HC(10) resonates al lower field as expected (peri‐position), whereas in the highly substituted congeners 1, 2, 3, 31, and 32, HC(9) is shifted paramagnetically, a fact which had lead to the erroneous assignment of structure 4 for natural plectranthon D.
Dehydration of Alkanones by Bare FeO+ Involves the ω/(ω − 1) Positions: Exclusion of Iron Carbenes as Intermediates. Preliminary Communication
Authors: Schröder Detlef , Eller Karsten , Schwarz Helmut 
Published: December 2004

AbstractFourier‐transform ion‐cyclotron‐resonance (FTICR) mass spectrometry has been used to uncover the mechanisms by which FeO+ dehydrates heptan‐4‐one (5a) and nonan‐5‐one (6a) in the gas phase. The study of isotopomeric ketones provides evidence that H2O loss is not due to a 1,1‐elimination, thus ruling out the intermediacy of high‐valent iron‐carbene species. Rather, H2O is generated in a formal 1,2‐elimination involving the ω/ω − 1 positions of the alkyl chain (‘remote CH bond activation’). In the consecutive alkene/H2O elimination, the olefins (ethylene from 5a and propene from 6a) originate from the terminal part of one alkyl chain, and the H‐atom is transferred to the FeO+ moiety in the course of this process, builds up together with an H‐atom from the ω/ω‐1 position of the other alkyl chain the H2O molecule. In either case, the O‐atom of H2O is provided by the FeO+ species.
Coralloidolide F, the First Example of a 2,6‐Cyclized Cembranolide: Isolation from the Mediterranean Alcyonacean Coral Alcyonium coralloides
Authors: D'Ambrosio Michele , Guerriero Antonio , Pietra Francesco 
Published: December 2004

AbstractThe Mediterranean alcyonacean Alcyonium ( = Parerythropodium) coralloides (Pallas, 1766) is shown here to contain coralloidolide F ( = (+)‐(3a‐R*, 7R*, 8R*, 9R*, 12R*, 12aS*)‐8,9‐epoxy‐1,3a,6,7,8,9,10,11,12,12a‐decahydro‐3a‐hydroxy‐12‐isopropenyl‐2,5‐dimethyl‐1‐oxocyclopfintacycloundecene‐9,7‐carbolactone; (+)‐2), the first example of a 2,6‐cyclized cembranolide. Structural assignments are mainly based on ID and 2D NMR and MS data.
Photorearrangement of 4,4‐Dimethylcyclohex‐2‐enones with Alkyl or Fluoro Substituents at C(5) and C(6): In Search of the Mechanism
Authors: Cruciani Giuliano , Margaretha Paul 
Published: December 2004

AbstractThe photorearrangement of cyclohex‐2‐enones 4a‐h to bicyclo[3.1.0]hexan‐2‐ones 5 and cyclopent‐2‐enones 6 (λ = 350 nm, MeCN) was investigated. Both the quantum yield (Φ−4 = 0.004– 0.024) and the product ratio (5/6 = 65:35–31:69) vary only over a rather small range, indicating the rearrangement to be relatively insensitive to substituents on C(5) or C(6). Compounds 4b, 4c, and 4g with just one alkyl group at either C(6) or C(5) rearrange selectively to the diastereoisomer 5 with alkyl group and three‐membered ring in trans‐configuration, while 6‐fluorocyclohex‐2‐enones 4d and 4f afford mixtures of diastereoisomeric bicyclohexanones. Mechanistic conclusions regarding an intermediate trimethylene biradical are presented.
Photochemical reactions. 152nd Communication. Photochemistry of acylsilanes; photolysis and thermolysis of cyclopropyl silyl ketones
Authors: Scheller Markus E. , Frei Bruno 
Published: December 2004

AbstractThe photolysis and thermolysis of the Cyclopropyl silyl ketones 3, 4, and 5 are described. On n,π* excitation, the silyl ketones 3 and 4 undergo a Norrish‐type‐II reaction involving γ‐H abstraction, cyclopropyl ring cleavage followed by retro‐enolization to the acylsilanes 6 and (E/Z)‐12, respectively. As a common product of 3 and 4, the dihydrofuran 7 is formed via the alternative C(α)‐C(β) cleavage of the cyclopropyl moiety. Compounds 6, 7, and (E/Z)‐12 are new types of acylsilane photoproducts. The irradiation of acylsilane 5 gave the analogous dihydrofuran 15 as the only product. On photolysis of 3 and 4, products 8A + B and 13A + B, derived from a siloxy carbene intermediate, were found as well. On thermolysis of 3 and 4, the acylsilanes 6 (80%), and (E)‐12 (33%) and (Z)‐12 (34%), respectively, are formed as the only products. Their formation may occur via a [1, 5] sigmatropic H‐shift. The thermolysis of 5 gave the diene 16 whose formation can be explained by insertion of a siloxycarbene into the neighboring cyclopropane leading to the cyclobutene 28 as thermally unstable intermediate.