bluuebunny/crossref_metadata_2025_split huggingface dataset:
Enantioselective Synthesis of Both Enantiomers of a Isoproterenol Analogue and of Di‐O‐pivaloylepinephrine
Authors: Solladié‐Cavallo Arlette , Simon‐Wermeister Marie‐Claude , Farkhani Dariouch 
Published: December 2004

AbstractAn asymmetric synthesis of the analogue 4a of isoproterenol and of di‐o‐pyvaloylepinephrine 3 which provides both enantiomers in optically pure state is reported. The key step of the method is the highly diastereo‐selective reduction (using DIBAL or DIBAL/ZnCl2) of a β‐ketosulfoxide 7 which leads, as desired, to the (S)‐ or the (R)‐configuration at C(1) (Schemes 4 and 5).
A New Synthesis of (2S,3R,4R)‐2‐(Hydroxymethyl)pyrrolidine‐3,4‐diol
Authors: Meng Qingchang , Hesse Manfred 
Published: December 2004

AbstractThe title compound 6 was synthesized from 2,3,5‐tri‐O‐benzyl‐D‐arabinofuranose (7) in three steps and 48% overall yield. Moreover, it was shown, in the case of γ‐hydroxy amide 9, that the Mitsunobu reaction is not suitable for the preparation of γ‐lactams, because O‐alkylation is predominant.
Biosynthesis of the Spermine Alkaloid Aphelandrine
Authors: Papazoglou Georgios , Sierra Jorge , Homberger Katharina , Guggisberg Armin , Woggon Wolf‐Dietrich , Hesse Manfred 
Published: December 2004

AbstractAphelandrine (1) is shown to be biosynthesized in the root cells of Aphelandra tetragona (VAHL) NEES from labelled putrescine (4), spermidine (5), and cinnamic acid (3). Whether spermine (6) and the (p‐hydroxycinnamoyl)spermidine 8 are precursors of 1 is uncertain, since the latter is hydrolysed to a large extent before incorporation, and the former is metabolized to 4 and 5. Methionine (7) is the source of the 3‐aminopropyl unit of 5 and 6.
Ionization Behavior and Ionization‐Dependent Conformation of Raclopride, a Dopamine D2 Receptor Antagonist
Authors: Carrupt Pierre‐Alain , Tsai Ruey‐Shiuan , Tayar Nabil El , Testa Bernard , de Paulis Tomas , Högberg Thomas 
Published: December 2004

AbstractRaclopride, an antipsychotic 6‐methoxysalicylamide (= 2‐hydroxy‐6‐methoxybenzamide) derivative, was shown by titrimetry and UV‐pholometry to exist in zwitterionic form at physiological pH. Calculations revealed that the neutral and zwitterionic forms differ considerably in their conformational behavior, the latter form being energetically favored by an intramolecular phenolate–ammonium ionic bond. These findings indicate that raciopride and other halogenated 6‐melhoxysalicylamides with a highly acidic phenolic group may not resemble other ortho‐methoxybenzamides in their stereoelectronic structure and mode of binding to the dopamine D2 receptor.
The Lewis basicity of the [Pt3(μ‐CO)3(PR3)3] clusters and the X‐ray crystal structure of [{ZnI2}{Pt3(μ‐CO)3(PPh(i‐Pr)2)3}]
Authors: Stockhammer Andreas , Dahmen Klaus‐Hermann , Gerfin Tobias , Venanzi Luigi M. , Gramlich Volker , Fetter Walter 
Published: December 2004

AbstractThe cluster compounds [Pt3(μ‐CO)3(PR3)3] act as Lewis bases towards the metal halides of Group XI, MX, Group XII, MX2, and Group XIII, MX3, to form cluster compounds of the composition [{MXn}{Pt3(μ‐CO)3(PR3)3}]. The X‐ray crystal structure, NMR and IR data are given for the compound [{ZnI}{Pt3(μ‐CO)3(PPh(i‐Pr)2)3}].
New Polycyclic Compounds from Photochemical Rearrangements of Some Substituted 2‐Azatricyclo[5.2.2.01,5]undeca‐4,8,10‐trien‐3‐ones
Authors: Mihova Tonya R. , Trifonov Latchezar S. , Dimitrov Valentin S. , Orahovats Alexander S. , Linden Anthony , Heimgartner Heinz 
Published: December 2004

AbstractThe products formed on UV irradiation of several tricyclic compounds (i.e. 3, 6, 8, 15, and 17, Schemes 2‐4) were studied in detail. A marked dependence of the reaction course on the type and site of substitution was found. Among the several light‐induced transformations, a novel rearrangement, i.e. 11 to 9 (Scheme 3) was identified. The formation of the polycyclic compound 13 on irradiation of 8a (Scheme 3) resulted from an unexpected skeletal rearrangement with dearomatization of one benzene ring. The structures of compounds 10, 11, and 13 were established by X‐ray crystallography (Figs. 1‐3). An attempt was made to give a general mechanistic picture of all observed photochemical results (Schemes 4‐6).
Transesterifications with 1,8‐Diazabicyclo[5.4.0]undec‐7‐ene/Lithium Bromide (DBU/LiBr) – Also Applicable to Cleavage of Peptides from Resins in Merrifleld Syntheses
Authors: Scebach Dieter , Thaler Adrian , Blaser Denis , Ko Soo Y. 
Published: December 2004

AbstractA mixture of the amidine base 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) and LiBr (preferably 0,5 and 5 equiv., resp.) turns out to be a highly efficient catalyst (at 0–25°) for saponifications (in THF/H2O) and transesterifications (in ROH). The scope and limitations of the method are determined using ca. two dozens of different ester/alcohol combinations (Schemes 2 and 3). The investigation is focused on peptides as substrates. Under carefully controlled conditions, no epirnerization occurs with N‐Boc‐ and N‐Z‐protected peptide esters, when methyl, ethyl, isopropyl, or allyl esters are the products, as shown for peptides containing up to six amino acids, with Ala, Len MeLeu, Asp(OEt), or Tyr at the C‐terminus (Scheme 3 and Tables 1 and 2). Hydrolytic and transesterifying detachments of Boc‐Leu‐Ala‐Gly‐Val‐OR and Boc‐Leu‐Ala‐Gly‐Phe‐OR (R = H, Me) from PAM and Wang resins (1–8 h at 0–25°, 2 equiv. of DBU, 5 equiv. of LiBr) can be achieved by this method without epimerization of the C‐terrninal stereogenic center; a comparison with other methods (HF, Ti(OR)4) is given (Schemes 4 and 5). Possible protecting‐group strategies involving the DBU/LiBr method are discussed (Table 3), Extensive experimental details are given.
Aromatic nucleophilic substitution. Part 3. Preparation of novel 9‐oxo‐9H‐thioxanthene‐ and 9‐oxo‐9H‐xanthenedicarboximides and ‐dicarboxylates
Authors: Fischer Walter 
Published: December 2004

AbstractBy aromatic nucleophilic substitution followed by intramolecular acylation, 9‐oxo‐9H‐thioxanthene‐ and 9‐oxo‐9H‐xanthene‐dicarboximides were prepared from nitro‐ or chlorophthalimides and the dianions of thiosalicylic and salicylic acids (Scheme). The 9‐oxo‐9H‐thioxanthene‐3,4‐dicarboximides were converted to 9‐oxo‐9H‐thioxanthene‐3,4‐dicarboxylic‐acid derivatives such as anhydride, esters, and further imides. Some of these derivatives proved to be excellent photosensitizers with special properties such as liquid aggregation form, H2O solubility, solubility in lipophilic organic solvents and polymers, or bathochromic shifts of the absorption wavelengths.
Dimeric Metal Complexes as Mediators for Radical CC Bond‐Forming Reactions
Authors: Giese Bernd , Thoma Gebhard 
Published: December 2004

AbstractIrradiation of dicarbonyl(η5‐cyclopentadienyl)iron dimer 1 or decacarbonyldimanganese (2) in the presence of alkyl halides leads to C‐centered radicals which can be trappedby alkenes and yields saturated and/or unsaturated addition products. Carbon radicals are generated via halogen abstraction by the initially formed metal‐centered radicals resulting from homolysis of the metal‐metal bond of dimeric mediators 1 and 2. No reaction occurs using octacarbonyldicobalt (3).
Base Hydrolysis of (Acidato)(Pentaamine) Complexes with Inert Metal Centers: Electronic Structure of the Intermediates, Requirements for Their Formation, and the Unique Reactivity of the Complexes of Cobalt(III)
Authors: Rotzinger François P. , Weber Jacques , Daul Claude 
Published: December 2004

AbstractMolecular‐orbital calculations have been performed for the conjugate base cis‐[Co(NH3)4(NH2)Cl]+ and trans‐[M(NH3)4(NH2)Cl]+ (M = CrIII, CoIII, and RhIII), the hexacoordinated intermediates cis‐ and trans‐[Co(NH3)4(NH2)…Cl]+, the square‐pyramidal and trigonal bipyramidal pentacoordinated intermediates apical‐[Co(NH3)4(NH2)]2+, basal‐[Co(NH3)4(NH2)]2+, and equatorial‐[M(NH3)4(NH2)]2+ (M = Co and Rh), respectively, using modified extended Hückel and SCF MS‐Xα methods. The LUMO of the conjugate bases is an antibonding metal centered dρ* orbital which is stabilized during the dissociative activation of the MCl bond. For the above conjugate bases, separations of the highest doubly occupied MO (HDOMO) and the LUMO of 1.3, 2.5, 1.8, and 2.5 eV, respectively, have been calculated using the SCF MS‐Xα model. Only in the conjugate base cis‐[Co(NH3)4(NH2)Cl]+ with the smallest HDOMO — LUMO gap, the singlet electronic structure of the ground state may be stabilized by changing into a triplet when the CoCl bond is activated. This situation is unique to (acidato)(pentaamine)cobalt(III) complexes with deprotonated amine ligand cis to the leaving group and the reason for the existence of intermediates and their reactivity as well. Base hydrolysis of the analogues CrIII and RhIII complexes – the latter taken to represent the second‐ and third‐row transition‐metal amines – is unlikely to proceed via intermediates; a concerted substitution process is expected to take place.
Metabolic products of microorganisms. Part 262. The absolute configuration of sphydrofuran, a widespread metabolite from streptomycetes
Authors: Bindseil Kai U. , Henkel Thomas , Zeeck Axel , Bur Daniel , Niederer Daniel , Séquin Urs 
Published: December 2004

AbstractSphydrofuran (1) was isolated from the culture filtrate of Streptomyces sp. (strain Gö 28 and Tü 3616) by chemical screening methods. Metabolite 1 is an anomeric and ring‐chain tautomeric mixture and could easily be transformed into the stable furan derivative 2 under acidic conditions. The constitution and relative configuration of 1 was established by X‐ray crystallography of its 8‐O‐methyl derivative 3a. The absolute configuration at C(4) of 1 and thus of the whole molecule (3R,4S,5S) could be determined starting from 2 using the Helmchen method.
Synthesis of Aristotelia‐type alkaloids. Part VIII. Synthesis of (±)‐aristolasicone
Authors: Güller Rolf , Dobler Markus , Borschberg Hans‐Jurg 
Published: December 2004

AbstractThe revised structure of the indole alkaloid aristolasicone (2) was confirmed through a convergent total synthesis of the racemic form of this metabolite. The key step involves a one‐pot condensation/cyclization reaction between 1‐(4‐methoxyphenylsulfonyl)‐1H‐indole‐2‐acetaldehyde (9) and (±)‐trans‐5‐(2,6‐difluorobenzyloxy)‐p‐menth‐l‐en‐8‐amine ((±)‐7). The resulting allohobartine derivative (±)‐13, obtained in 84% yield, was deprotected and oxidized to (±)‐alloscrratenone ((±)‐15) which cyclized smoothly to the target molecule (±)‐2 upon exposure to BF3 · Et2O.
Synthesis of New Phospholipids Linked to Steroid‐Hormone Derivatives Designed for Two‐Dimensional Crystallization of Proteins
Authors: Lebeau Luc , Oudet Pierre , Mioskowski Charles 
Published: December 2004

AbstractThe synthesis of phospholipids 1n–3n, rationally designed for two‐dimensional crystallization of progesterone and estradiol receptors, is reported. The structure of these lipids provides them with essential properties such as fluidity and stability when spread into monolayers at the air/H2O interface, affinity for the protein to be crystallized, and accessibility of the ligand under the lipid monolayer.
Enzymatic Formation of Raucaffricine, the Major Indole Alkaloid of Rauwolfia serpentina Cell‐Suspension Cultures
Authors: Ruyter Carl Michael , Stöckigt Joachim H. H. 
Published: December 2004

AbstractThe major alkaloid from Rauwolfia serpentina cell‐suspension cultures, the glucoalkaloid raucaffricine (2), was enzymatically formed from vomilenine (1) and UDPG in presence of microsomal‐bound enzyme. This glucosyltransferase exhibits a relatively high substrate specificity with strong preference for 1 and UDPG. The apparent Km values for 1 and UDPG were 40 μM and 0.8 mM, respectively, for raucaffricine formation. Optimum transferase activity was observed at 50° and pH 6.3. The taxonomic distribution of this enzyme seems to be very limited because transferase can he exclusively detected in raucaffricine‐producing plant cells.
Synthese des Makrolid‐Antibiotikums (−)‐A26771B mit Photolactonisierung als Schlüsselreaktion und Computersimulation als effektive Optimierungshilfe
Authors: Quinkert Gerhard , Küber Frank , Knauf Wolfgang , Wacker Manfred , Koch Uwe , Becker Heinrich , Nestler Hans Peter , Dürner Gerd , Zimmermann Gottfried , Bats Jan W. , Egert Ernst 
Published: December 2004

AbstractSynthesis of the Macrolide Antibiotic (−)‐A2677IB Using Photolactonization as a Key Reaction and Computer Simulation as an Effective Aid in OptimizationThe title compound has been synthesized in a sequence of 21 steps starting from phenol. The cyclic skeleton has been built up in the earlier part of the synthesis by photolactonization. This reaction supplies C(6), with a functionality, not present in the target structure, but useful for oxygenation at C(4) and generation of the stereogenic center C(5). The Barton/McCombie reaction provides a product deoxygenated at C(6), but only after an adjacent oxirane ring has been opened at the cost of an increase in the number of overall steps. X‐Ray structural information and computational modelling of appropriate molecules proved to be essential to selection of optimal conditions for various transformations.
Characterization of Nitrosocarbonyl Compounds by Neutralization‐Reionization Mass Spectrometry
Authors: O'Bannon Patrick E. , Sülzle Detlev , Schwarz Helmut 
Published: December 2004

AbstractThe neutral molecules nitrosoformaldehyde (1a), 1‐nitrosoacetaldehyde (1b), and 2,2,2‐trifluoro‐1‐nitrosoacetaldehyde (1c) have been generated and detected by neutralization‐reionization mass spectrometry (NR‐MS).
Nucleotides. Part XXXVIII.. Syntheses and characterization of phosphorothioate analogues of (2′–5′) adenylate dimer and trimer and their 5′‐O‐monophosphates
Authors: Charubala Ramamurthy , Pfleiderer Wolfgang 
Published: December 2004

AbstractThe chemical syntheses of the phosphorothioate of (2′–5′)adenylate dimer (see 6a, 6b) and trimer (see 11a, 11b, 12a, 12b) as well as of their 5′‐monophosphates (see 15a, 15b, 16a, 16b) using the phosphoramidite method are described. The resulting diastereoisomer mixtures were separated into the pure components by chromatographical means. All synthetic intermediates were characterized by TLC, elemental analysis, and UV and 1H‐NMR spectra.
The Reaction of 9‐Bromoanthracene with Benzenethiolate Anion in Tetraglyme: Evidence against a competing electron‐transfer mechanism
Authors: Baumgarner Charles D. , Malen Alyssa H. , Pastor Stephen D. , Ali NabiRahni M. 
Published: December 2004

AbstractThe reaction of 9‐bromoanthracene (1) with benzenethiol(ate) in tetraglyme proceeds by a SNAr mechanism. The concurrent formation of anthracene is not due to a competing single‐electron‐transfer pathway involving either benzenethiol or benzenethiolate anion.
1,2‐Stereoinduction in Radicals and Anions: A comparison between hydrogen abstraction and protonation
Authors: Erdmann Peter , Schäfer Jörg , Springer Ronald , Zeitz Heinz‐Georg , Giese Bernd 
Published: December 2004

AbstractChiral enolates 5 and 10, generated by radical addition and subsequent reduction, show diastereofacial selectivity during protonation. In the presence of substituted amines, diastereoselectivity is enhanced and becomes comparable to radical stereoselection. Diastereoselectivities up to 99:1 and yields up to 90% are reached.
Synthesis of Selectively Substituted Lipocyclopolyamines
Authors: Brand Guy , Hosseini Mir Wais , Ruppert Romain 
Published: December 2004

AbstractA general synthetic strategy for the preparation of polyazamacrocycles containing long aliphatic chains is reported. These compounds are 22‐ or 24‐membered hexaazamacrocycles incorporating two diethylenetriamine subunits (H2NCH2CH2NHCH2CH2NH2) linked together either by two CH2CH2OCH2CH2 (see 4–6), two tetramethylene (see 7), or two pentamethylene (see 8) fragments. Whereas for compound 4, one of the centrally located amine functionalities of the triamine subunit bears a hexadecyl chain, for compound 5, two of such chains are centrally attached to both subunits. In macrocycles 6–8, all six amino functionalities bear hexadecyl chains. Compounds 4–8 are potential candidates for the construction of selective chemical sensors such as specific electrodes for adenosine mono‐, di‐, and tri‐phosphate. They may also be used for nucleoside polyphosphates extraction and/or transport.
Spiro[1,3‐benzoxathiepin‐4(5H),1′‐cyclohexa[2,4]diene]‐2,2′‐dione, a Novel Heterocyclic Ring System
Authors: Kolly Sandra , Meier Hansrudolf , Winkler Tammo 
Published: December 2004

AbstractThe 4,4′,6,6′‐tetrasubstituted 2,2′‐alkylidenebis(phenols) 1 reacted with CISCOI to give spiro[1,3‐benzoxathiepin‐4(5H), 1′‐cyclohexa[2,4]diene]‐2,2′‐diones 4, together with cyclic carbonates 5. The structures of the products were elucidated mainly by 13C‐NMR and 1H‐NMR spectroscopy.
Aspidosperma Alkaloids. A New Didehydrodimerization Mode of β‐Anilinoacrylic Alkaloids by Anodic Oxidation
Authors: Palmisano Giovanni , Danieli Bruno , Lesma Giordano , Santagostino Marco , Fiori Giorgi , Toma Lucio 
Published: December 2004

AbstractA regio‐ and stereoselective didehydrodimerization procedure, whose key step involves the anodic oxidation, allows the conversion of β‐anilinoacrylic alkaloids belonging to the Aspidosperma class, typified by tabersonine (1) and its 3‐oxo derivative 2, into the hitherto unknown 16,10′‐didehydro dimers 3 and 7, respectively.
The H/D Exchange and Hydrogenolysis of Butane on an Alumina‐Supported Nickel Catalyst
Authors: Öz Hami , Gäumann Tino 
Published: December 2004

AbstractThe exchange of butane with deuterium on a nickel/alumina catalyst is studied over a wide range of temperatures (50–230°) and for three hydrocarbon/deuterium ratios. The initial distribution is used to propose a reaction scheme by considering different adsorbed species such as: alkyl‐(α), alkene‐(α, β) and allyl‐(α,β,γ) species, where α, β, and γ are different C‐atoms. It is assumed that each adsorbed species can either desorb directly while exchanging one, two, and three H‐atoms or undergo multiple exchange before desorption. It is shown that by reducing the activity of the catalyst a back‐exchange of the fully deuterated species sets in a lower temperature; a systematic deviation of all model calculations for the estimation of the concentrations of the C4HD9 and C4D10 is, thus, explained. The exchange of CH3CD2CD2CH3 and CD3CH2CH2CD3 with H2 and D2 is also studied, and similar results are obtained as with exchanges of CH3CD2CH3 and CD3CH2CD3. With the help of very‐high‐resolution mass spectra, it is shown that the initial degree of deuteration, i. e. the degree of deuteration after one adsorption step, is 84% for the methylene and 57% for the methyl group respectively. The hydrogenolysis reactions of butane is studied in the same system between 180° and 230°. A wide range of conversion is covered, and the product distributions are fitted to kinetic equations in order to obtain the initial rate constants. On the nickel/alumina catalyst one or several C–C bonds are broken before the desorption of the species. Under the conditions used in these experiments, the surface cracking is the rate‐limiting step. An isotope effect is observed for the decomposition in D2, the production of propane being favored.
Phenols and Quinones from Seeds of Different Iris Species
Authors: Marner Franz‐Josef , Horper Wolfgang 
Published: December 2004

AbstractThe seed oils of I.pseudacorus LINN., I.sibirica LINN., and I.missouriensis NUTTAL. contain appreciable amounts of phenols, quinones, and resorcinols, substituted with homologous alkyl or alkenyl side chains. The structure elucidation of these compounds is described, and their possible biosynthesis is discussed.
New 2,2′‐Bipyridine Derivatives and Their Luminescence Properties with Europium(III) and Terbium(III) Ions
Authors: Mukkala Veli‐Matti , Kankare Jouko J. 
Published: December 2004

AbstractTwenty differently substituted 2,2′,2″,2‴ ‐[(2,2′‐bipyridine‐6,6′‐diyl)bis(methylenenitrilo)]tetrakis(acetc acids) 75–94 were synthesized with the purpose of developing new markers to be used in bioaffinity assays based on the unique luminescence properties of EuIII and TbIII ions. The relative luminescence yields, excitation maxima, and emission decay constants were determined for the corresponding EuIII and TbIII chelates. The substituents at the bipyridine moiety had a significant effect on the luminescence properties: the best relative luminescence yields R were obtained for ligands with electron‐donating substituents (e.g. Me, Ph), electron‐withdrawing substituents (e.g. NO2, COOH) had a reverse effect. However, no clear correlation between the relative luminescence yields and the substituent parameters was found.
An Enantioselective Approach to the Taxanes: Direct access to functionalized cis‐tricyclo[9.3.1.03,8]pentadecanes via α‐hydroxy ketone and Wagner‐Meerwein rearrangements
Authors: Paquette Leo A. , Elmore Steven W. , Combrink Keith D. , Hickey Eugene R. , Rogers Robin D. 
Published: December 2004

AbstractThe feasibility of the titled reactions for the rapid, enantioselective synthesis of cis‐tricyclo[9.3.1.03,8]pentadecane precursors to taxusin and taxol has been examined. The catalysts most well suited to inducing the appropriate 1,2‐shifts have been identified. To a great extent, the rearrangement products are formed as a direct consequence of appropriate structural features (kinetic phenomenon) and strain minimization (thermodynamic driving force). Complementary MM2 calculations of the global minimum in each series provided indications that were completely in line with the experimental observations. Sophisticated NMR studies and X‐ray crystallographic determinations were coordinated to remove any ambiguity of product structure and solid‐state conformation.
Conformational Studies of Marine Polyhalogenated α‐Chamigrenes Using Temperature‐dependent NMR spectra inverted‐chair and twist‐boat cyclohexane moieties in the presence of an axial halogen atom at C(8)
Authors: Guella Graziano , Chiasera Giuseppe , Pietra Francesco 
Published: December 2004

Abstractα‐Chamigren‐3‐one (+)‐8 bearing an axial CI‐atom at C(8) exists as a largely dominant conformer with Me—C(5) at the envelope‐shaped enone ring pointing away from CIax−C(8) at the cyclohexane ring (= B) in the ‘normal’ chair conformation, as shown by 1H‐NMR. In contrast, the α‐chamigren‐3‐ols (+)‐9 and (+)‐10, obtained from hydride reduction of (+)‐8, show a temperature‐dependent equilibrium of conformers where the major conformers have ring B in the inverted‐chair (and twist‐boat for (+)‐9) conformation to avoid repulsions between Me−C(5) and CIax–C(8) (Scheme 1). This is in agreement with the conformation of the epoxidation product (+)‐12 of (+)‐9 where Me–C(5) is pushed away from CIax–C(8) in a ring‐B chair similar to that of (+)‐8 (Scheme 2). Introduction of a pseudoequatorial Br‐atom at C(2) of (+)‐8, as in enone (+)‐15 (Scheme 3), does not affect the conformation; but a pseudoaxial BrC(2) experiences repulsive interactions with Heq–C(7), as shown by the 1H‐NMR data of the isomeric enone (+)‐16 where the ‘normal’‐chair conformer Cβ‐16 is in an equilibrium with the inverted chair conformer ICβ‐16 (Scheme 3). These results and the accompanying paper allow a unifying view on the conformational behavior of marine polyhalogenated α‐chamigrenes. This view is supported by the acid‐induced isomerization of α‐chamigrene (+)‐9 (inverted chair) to β‐chamigrene (+)‐17 (‘normal’ chair; Scheme 4), the driving force being the lesser space requirement of CH2C(5) than of Me–C(5). This explains why β‐chamigrenes are so common in nature.