bluuebunny/crossref_metadata_2025_split huggingface dataset:
Ketonic complexes of acetylacetone and ethyl acetoacetate with divalent transition metals
Authors: Driessen W. L. , Everstijn P. L. A. 
Published: September 2010

AbstractSeveral new coordination compounds are reported with 2,4‐pentanedione (acacH) and ethyl 3‐oxobutanoate (etaaH) as the neutral bidentate oxygen donor ligands, viz. [M(acacH)3]‐ (InCl4)2 and [M(etaaH)3](InCl4)2 where M is Mg(II), Mn(II), Fe(II), Co(II), Ni(II) and Zn(II). In the complexes the ligands are present in their ketonic form. The metal ions are surrounded octahedrally by six oxygen atoms.
A frontier orbital description of the reaction of singlet oxygen with simple aromatic systems
Authors: van den Heuvel C. J. M. , Verhoeven J. W. , de Boer Th. J. 
Published: September 2010

AbstractFrontier molecular orbital theory has been applied to the [4 + 2]‐cycloaddition of singlet oxygen, to mono‐ and dimethylnaphthalenes and to a number of other polycyclic aromatic compounds. Semilogarithmic plots of rate constants versus second order perturbation energies are found to be linear. The good correlations are due partly to the fact that singlet oxygen is such a small dienophile.
Chemistry of electron‐rich conjugated polyenes VI. A comparative study of the rates and mechanism of [4 + 2]‐cyclo‐additions between alkoxy‐1,3‐butadienes and β,β‐dicyanostyrenes
Authors: Broekhuis A. A. , Scheeren J. W. , Nivard R. J. F. 
Published: September 2010

AbstractThe kinetics of [4 + 2] cycloadditions in several solvents have been studied for reactions between ring‐substituted β,β‐dicyanostyrenes (2a–j) and a series of methoxybutadienes (1a–e), differing in electron‐donating properties, conformational preference and polarity. All reactions studied gave linear Hammett plots with positive p values when log k/k0 values were plotted against σ+ substituent constants. Solvent effects appeared to be small.The results have been discussed on the basis of differences in the HOMO‐LUMO interaction and in conformational factors for differing pairs of reactants. In this way more detailed insight has been obtained into variations in the nature (olefin‐like or product‐like) and polarity of the transition state of these reactions.
The crystal structure of copper(II) (R,S)‐1‐amino‐2‐propanol chloride
Authors: Nieuwpoort G. , de Kok A. J. , Romers C. 
Published: September 2010

AbstractCopper(II) (R,S)‐1‐amino‐2‐propanol chloride, Cu(C3NOH9),Cl2, is monoclinic. The lattice constants at 22°C are a = 8.050(5), b = 10.930(6), c = 8.120(6) Å, β = 92.20(6)°, Z = 4 and μ(MoKα) = 37.95 cm−1.Extinctions 0k0 with k odd indicate the space group P21, but weak reflections h0l with l odd demonstrate that the symmetry almost corresponds to P21/c. The structure was solved by Patterson methods. Least‐squares refinement in P21/c was unsatisfactory but refinement in P21 resulted in a final R value of 0.038 for 2192 reflections, 1772 of which have intensities greater than 2σ(I). The structure is disordered and shows three conformations of the ligand, i.e. the structure contains as major form, (58%), ligands A and Ā and as minor form, (42%), ligands B and C. In all cases the copper atoms are coordinated octahedrally by four chlorine atoms, one nitrogen and one oxygen atom. Two chlorine atoms are bound at 2.273(1) Å and 2.247(1) Å and two at 2.993(1) Å and 3.181(1) Å.
The chemistry of small ring compounds. Part 43. Synthesis of 1‐substituted cyclopropyl sulfides
Authors: Jorritsma R. , Steinberg H. , de Boer Th. J. 
Published: September 2010

Abstract1‐Hydroxy‐ and 1‐halocyclopropyl sulfides can be conveniently prepared starting from cyclopropanone. Cyclopropyl sulfides with halogen or a dimethylsulfonium group in the α‐position can be transformed into 1‐substituted cyclopropyl sulfides with a variety of nucleophiles. This means that the usual difficulties encountered with nucleophilic substitution at three‐membered rings ‐ i.e. low rates and ring opening — are largely removed by the presence of an alkylthio group at the same ring carbon atom as the leaving group.
Photo‐induced electron transfer from NADH and other 1,4‐dihydronicotinamides to methyl viologen
Authors: Martens F. M. , Verhoeven J. W. 
Published: September 2010

AbstractThe photo‐induced oxidation of NADH and its model compound, 1‐benzyl‐1,4–dihydronicotinamide (BNAH), by N,N′‐dimethyl‐4,4′‐bipyridinium dichloride (methyl viologen: MV+ +) was studied in homogeneous and in micellar solution. In the overall, slightly endoergonic, reaction: NADH + 2 MV+ + + hv → NAD+ + 2 MV +• + H+ the dihydropyridines lose a hydride equivalent via a stepwise e,H+,e sequence in which only the initial one‐electron transfer requires photostimulation. It is shown that the singlet‐excited state of the dihydropyridine is the photoreactive intermediate in this initial electron transfer process and that the overall quantum yield depends critically on the ratio of the rates for ionic dissociation and back‐electron transfer in the encounter complex between this excited state and MV+ + molecules. This ratio can be influenced dramatically by changing the solvent medium. In aqueous solution (for NADH) and especially in anionic micellar solution (for BNAH) strong ground state association between MV+ + and the dihydropyridines occurs. Evidence for this association is provided by the appearance of a long‐wavelength charge transfer absorption, as well as by the large deviations from a Stern‐Volmer behaviour observed in the quenching of the dihydropyridine fluorescence by MV+ +. The ground state complexation appears to hamper ionic dissociation of the excited complex. This is evident from the low quantum yield for photooxidation of such systems, upon excitation of the dihydropyridine chromophore (365 nm), and the complete absence of such oxidation upon their excitation within the charge transfer absorption (546 nm). The data obtained are shown to allow for a complete thermodynamic analysis of the Gibbs free‐energy changes involved in loss of a hydride equivalent from a 1,4‐dihydropyridine via a stepwise e,H+,e mechanism. The consequences of this analysis for the mechanism of reactions in which 1,4‐dihydropyridines transfer a hydride equivalent to various substrates under thermal conditions are discussed.
Fluorination of 2‐bromo‐4,5‐dimethylphenol with xenon difluoride
Authors: Koudstaal H. , Olieman C. 
Published: September 2010

AbstractFluorination of 2‐bromo‐4,5‐dimethylphenol (1) with xenon difluoride, using boron trifluoride etherate as catalyst, gave 2‐bromo‐4,5‐dimethyl‐4‐fluorocyclohexa‐2,5‐dienone (2), 2‐bromo‐3,4‐dimethyl‐6‐fluorophenol (3) and 6‐bromo‐3,4‐dimethyl‐2‐fluorophenol (4). The products were isolated by means of preparative reverse‐phase HPLC and identified by 1H and 13C NMR spectroscopy.
A convenient, stereospecific synthesis of racemic juvenile hormones and some of their analogues via vinylcuprates
Authors: Kleijn H. , Westmijze H. , Meijer J. , Vermeer P. 
Published: September 2010

AbstractThis paper describes a stereospecific synthesis of nine 2E,6E, 10(Z)‐trienyl esters R1R2CCH(CH2)2CR3CH(CH2)2CR4CHCO2Me (1a‐i) and their selective conversion into the corresponding epoxides magnified image (7a‐i), including the natural occurring hormones JHI‐III. The trienyl system of 1 has been reached stereospecifically by allowing specifically substituted vinylcuprates, [R1R2CCHCuY]⊖, to react with 4‐iodo‐1‐butyne, followed by conversion of the formed enynes R1R2CCH(CH2)2CCH (3) into new cuprates of the type thus [R1R2CCH(CH2)2CR3CHCuY]⊖. These cuprates furnish, upon reaction with 4‐iodo‐l‐butyne, the dienynes R1R2CCH(CH2)2CR3CH(CH2)2CCH (5). From 5, vinylcuprates [R1R2CCH(CH2)2CR3CH(CH2)2CR4CHCuY]⊖ have been obtained which, upon reaction with carbon dioxide followed by methylation, gave compounds 1. For the preparation of the vinylcuprates from the 1‐alkynes we generally used a new alkylcopper(I) species of the type R3Cu2MgX. The epoxidation of 1 into 7 was performed using a literature procedure.
The mechanism of the photocyclization of 1,4‐diarylbutenynes into aryl‐substituted aromatics. The influence of amines and oxygen
Authors: van Arendonk R. J. F. M. , de Violet P. H. Fornier , Laarhoven W. H. 
Published: September 2010

AbstractThe formation of a 1,2,4‐cyclohexatriene derivative (cf. 7) from the singlet‐excitedcis‐butenyne is thought to be the primary reaction step in the photocyclization of 1,4‐diarylbutenynes. This very reactive intermediate can yield the end‐product along various pathways: (i) directly via a [1,5] hydrogen shift; (ii) via radical intermediates, formed either by acceptance of a hydrogen atom from the solvent (e.g.benzene, hexane) or by release of a hydrogen atom in the presence of oxygen; (iii) via ionic intermediates, formed either by acceptance of a proton (e.g.from methanol) or by release of a proton in the presence of amines. The rates of the cyclization are approximately equal for various butenynes. Differences in product quantum yields are mainly due to differences in the rates of the ring‐opening reaction of the cyclic allene. Using laser flash photolysis two transients of the cyclization could be observed. The effect of amines on the transients is described.
The reaction of 2‐butynedinitrile with 2,3,4,5‐tetramethylfuran
Authors: Hall R. H. , Harkema S. , den Hertog H. J. , van Hummel G. J. , Reinhoudt D. N. 
Published: September 2010

Abstract2‐Butynedinitrile reacts by (2 + 4)‐cycloaddition with one or two equivalents of 2,3,4,5‐tetramethylfuran to give 1,4,5,6‐tetramethyl‐7‐oxabicyclo[2.2.1]hepta‐2,5‐diene‐2,3‐dicarbonitrile (1) and 1α,4α,4aα, 5β,8β,8aα‐hexahydro‐1,2,3,4,5,6,7,8‐octamethyl‐1,4:5,8‐diepoxy‐naphthalene‐4a,8a‐dicarbonitrile (5), respectively. The presence of aluminium(III) chloride has no effect on the reactions, unlike the corresponding reaction with the analogous thiophene. On irradiation, 1 is converted into 2,4,6,7‐tetramethyl‐3‐oxatetracyclo[3.2.0.02,7.04,6]heptane‐1,5‐dicarbonitrile (2) in good yield. The structure of 5 has been determined by a single‐crystal X‐ray analysis.
Studies on polypeptides. XXXV. Synthesis of a partially protected heptapeptide, corresponding to the sequence 14–20 of bovine pancreatic ribonuclease
Authors: Hoogerhout P. , Schattenkerk Cecile , Kerling K. E. T. 
Published: September 2010

AbstractThe partially protected heptapeptide, Boc—Asp(OtBu)—Ser—Ser‐Thr‐Ser‐Ala‐Ala‐OH, corresponding to the sequence 14‐20 of bovine pancreatic ribonuclease, was prepared by a combination of solid‐phase synthesis using Nα‐(9‐fluorenylmethoxycarbonyl) amino acids and synthesis in solution by the active ester method. The overall yield was 38%.
Synthesis of oligoribonucleotides with sequences identical to the nucleation region of Tobacco Mosaic Virus RNA: Preparation of AAG, AAGAAG and AAGAAGUUG via phosphotriester methods
Authors: den Hartog J. A. J. , Wille G. , van Boom J. H. 
Published: September 2010

AbstractThe synthesis of the oligoribonucleotides AAG (23), AAGAAG (24) and AAGAAGUUG (25), corresponding to fragments of the nucleation region of Tobacco Mosaic Virus RNA, is described. The assemblage of the oligomers 23–25 was effected using a phosphotriester method which was based on the use of the monofunctional phosphorylating agents 2,2,2‐trichloroethyl 2‐chlorophenyl phosphorochloridate (7) and the analogous 2,2,2‐tribromoethyl 2‐chloro‐4‐tert‐butylphenyl phosphorochloridate (8). The levulinyl function, which was introduced into the monomeric building blocks 1 (see Scheme 1) by an improved procedure, was used for the protection of the 5′‐hydroxyl group of phosphotriester intermediates. Deblocking of the fully protected oligomers 15f, 17g and 19h with fluoride ions, followed by ammonia and acid treatment, afforded the desired derivatives AAG, AAGAAG and AAGAAGUUG, respectively, in good yields.
Synthesis of three new compounds structurally related to damascone and ionone
Authors: Witteveen Jan G. , van der Weerdt Anton J. A. 
Published: September 2010

Abstract1‐(3,6,6‐Trimethyl‐1,3‐cyclohexadien‐1‐yl)‐2‐buten‐1‐one (1), a β‐damascenone isomer, was synthesized from 3,6,6‐trimethyl‐2‐cyclohexenone (14). This starting ketone 14 was obtained in good yield from 3‐methyl‐2‐butanone (11) and 3‐buten‐2‐one (12). Ethynylation of 14 with 3‐butyn‐2‐ol afforded acetylenic diol 20. Acid‐catalyzed rearrangement of 20 yielded
1‐(3,6,6‐Trimethyl‐1‐cyclohexen‐1‐yl)‐2‐buten‐1‐one (2), a β‐damascone isomer, was prepared from 1 by hydrogenation. Ketone 2 is shown to be identical to a naturally occurring compound found in fusel oil.
4‐(3,6,6‐Trimethyl‐1‐cyclohexen‐1‐yl)‐3‐buten‐2‐one (3), a β‐ionone isomer, was obtained by condensation of 3,6,6‐trimethyl‐1‐cyclohexenecarboxaldehyde (29) with acetone. Aldehyde (29) was synthesized from 2,2,5‐trimethylcyclohexanone (16) by reaction of its tosylhydrazone 28 with n‐butyllithium and DMF.
The compounds 1–3, 17–21 and 25–29 are new. 1.
Prenylation of methoxyhydroquinone in aqueous acid solution and an alternative synthesis of precocene II
Authors: Uchiyama M. , Overeem J. C. 
Published: September 2010

AbstractThe acid‐catalyzed condensation of methoxyhydroquinone with 2‐methyl‐3‐buten‐2‐ol leads to 2‐methoxy‐5‐(3‐methyl‐2‐butenyl)hydroquinone (4) and 2‐(1,1‐dimethylallyl)‐5‐methoxy‐hydroquinone (7). Compound 4 can easily be converted into precocene II (1, R  Me).
Acid‐catalysed isomerisation of epoxides to allylic alcohols
Authors: van Zon A. , Huis R. 
Published: September 2010

AbstractThe acid‐catalysed isomerisation of epoxides containing tertiary ring carbon atoms results in the selective formation of the corresponding allylic alcohols if the reaction is carried out in methylene chloride or chloroform in the presence of small quantities of sulfuric acid (< 10 mmol/l) or methanesulfonic acid (< 100 mmol/l). The zero‐order kinetics of the isomerisation have been rationalised by a mechanism in which a rapidly and quantitatively formed epoxide‐acid intermediate slowly decomposes into allylic alcohol and acid.
Reactions of overcrowded polymethylbenzenedisulfonic acids in concentrated aqueous sulfuric acid; protiodesulfonation and intramolecular sulfonic anhydride formation
Authors: Koeberg‐Telder Ankie , Cerfontain Hans 
Published: September 2010

AbstractThe reaction of the three tetramethylbenzenedisulfonic acids and some lower homologues in concentrated aqueous sulfuric acid has been studied at 25deg;C. 3,4,5,6‐Tetramethylbenzene‐1,2‐disulfonic acid undergoes both intramolecular disulfonic anhydride formation (the rate of which strongly increases upon increasing the sulfuric acid acidity) and protiodesulfonation (the rate of which decreases slightly with increasing acidity) while the other disulfonic acids only undergo protiodesulfonation. The observed rate order for protiodesulfonation within the tetramethylbenzenedisulfonic acids [1,2‐ > 1,3>> 1,4‐(SO3H)2] differs strongly from that predicted on the basis of the additivity principle of substituent effects [1,3>>;1,4‐ > 1,2‐(SO3H)2]. The strongly enhanced reactivity of the 1,2‐disulfonic acid is ascribed to a greater relief of steric strain in the 1,2‐than in the other disulfonic acids, the sulfo group being more bulky than the methyl. Also, within the group of tri‐ and tetra‐methylbenzene‐1,3‐disulfonic acids, the rate of desulfonation increases with an increase in the degree of overcrowding around the sulfo group which is desulfonating. Mechanisms for the two types of reaction of the tetramethylbenzenedisulfonic acids are proposed, based on the variation of the rate constants with the sulfuric acid acidity. It is concluded that the substrate entity undergoing protiodesulfonation is the disulfonate dianion and further that the entity undergoing cyclization to the 1,2‐disulfonic anhydride is the monoprotonated 1,2‐disulfonic acid.
Synthesis of DNA fragments containing 5‐methyl‐deoxycytidine residues
Authors: van der Marel G. A. , Wille G. , Westerink H. , Wang A. H.‐J. , Rich A. , Mellema J. R. , Altona C. , van Boom J. H. 
Published: September 2010

AbstractA properly protected 5‐methylated deoxycytidine (5mC) derivative has been synthesized and applied to assemble DNA fragments containing 5mCG, 5mCC and 5mC5mC sequences.
A study of mixed RuO2/TiO2 catalysts for the photochemical oxidation of water by persulfate ions, sensitised by ruthenium(II)tris(2,2′‐bipyridine)ions
Authors: Thewissen D. H. M. W. , Eeuwhorst‐Reinten M. , Timmer K. , Tinnemans A. H. A. , Mackor A. 
Published: September 2010

AbstractMixed RuO2/TiO2 catalysts with a varying RuO2/TiO2 ratio have been examined in the oxidation of water by persulfate ions under illumination with visible light, sensitised by Ru (bipy)32+. An optimum in the rate of oxygen evolution is found when the RuO2/TiO2 ratio amounts to about 10/90. A catalyst, which was prepared by precipitation of relatively small amounts of RuO2 on TiO2, displays a considerably higher activity in the photochemical oxidation of water than the mixed RuO2/TiO2 catalysts.
A simple approach to the synthesis of a membrane teichoic acid fragment of Staphylococcus aureus
Authors: Oltvoort J. J. , van Boeckel C. A. A. , de Koning J. H. , van Boom J. H. 
Published: September 2010

AbstractThe triflate procedure of Schmidt et al. enabled us to synthesize the glycolipid, 1,2‐di‐O‐stearoyl‐3‐O‐[6‐O‐(2,3,4‐tri‐O‐benzyl‐(3‐D‐glucopyranosyl)‐2,3,4‐tri‐O‐benzyl‐ß‐D‐glucopyranosyl)‐2,3,4‐tri‐O‐benzyl‐ß‐D‐glucopyranosyl]‐sn‐glycerolThe prefix sn stands for stereospecific numbering in accordance with the IUPAC recommendation, see: Eur. J. Bio‐chem. 2, 127 (1967).
. Phosphorylation of this compound with an activated and properly protected phosphatidyl derivative gave a phosphotriester derivative which, after deblocking of all protective groups, afforded a naturally occurring teichoic acid fragment.
Adsorption and Mössbauer studies of partially dehydrated FeCaA zeolites
Authors: van de Vloed G. , de Roy G. L. , Verhaert I. , Vansant E. F. 
Published: September 2010

AbstractThe physicochemical properties of the iron exchanged A‐type zeolites are affected by the degree of dehydration of the samples. Mössbauer and adsorption techniques were used to investigate the cation distributions in these zeolites. At lower degrees of hydration, a significant number of iron ions are located inside the sodalite cavities. A systematic study of the adsorption of probe gases, such as CH4, CO and CO2, was subsequently carried out and the results were correlated to the structural features of the dehydration.
Photocycloadditions of xanthenethione to allenic compounds
Authors: Kamphuis J. , Hupkes J. G. , Visser R. G. , Bos H. J. T. 
Published: September 2010

AbstractThe (2 + 2)‐ and (4 + 2)‐cycloaddition of triplet xanthenethione to phenyl‐, methoxy‐tert‐butoxy‐, (methylthio)‐, (tert‐butylthio)‐ and 1,1‐dimethyl‐allene gives rise to thietanes 3 and 4, and to thiopyran derivative 5, respectively. The mechanism of the reaction is discussed. In the case of thietane 3 (X = Ph), prototropy to the stable thiete 6 has been observed.
A model study of the role of the CONH2‐group in the redox co‐enzyme NAD(P)+‐NAD(P)H. chirality in 2,4‐dimethyl‐3‐carbamoylpyridinium cations
Authors: van Hooff H. J. G. , van Lier P. M. , Bastiaansen L. A. M. , Buck H. M. 
Published: September 2010

AbstractExperimental verification of chirality in 2,4‐dimethyl‐3‐carbamoylpyridinium cations.
Conversion of 2‐alkylcyclopentanones into 2‐alkyl‐2‐cyclopentenones with hydrated ferric chloride and cupric chloride
Authors: Cardinale G. , Laan J. A. M. , Russell S. W. , Ward J. P. 
Published: September 2010

AbstractHydrated ferric chloride, FeCl3·6 H2O, in a polar, hydroxylic solvent converted 2‐alkylcyclopentanones directly into 2‐alkyl‐2‐cyclopentenones on heating in the presence of air. Yields of isolated products were about 40 %. The scope of the reaction is discussed. With cupric chloride, chloroketones were formed as intermediates, which could be dehydrochlorinated to 2‐alkyl‐2‐cyclopentenones in similar yields.
Synthesis of 1‐isocyano‐1‐tosylalkenes using lithio‐α‐(trimethylsilyl)tosylmethyl isocyanide
Authors: van Leusen Albert M. , Wildeman Jurjen 
Published: September 2010

AbstractThe previously unreported lithio derivative of α‐(trimethylsilyl)tosylmethyl isocyanide (4) has been used in an improved method for the synthesis of 1‐isocyano‐l‐tosylalkenes (1) by reaction with aromatic aldehydes, with α,β‐unsaturated aldehydes and with cyclobutanone (Peterson reaction). Different results are obtained with chloral.
The reaction of some polycyclic halides with lithium p,p′‐di‐tert‐butylbiphenyl
Authors: Stapersma J. , Kuipers P. , Klumpp G. W. 
Published: September 2010

Abstract3‐Chloroquadricyclane (1), 2(4)‐chlorosemibullvalene (2a), a series of 2‐norbornyl halides and 4,4‐dichlorotetracyclo[3.3.0.02.8.03.6]octane (4) have been treated with lithium p,p′‐di‐tert‐butylbiphenyl. Lithiation (2, norbornyl halides), lithiation with rearrangement (4) and lithiation with rearrangement followed by reduction to the cycloheptatrienyl trianion (1 → 14) have been found to occur.
Synthesis and photoisomerisation of 2,3,17,18,22‐pentamethyl‐10,23‐dihydro‐1,19‐[21H,24H]‐bilindione, an unsymmetrical bilirubin model compound
Authors: de Groot J. A. , van der Steen R. , Fokkens R. , Lugtenburg J. 
Published: September 2010

AbstractThe unsymmetrical bilirubin model compound 2,3,17,18,22‐pentamethyl‐ 10,23‐dihydro‐1,19‐[21H,24H]‐bilindione 1c has been prepared in good yield. Irradiation of 1c leads to a photostationary state which contains three photoproducts in addition to the starting material. The photoproducts have been isolated and fully characterized as the three possible geometric isomers of 1c. Short irradiation of each of the photoisomers shows that a one‐photon one‐double‐bond isomerisation takes place.
Phosphotriester synthesis of DNA fragments on cellulose and polystyrene solid supports
Authors: van der Marel G. A. , Marugg J. E. , de Vroom E. , Wille G. , Tromp M. , van Boeckel C. A. A. , van Boom J. H. 
Published: September 2010

AbstractThe synthesis of a total of twelve DNA fragments on the solid supports cellulose or polystyrene via a phosphotriester approach is described. Dimer and trimer blocks, protected at the 5′‐end with the levulinoyl group, were used for chain extension in the presence of the activating agent (2,4,6‐triisopropylbenzenesulfonyl)‐3‐nitro‐1,2,4‐triazole or (mesitylenesulfonyl)‐3‐nitro‐1,2,4‐triazole. The yields of the coupling procedures are normally high and the final unprotected DNA fragments released from the solid support could easily be isolated by Sephadex G50 column chromatography.
The regiochemistry of organocopper‐induced substitution of 2,4‐pentadiynyl methanesulfinates
Authors: Oostveen E. A. , Elsevier C. J. , Meijer J. , Vermeer P. 
Published: September 2010

AbstractThe reaction of alkylcopper(I) compounds, RCu, with 2,4‐pentadiynyl methanesulfinates, R′CC‐CCCR″R′″OS(O)Me (1), shows a regiochemistry that depends on the size of R, R′, R″ and R′″. If the group R′ is methyl or trimethylsilyl, the reaction is regiospecific and leads to pure allenynes R′CCC(R)CCR″R′″ (2). Mixtures of allenynes 2 and pentatetraenes RR′CCCCCR″R′″ (3), however, may arise when 1, in which R′ is hydrogen, is used as substrate.