Tetrahedron Letters Research Articles

Bibliometric Study of Microwave-Assisted Synthesis of Flavanone Derivatives

This study aims to describe various studies related to synthesizing flavanone derivatives using the microwave-assisted method with a bibliometric approach. Metadata information was collected from Scopus on June 30th, 2023, with three keywords (microwave-assisted OR microwave, synthesis, and flavanone) searched for article titles, abstracts, and keywords. Analysis and research mapping were carried out with VOSviewer. Of the 33 articles relevant for analysis, 15 keyword clusters were obtained, most of which contained a list of flavanone derivatives. Interestingly, none of these clusters contain keywords for well-known compounds from the flavanone group, such as pinostrobin, pinocembrin, or hesperetin. In other words, there is an excellent opportunity to obtain novelty for microwave-assisted derivatization studies of flavanones. The chances of publication of these studies are greater in Chemistry of Heterocyclic Compounds (Springer Nature), Oriental Journal of Chemistry (Scientific Publishers), and Tetrahedron Letters (Elsevier), each with three documents. Meanwhile, most researchers on this topic come from India with 13 documents. This information allows researchers on this topic to determine potential flavanones that have the opportunity to be derivatized by the microwave-assisted method.

The Effect of Surface-Abundant Hydrogen Bonding on the Electrolyte Reduction for the Stable SEI in Lithium Metal Batteries

Lithium (Li) metal anodes (LMAs) have been attracted world-wide attention as an ideal anode because of its extra-high theoretical capacity (3860 mAh g-1) and low electrode potential (-3.04 V vs S.H.E.). However, the dendritic growth of Li and low Coulombic efficiency (CE) are still hindering their practical uses [1]. To date, numerous methods such as construction of artificial solid electrolyte layer (ASEI) [2], adoption of 3D current collector [3], and tuning of the electrolyte composition [4] have been proposed to prevent Li dendrite growth and increase the CE. Among them, introducing functional additives is one of the most efficient approaches for practical application considering its cost-effectiveness. Until now, various functional additives were introduced to form stable and robust SEI layer in LMBs [4]. Among them, lithium nitrate (LiNO3) is considered as the most efficient electrolyte additive, ensuring high coulombic efficiency (CE) as well as long lifespan of LMBs. When LiNO3 is dissolved in the electrolyte, NO3 - anions are mainly reduced to form inorganic species such as Li3N, which has a high ionic conductivity and mechanical strength. As such species contribute to the construction of the robust and ionic-conductive SEI layer, and hence the reduction of NO3 - is important for stable Li cycling. In this regard, many researchers have focused on increasing reduction of NO3 - by using high-concentration LiNO3 [4], or adding solubilizer to increase more NO3 - in the electrolyte [5]. However, those remedies are still insufficient because most of them increase the viscosity of electrolyte leading to low kinetics, hence a novel and more efficient way to increase NO3 - reduction is needed for practical application.On the other hand, recent researches have reported that the preferential reduction of specific anions is possible by regulation of inner Helmholtz plane (IHP) structure [6]. For instance, Huang et al. reported that intermolecular force between PF6 - anions and surface adsorbent tris(trimethylsilyl) borate could derive in PF6 --abundant IHP, successfully resulted in LiF-rich SEI layer to increase the stability of LMA [6]. Inspired by those works, we expected that NO3 --derived SEI layer would be achieved by using surface adsorbent showing strong intermolecular interaction with NO3 -.In this context, we introduce the adoption of thiourea (TU) as a catalytic additive for the LiNO3 reduction during the SEI formation. Due to its unique molecular structure, addition of TU could induce NO3 - derived SEI layer. Firstly, TU could adsorb onto metallic surface by its S atom. Meanwhile, thiourea could form hydrogen bonding with NO3 - anion by its N-H bonds [7]. Hence in the presence of TU, we suggest that NO3 --abundant electrode surface would be achieved by interaction between TU-NO3 -, resulting in Li3N-rich SEI layer.The adsorption behavior of TU on the Cu electrode was investigated by potential of zero charge (PZC) measurement (Figure 1(a)). As the TU concentration increases, PZC decreases, indicating more surface coverage by TU. Figure 1(b) shows 1H NMR spectra of electrolytes with different components. Upshift displacement of N-H bond of TU were detected after addition of DME and LiTFSI, indicating that intramolecular H-bond of TU were weakened. By contrast, downshift displacement appeared when LiNO3 was added, which means NO3 - would form strong hydrogen bonding with TU. Furthermore, linear scanning voltammetry (LSV) curves at different concentration of TU were measured to investigate the effect of TU on electrolyte reduction (Figure 1(c)). The distinct peaks at 1.6 V and 1.3 V in the cell with 5 wt% LiNO3 indicate reduction of LiNO3 and LiTFSI, respectively. Interestingly, in the presence of TU, negative potential shift and increased current of those redox peaks were shown, indicating that the TU significantly increases the LiNO3 reduction.Importantly, from the XPS analysis, it was found that more abundant Li3N components are in the ASEI layer with TU than that without TU, implying that TU accelerates the reduction of LiNO3 (Figure 2(a-b)). As a result, Li|Cu@ASEI with TU shows better cyclability and higher average CE of 96.44% during 80 cycles compared to Li|Cu@NSEI and Li|Cu@ASEI w/o TU (Figure 3). In addition, morphological and chemical investigation on the favorable ASEI layers assisted by TU, and its electrochemical performance in LMBs will be discussed in this presentation.[1] Cheng et al, Chem. Rev, 117, 10403, 2017.[2] Lopez Jeffrey, et al. JACS 140.37 (2018): 11735-11744.[3] Yang Chun-Peng et al. Nature communications 6.1 (2015): 1-9.[4] Kang et al. Journal of Power Sources 490 (2021): 229504.[5] Zhang et al. Advanced Materials 32.24 (2020): 2001740.[6] Huang et al. Angewandte Chemie. 60.35 (2021): 19232-19240.[7] Nishizawa et al. Tetrahedron letters 36.36 (1995): 6483-6486. Figure 1

Sir Robert Robinson—Minor Prophet, a most senior referee, a story of darkness and light

The story of the Tetrahedron Letters affair in 1973–1974 is described and lessons for Editors, Referees, and Authors are drawn from the TL affair and related correspondence of Sir Robert Robinson (Sir RR) (Nobel Laureate), a Minor Prophet. The TL affair centered at two related papers authored by Israel Agranat, et al. on reversible Friedel-Crafts acylations and acyl rearrangements. The first paper (A) was published in J. C. S. Chem. Comm. (1973); it was followed by the second paper (B), which contained a reference to A “in the press.” It was originally accepted for publication in TL, following a favorable review. However, a month later, it was rejected in view of the reviewer’s subsequent allegation of lack of novelty due to A. The TL Editor Professor Alan Roy Katritzky relied also on the opinion of one of the journal’s most senior referees, Sir Robert Robinson, the founder of Tetrahedron and Tetrahedron Letters and the Chairman of their Honorary Advisory Boards, who in 1973 was blind. The TL affair led to correspondences of Sir RR with Professor Ernst David Bergmann and with the Author. Paper B was published (after modifications) in 1974 in Synthetic Communications.

Application of Bibliometric Analysis to Letters Journals in Organic Chemistry

ABSTRACT Subject librarians are well-positioned to apply and teach searching techniques using online library resources for assisting researchers and scholars on where to publish. A bibliometric analysis was used to compare four premier letters journals in organic chemistry: Bioorganic & Medicinal Chemistry Letters, Letters in Organic Chemistry, Organic Letters, and Tetrahedron Letters. The objective of this study is to develop a routine methodology for evaluating and comparing journals with a similar scope based on researchers’ immediate needs. The study provides a practical guide to help organic chemists decide where to submit a short article for publication. For initial comparative purposes, general parameters, such as subscription rate, open access availability and article processing charge, field categories, language, maximum page length, citation style, peer-review type, review time, publication frequency, impact factor, journal h-index, acceptance rate, and database coverage were obtained from various online resources. Between 1999 and 2016, a total of 20,675 articles from Bioorganic & Medicinal Chemistry Letters, 1,732 articles from Letters in Organic Chemistry, 24,889 articles from Organic Letters, and 34,380 articles from Tetrahedron Letters were retrieved from the advanced search option in Web of Science Core Collection (Science Citation Index Expanded). The articles were examined by year, language, country, and organization. The VOSviewer software was employed for an in-depth map analysis of the research topics covered in each journal derived from key terms in article titles. The SciFinder database was used to compare similarities and differences from the articles in each letters journal through Chemical s indexing.

Trends in Pharmaceutical Scholarly Communications from India, China and United States a Comparative Study


 
 
 The present study throws light on the scholarly communication of three leading countries in the field of pharmaceutical research as reflected in SCOPUS database during 1998-2017. In terms of publication output, United States with two leading Asian countries, China and India leads with 40.54 per cent share of the global research publication share in pharmaceutical sciences. The global outcome of scholarly communication in the field of pharmaceutical research is 1395221. The study mainly focuses both on qualitative and quantitative research growth of United States, China and India in terms of output of scholarly communication, citation impact, relative research effort, common sources used for publications and research collaboration. The growth pattern of three leading countries is highly chaotic. The relative research effort of India and China increased during 2008-2017 while in United States, it was decreased. Bioorganic and Medicinal Chemistry, Tetrahedron Letters and Tetrahedron are the common source of communication in these three countries. All the three countries show the positive shift in international collaboration during 1998-2002 to 2012-2017 in pharmaceutical research.
 
 

Introducing article numbering to Tetrahedron Letters

Introducing article numbering to Tetrahedron Letters

Impedance Spectroscopy and Differential Electrochemical Mass Spectrometry As Tools for Characterization of Mediated Oxidation of Organic Pollutants

Persistent Organic Pollutants (POP) are hazardous environmental contaminants of mainly synthetic origin, highly toxic and chemically inert. They accumulate in the environment and in living organisms. Most POPs do not ignite themselves and they cannot be combusted. Therefore it is very important to find an alternative processes of POP neutralization. Mediated Electrochemical Oxidation has been proven to be an efficient approach, in which those dangerous substances are electrochemically combusted under ambient conditions. Mediated electrochemical oxidation is an indirect electrolysis, where pollutants are oxidized through the mediation of some electrochemically generated redox reagents, which act as an intermediary for electrons shuttling between the electrode and the target molecule. This technique enables the possibility to oxidize an (electro)inactive compounds.1 Recently, we have derived the exact analytical solution for impedance of catalytic processes involving redox mediators. The results obtained for general case in the rigorous treatment provide a powerful research tool that can be used in studies of many important catalytic processes.2 Unusual impedance characteristics have been found for systems having unequal diffusivities of the oxidized and reduced forms of the redox mediator. Depending on the ratio of diffusion coefficients the spectra are either markedly deformed at low frequency range or an inductive loop appears at the complex plane. It is impossible to fit these spectra in the whole frequency range, neither with the Gerischer impedance nor with any other known transfer function.2 We show that the key parameters of the studied systems, such as both heterogeneous and homogeneous rate constants, which are limiting factors for the electrochemical combustion efficiency, can be readily obtained even from a single impedance spectrum.2 In this research, silver(II) compounds were used as redox mediators for electrochemical combustion of organic molecules.3 As reported by us previously, the redox potential of the system Ag(II)/Ag(I) can reach up to 2.9V in sulfuric acid media.4 Moreover the average lifetime of the produced radical oxidant was estimated to be in order of seconds.5 Initially, investigations were carried out for a model organic molecule - acetic acid, which is known to be difficult to oxidize, and next for 1,2-dichlorobenzene an organic compound from POP group. The catalytic activity of Ag(II) was characterized mainly by using electrochemical methods with a particular emphasis placed on Electrochemical Impedance Spectroscopy (EIS). We have also performed analysis of volatile products by using mass spectrometry coupled with electrochemical measurements DEMS (Differential Electrochemical Mass Spectroscopy). This technique allows to detect in situ volatile electrooxidation products and/or intermediates. The latter were monitored for catalytic electrooxidation of acetic acid and 1,2-dichlorobenzene in function of electrode potential, for both compounds a large amount of CO2 was detected. 1. M. Panizza and G. Cerisola, Chemical Reviews, 2009, 109, 6541-6569. 2. R. Jurczakowski and P. Polczynski, J Phys Chem C, 2014, 118, 7980-7988. 3. M. Fleischmann and D. Pletcher, Tetrahedron Letters, 1968, 9, 6255-6258. 4. P. Polczynski, R. Jurczakowski and W. Grochala, Chem Commun, 2013, 49, 7480-7482. 5. P. Polczynski, R. Jurczakowski and W. Grochala, J Phys Chem C, 2013, 117, 20689-20696.

Synthetic Studies Towards (-) Mesembrine

Mesembrine is a naturally occurring alkaloid which has been isolated from Sceletium tortosum, family Aizoaceae. It has a therapeutic value in the treatment of depression as selective serotonin reuptake inhibitor. Because of its challenging chemical features such as a cis-3a-aryloctahydroindole moiety with syn configuration at two bridge-head stereogenic centers. [1] Several synthetic studies have been published to control the construction of the sterically hindered, benzylic quaternary stereogenic center at C-3a. [2] By utilizing Michael addition on chiral enolate derived from benzyl oxazoline, the desired stereogenic center at carbon 3a was created early in the synthesis followed by carbene insertion and intramolecular aldol reactions to complete the aryloctahydroindole scaffold. The complete details of synthetic transformations will be discussed. Acknowledgements: This research is supported in part by The United States Department of Agriculture, Agricultural Research Service, Specific Cooperative Agreement No. 58 – 6408 – 1-603 – 07

Novel System Based on Divalent Silver for Mediated Electrochemical Oxidation of Persistent Organic Pollutants

Persistent Organic Pollutants (POP) are hazardous environmental contaminants of mainly synthetic origin, highly toxic and chemically inert. They accumulate in the environment and in living organisms. The highest concentrations are found in animals at the top of the food chains. The most commonly human health effects related to POPs are skin conditions, liver damage, impaired fetal growth1 and further dysfunctions in intellectual development of the child.2 Most POPs do not ignite themselves and they cannot be combusted. Incineration process result in incomplete combustion despite a large excess of fuels and oxygen. This lead to the formation of dioxins, furans and PCBs, which are known human carcinogens. Therefore it is very important to find an alternative processes of POP neutralization. Mediated Electrochemical Oxidation has been proven to be an efficient approach, in which those dangerous substances are electrochemically combusted under ambient conditions. Mediated electrochemical oxidation is an indirect electrolysis, where pollutants are oxidized through the mediation of some electrochemically generated redox reagents, which act as an intermediary for electrons shuttling between the electrode and the target molecule. This technique enables the possibility to oxidize an (electro)inactive compounds.3 In this research, for mediated electrochemical oxidation, we have chosen silver(II) compounds as redox mediators.4 As reported by us previously, the redox potential of the system Ag(II)/Ag(I) can reach up to 2.9V in sulfuric acid media.5 Moreover the average lifetime of the produced radical oxidant was estimated to be in order of seconds.6 The catalytic activity of Ag(II) was characterized mainly by using electrochemical methods with a particular emphasis placed on Electrochemical Impedance Spectroscopy (EIS). Recently, we have derived the exact solution for the analysis of catalytic systems. Kinetics of the pseudo-first order chemical reaction with depolarizer reproducing was included in the model using the proposed element.7 Here we show that the key parameters of the studied systems, such as both heterogeneous and homogeneous rate constants, which are limiting factors for the electrochemical combustion efficiency, can be readily obtained even from a single impedance spectrum.7 Exemplary results for electrocatalytic oxidation of 1,2-dichlorobenzene are shown in Figure 1. Before the addition of the pesticide, two semicircles are observed on the Niquist plot of impedance registered in sulfuric acid containing silver(I) (-○-). The first semicircle, related to the coupling of the charge transfer resistance with the double layer capacitance, is followed by the second skewed semicircle characteristic for the Gerischer Impedance and related to catalytic oxidation of the acid.6 The determined homogenous reaction rate constant for the latter is equal to k=0.0100±0.0003 dm3∙mol-1∙s-1. The nonzero value is related to a finite half-life time of the electrogenerated oxidizer. After addition of the pesticide (-●-), the second semicircle decreases markedly. According to our theoretical model, this is related to a higher rate constant of the chemical homogeneous step.7 First semicircle is also smaller because the resistance of the charge transfer is inversely proportional to the square root of the homogeneous reaction rate constant.7 The homogenous reaction rate constant of pesticide oxidation is equal to 120±5 dm3∙mol-1∙s-1, which is four order of magnitude higher than that observed in pure sulfuric acid. This result indicates that electrogenerated Ag(II) effectively oxidize 1,2-dichlorobenzene. Products and possible mechanism of this reaction will be discussed. 1. C. Koopman-Esseboom, D. C. Morse, N. Weisglas-Kuperus, I. J. Lutkeschipholt, C. G. Van Der Paauw, L. G. Tuinstra, A. Brouwer and P. J. Sauer, Pediatric res, 1994, 36, 468-473. 2. J. L. Jacobson, S. W. Jacobson and H. E. Humphrey, J pediatrics, 1990, 116, 38-45. 3. M. Panizza and G. Cerisola, Chem Rev, 2009, 109, 6541-6569. 4. M. Fleischmann and D. Pletcher, Tetrahedron Letters, 1968, 9, 6255-6258. 5. P. Polczynski, R. Jurczakowski and W. Grochala, Chem. Commun., 2013, 49, 7480-7482. 6. P. Polczynski, R. Jurczakowski and W. Grochala, J Phys. Chem. C, 2013, 117, 20689-20696. 7. R. Jurczakowski and P. Polczynski, J Phys. Chem. C, 2014, 118, 7980-7988. Figure 1

Gerhard Quinkert (1927-2015).

Gerhard Quinkert (1927-2015).

The π-complex in the benzidine rearrangement: a molecular orbital analysis.

Michael Dewar[1] famously implicated a so-called π-complex in the benzidine rearrangement, back in the days when quantum mechanical calculations could not yet provide a quantitatively accurate reality check. Because this π-complex actually remains a relatively unusual species to encounter in day-to-day chemistry, I thought I would try to show in a simple way how it forms. I am actually illustrating it with the benzidine rearrangement of monoprotonated PhNHOPh, which I dealt with in the previous post, if only because the energy of this π-complex relative to monoprotonated PhNHOPh is amazingly low (in other words, it is not one of these high energy molecules which only exist in the virtual world of computational modelling). The mechanism can be conceptually broken down to considering how the N-O bond can be cleaved in one of three ways. Route A is the homolytic route to give a 4-biradical (in one of the possible resonance forms), which of course can couple to form a 4,4′-biphenyl. Route B is a heterolytic route in which the two electrons from the N-O σ-bond are retained by 1, whilst for route C this electron pair is retained by 4. These two fragments can then interact in several ways to form the π-complex. Here I will illustrate just the two closed shell options (B/C), whilst recognising that there may also be contribution from the open shell biradical (in water as solvent, the two ionic configurations are clearly going to be stabilised by solvation and so may contribute relatively more than the non-polar radical-pair ). Route B (green), overlapping the HOMO of 1 with the LUMO of 2 to create a new π-MO to be occupied by the two electrons extracted from the N-O σ-bond (a similar promotion of a σ- to a π-pair was noted in this post). Route C (red), overlapping the HOMO of 4 with the LUMO of 3 to achieve the same result. Route B LUMO of 2. Click for 3D. HOMO for π-complex. Click for 3D. HOMO of 1. Click for 3D. Route C LUMO of 3. Click for 3D. HOMO for π-complex. Click for 3D. HOMO of 4. Click for 3D. The relative weight of these two combinations is largely determined by the difference in energies between the two HOMO/LUMO pairs and their overlap. ΔE is different for the two combinations, being 0.021 Hartree (route B) and 0.091 (route C), with lower being better. The overlap of the HOMO/LUMO (in either orbital combination) is almost perfect for the face-to-face π-stacking of the complex. Note that this π-π-stacked arrangement in effect returns some electrons to the N-O region, in what is now called σ-π conjugation, and which used to be called hyperconjugation (it also resembles the conjugation of a Si-C bond with a phenyl ring in the Wheland intermediate). References M. Dewar, and H. McNicoll, "Mechanism of the benzidine rearrangement", Tetrahedron Letters, vol. 1, pp. 22-23, 1959. http://dx.doi.org/10.1016/S0040-4039(01)82765-9 <!-- kcite-section 9186 --&gt

Erratum to “The origin of homochirality in amino acids and sugars on prebiotic Earth” [Tetrahedron Letters 52 (2011) 2028-2032

Erratum to “The origin of homochirality in amino acids and sugars on prebiotic Earth” [Tetrahedron Letters 52 (2011) 2028-2032

Bacterial Pericarditis due to Providencia stuartii

A 58-year-old man was admitted for relapsing pericarditis. His past medical history included a transient ischemic attack when he was 47 years old, radical right nephrectomy for neoplasia 8 years earlier, and hypertension. One year earlier, a diagnosis of pleuropericarditis was made in another hospital. The patient was febrile and complained of chest pain; an echocardiogram showed thickening of parietal pericardium, hyperechoic matter on visceral pericardium, and the presence of pericardial and pleural effusion, without hemodynamic impairment. Concomitantly, the patient had a dental abscess; cultures from this abscess were negative during antibiotic therapy. He was treated empirically with piperacillin/tazobactam for 15 days and then with amoxicillin/clavulanate for ≈1 month and …

A content analysis of referees’ comments: how do comments on manuscripts rejected by a high-impact journal and later published in either a low- or high-impact journal differ?

Using the data of a comprehensive evaluation study on the peer review process of Angewandte Chemie International Edition (AC-IE), we examined in this study the way in which referees’ comments differ on manuscripts rejected at AC-IE and later published in either a low-impact journal (Tetrahedron Letters, n = 54) or a high-impact journal (Journal of the American Chemical Society, n = 42). For this purpose, a content analysis was performed of comments which led to the rejection of the manuscripts at AC-IE. For the content analysis, a classification scheme with thematic areas developed by Bornmann et al. (2008) was used. As the results of the analysis demonstrate, a large number of negative comments from referees in the areas “Relevance of contribution” and “Design/Conception” are clear signs that a manuscript rejected at AC-IE will not be published later in a high-impact journal. The number of negative statements in the areas “Writing/Presentation,” “Discussion of results,” “Method/Statistics,” and “Reference to the literature and documentation,” on the other hand, had no statistically significant influence on the probability that a rejected manuscript would later be published in a low- or high-impact journal. The results of this study have various implications for authors, journal editors and referees.

Preface to the Tetrahedron Letters 50th Anniversary Special Issue

Preface to the Tetrahedron Letters 50th Anniversary Special Issue

WITHDRAWN: Lanthanum metal-assisted cyclopropanation of alkenes with gem-dihaloalkanes

The Publisher regrets that this article is an accidental duplication of an article that has already been published in Tetrahedron Letters , doi: 10.1016/j.tetlet.2007.06.168 . The duplicate article has therefore been withdrawn.

RETRACTED: Regioselective 1,2-hydroxy and methoxy iodination of alkenes by molecular iodine and aqueous hydrogen peroxide

This article has been retracted at the request of the authors and editor. Please see http://www.elsevier.com/locate/withdrawalpolicy . Reason: Subsequent to the publication of the article in Tetrahedron Letters 47 (2006) 1997–2001, it has come to the knowledge of the authors and editor that similar research work had already been published by Professor Marjan Jereb et al. in Green Chemistry 7 (2005) 100–104. The editor and authors have, therefore, requested the article be retracted from Tetrahedron Letters and apologize for the oversight.

Statistical relationships between downloads and citations at the level of individual documents within a single journal

Statistical relationships between downloads from ScienceDirect of documents in Elsevier's electronic journal Tetrahedron Letters and citations to these documents recorded in journals processed by t...

Statistical relationships between downloads and citations at the level of individual documents within a single journal

AbstractStatistical relationships between downloads from ScienceDirect of documents in Elsevier's electronic journal Tetrahedron Letters and citations to these documents recorded in journals processed by the Institute for Scientific Information/Thomson Scientific for the Science Citation Index (SCI) are examined. A synchronous approach revealed that downloads and citations show different patterns of obsolescence of the used materials. The former can be adequately described by a model consisting of the sum of two negative exponential functions, representing an ephemeral and a residual factor, whereas the decline phase of the latter conforms to a simple exponential function with a decay constant statistically similar to that of the downloads residual factor. A diachronous approach showed that, as a cohort of documents grows older, its download distribution becomes more and more skewed, and more statistically similar to its citation distribution. A method is proposed to estimate the effect of citations upon downloads using obsolescence patterns. It was found that during the first 3 months after an article is cited, its number of downloads increased 25% compared to what one would expect this number to be if the article had not been cited. Moreover, more downloads of citing documents led to more downloads of the cited article through the citation. An analysis of 1,190 papers in the journal during a time interval of 2 years after publication date revealed that there is about one citation for every 100 downloads. A Spearman rank correlation coefficient of 0.22 was found between the number of times an article was downloaded and its citation rate recorded in the SCI. When initial downloads—defined as downloads made during the first 3 months after publication—were discarded, the correlation raised to 0.35. However, both outcomes measure the joint effect of downloads upon citation and that of citation upon downloads. Correlating initial downloads to later citation counts, the correlation coefficient drops to 0.11. Findings suggest that initial downloads and citations relate to distinct phases in the process of collecting and processing relevant scientific information that eventually leads to the publication of a journal article.

Structural identification of sedimentary C 21 and C 22 highly branched isoprenoid alkanes

C 21 and C 22 highly branched isoprenoid (HBI) alkanes occurring in high relative abundance in lagoonal sediments of Abu Dhabi have been unambiguously identified as 2,6,10-trimethyl-7-(3-methylpentyl)dodecane and 3,7,11-trimethyl-6-(3-methylpentyl)tridecane, respectively, using NMR spectroscopy. A second C 21 HBI isomer is tentatively identified as 3,7,11-trimethyl-6-(3-methylbutyl)tridecane, on the basis of comparison of its mass spectral fragmentation with those of fully identified HBIs. The structures of these three components are formed by extension of the “parent” C 20 HBI alkane, first identified in Rozel Point Oil by Yon et al. [Yon, D.A., Maxwell, J.R., Ryback G., 1982. 2,6,10-trimethyl-7-(3-methylbutyl)dodecane, a novel sedimentary biological marker. Tetrahedron Letters 23, 2143–2146], by one or two carbon atom(s), respectively, at the terminal carbon atom of one or two of the T-branches. In contrast to the C 20 HBI alkane present in the oil, 13C NMR spectroscopy did not indicate the presence of diastereoisomers for the C 21 and C 22 HBIs in sediments, indicating stereospecific enzyme-controlled biosynthesis of these components by an unidentified biological source. The stable carbon isotopic compositions of the major C 20–C 22 HBIs are identical, suggesting a common biological source.