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Bridgehead alkenes are polycyclic molecules bearing at least one C=C bond that includes a bridgehead carbon atom. For small bicyclic systems, these bonds are highly strained due to geometric constraints placed on the sp2 hybridized carbon atoms. These small, strained molecules have been termed "anti-Bredt" alkenes. β-halo carbanions have served as convenient precursors to bridgehead alkenes in experimental studies. We observed that upon attempted computational geometric optimizations (ωB97X-D/aug-cc-pVDZ) of the precursors, spontaneous elimination of the halide occurs along with formation of the anti-Bredt alkene in many cases. Such computational eliminations were shown to faithfully mimic experimentally obtained results. Computational elimination was not observed for [1.1.1] or [2.1.1] frameworks, in agreement with predictions that these bridgehead alkenes are too strained to be formed. However, computational elimination from the [2.2.1] framework was observed to form 1-norbornene, a compound suggested in experimental work to be a reactive intermediate. Similarly, [3.1.1] frameworks and higher led to eliminations upon computational geometric optimization, in agreement with experimental findings. Natural bond order (NBO) calculations of the starting geometries proved to be excellent predictors as to whether elimination would take place. Those precursor compounds exhibiting delocalization energies in the order of 10 kcal/mol between the lone-pair electrons of the carbon atom and σ*C-Br were generally found to undergo elimination. Thus, computational optimization of β-halo substituted bicyclic precursor anions can be used to predict whether strained anti-Bredt alkenes are likely to be formed, thereby saving valuable time and costs in the experimental lab. [ABSTRACT FROM AUTHOR]

Understanding how, and from where, invasive species were introduced is critical for revealing the invasive mechanism, explaining the invasion success, and providing crucial insights for effective management. Here, we combined a phylogeographic approach with ecological niche modeling comparisons to elucidate the introduction mode and source of Nicotiana glauca, a native South American species that is now invasive worldwide. We tested three different scenarios based on the invasion source—random native, restricted native, and bridgehead invasive—considering genetic diversity and climatic niche comparisons among native and invaded areas. We found three genetic lineages geographically and climatically differentiated within the native range. Only one of these genetic groups contained the invasive haplotypes, but showed no climatic niche overlap with any invaded area. Conversely, one invaded area located in western South America, with more genetic diversity than other invaded areas but less than the native range, showed climatic niche overlap with almost all other invaded areas worldwide. These findings indicate that N. glauca first likely invaded the southernmost areas beyond its native range, forming a bridgehead invasive source, from which the species subsequently invaded other regions around the world. Invasiveness would have been fostered by changes in the environmental preferences of the species in the bridgehead area, towards drier, colder and less seasonal climates, becoming the actual source of invasion to areas climatically similar throughout the world. The fine scale resolution analyses combining genetic and climatic approaches within the native range were essential to illuminating the introduction scenario of this invasive species. [ABSTRACT FROM AUTHOR]

Invasions by fungal plant pathogens pose a significant threat to the health of agricultural ecosystems. Despite limited standing genetic variation, many invasive fungal species can adapt and spread rapidly, resulting in significant losses to crop yields. Here, we report on the population genomics of Colletotrichum truncatum, a polyphagous pathogen that can infect more than 460 plant species, and an invasive pathogen of soybean in Brazil. We study the whole‐genome sequences of 18 isolates representing 10 fields from two major regions of soybean production. We show that Brazilian C. truncatum is subdivided into three phylogenetically distinct lineages that exchange genetic variation through hybridization. Introgression affects 2%–30% of the nucleotides of genomes and varies widely between the lineages. We find that introgressed regions comprise secreted protein‐encoding genes, suggesting possible co‐evolutionary targets for selection in those regions. We highlight the inherent vulnerability of genetically uniform crops in the agro‐ecological environment, particularly when faced with pathogens that can take full advantage of the opportunities offered by an increasingly globalized world. Finally, we discuss "the means, motive and opportunity" of fungal pathogens and how they can become invasive species of crops. We call for more population genomic studies because such analyses can help identify geographical areas and pathogens that pose a risk, thereby helping to inform control strategies to better protect crops in the future. [ABSTRACT FROM AUTHOR]

Starting from a passage of Demosthenes’ Fourth Philippic, the essay examines the career of Hermias of Atarneus, in particular the controversial issue of his relationship with Philip II of Macedonia. In this regard, an attempt is made to reject the claim that the arrival of Aristotle at Hermias was desired by Philip to secure a bridgehead for the expedition to Asia, whereas there is no reason to believe that this expedition had already been planned in the first half of the Forties. In fact, it is reasonable to think that the relationship with Hermias, which probably began after Aristotle’s arrival at the Macedonian court in 343, was of no particular importance to Philip.

While Western and Eastern geopolitical identities have been present throughout Eastern Europe, Hungary stands out in the extent to which it has been torn between such self-images. Informed by critical geopolitics and borderscaping, this article analyzes how and why two (seemingly) irreconcilable notions – Christian bulwark and Turanism – have been co-existing in that country. By drawing on (geo)political communications, (geo)economic indicators, socio-cultural dynamics as well as public perceptions, the study shows that these two geopolitical traditions are not just competing or existing parallelly but have at times even been reinforcing one another; historically through cultural fusion and recently through a third self-image of a bridgehead linking East and West. Indeed, despite the dominant discourse of a thousand-year-old Christian state, Eastern elements such as nomadism and paganism have long been present in Hungarian society, even incorporating some Christian myths. At other times, the self-image of a Christian bulwark was indeed applied to defend the country (even Europe) against real and perceived enemies from the East or South. Turanism shares a common enemy: liberalism and cosmopolitanism. Yet it emphasizes the opposite geopolitical orientation: through its Occidentophobia it has been trying to orient Hungarians away from the West toward their real or imagined kin in Eurasia. Today, both Christian bulwark and Turanism are increasingly embraced by the main government party e.g. through its Eastern Opening policy, the gains of which however remain unclear. Whereas both geopolitical ideologies have served to legitimize Hungary's illiberal turn, the country remains an eastern periphery of the West. [ABSTRACT FROM AUTHOR]

The breakaway republic - de facto state - of Transnistria has steadily been edging closer and closer to Russia, but the Kremlin does not seem all that enthusiastic. The EU ought to take specific action to prevent Moldova from becoming the next victim of Russian aggression. Transnistria could represent a Russian, bridge head in the East of Europe. There is a real chance that the breakaway region in the Eastern part of the country, the Transnistrian republic, might follow in the footsteps of Crimea and be accepted into the Russian Federation as a new federal subject. The annexation of Crimea by force dramatically changes the security situation in the Black Sea region.

The article analyses the cases of military actions in the territory of Western Lithuania and Latvia from 1915 to 1945. At the same time, attention is paid to the operational plans of the Lithuanian and Latvian military forces in the second half of the 1930s. Using this method of analysis, based on military history research, an alternative defence concept for small countries whose defence depends on allies not only with ground capabilities but also with naval military capabilities and capabilities to conduct air operations, is disclosed. The strategic directions in which armies invade and retreat in the Eastern Baltic region in the first half of the twentieth century are essentially the same. Only changing technologies, capability of mobility and firepower, and growing/increasing opportunities have affected the pace and geography of military action. In 1915, the Russian army did not concentrate its forces in Courland, and through it the German army saw the possibility of manoeuvring in the northern wing. This helped to defeat the enemy on the Venta-Dubysa river line and take advantage of the success and maintain the initiative to occupy the entire territory of Lithuania by the autumn and reach the left bank of the Daugava. These operations influenced the military planning of interwar Lithuania and Latvia. In 1941, the German forces invaded to the east in the same direction as in 1915, the only difference being that the enemy failed to stop them in Samogitia and Courland, although there was some stopping at Raseiniai near Dubysa. In 1944, the Red Army (RA) broke into the Baltic Sea near Palanga - which is what it failed to do in 1919 - forming the Courland pocket, a bridgehead in which German forces, defending the sea with a sea connection to Germany, were cut off for seven months, from 10 October to 9 May 1945. The Curonian Boiler/bridgehead forces surrendered only after the German capitulation. Klaipėda was defended by the German military for more than 3 months: from October 10, 1944 to January 28, 1945, when there was a threat that the land route through the Curonian Spit would be interrupted; they abandoned the city and left it to the Red Army without a battle. [ABSTRACT FROM AUTHOR]

This article contributes to the literature on the mechanisms, rhetoric, and limits of mid-Victorian expansion by asking how far late Tokugawa Japan was subject to forms of British imperialism. In September 1862 a British merchant was murdered on the high road between Edo and Kyoto; a year later, a British fleet bombarded Kagoshima in retaliation. By engaging with John Darwin's concept of the 'bridgehead', this article examines the circumstances in which a lonely death on the frontiers of British commerce could be transformed into a Victorian 'outrage'. It considers what we stand to gain by bringing an imperial history perspective to bear on what remains, for most imperial historians, a largely forgotten conflict. In positing Yokohama as a bridgehead that could gain only fitful purchase in London, it asks new questions about the conduct of 'gunboat diplomacy' and the fault lines of mid-Victorian expansion; the place of Japan in British political imaginaries; the nature of informal empire; and the discourses buffeting British expansion in the turbulent 1860s. [ABSTRACT FROM AUTHOR]

Toward the total synthesis of acremoxanthone A, a model study has revealed a convergent approach to construct the ABCDE ring system. The key steps include: (1) an effective construction of the bicyclo[ 3.2.2]nonane skeleton, (2) protocol for generating the bridgehead anion and trapping, and (3) 1,3-dipolar cycloaddition of a nitrile oxide to the internal alkene. [ABSTRACT FROM AUTHOR]

An unprecedented access to strained tetracyclic bridgehead alkenes by reaction of easily accessible ortho-alkynylarylaldehydes with conjugated dienes is described. The process involves a chemo- and stereo-selective, gold-catalyzed, tandem intermolecular [3+2] cycloaddition/Prins-type ring-closing reaction that allows generating structural complexity in a straightforward manner. This approach for the preparation of anti-Bredt compounds is synthetically superior to those previously reported: the procedure is easy to implement, operates under mild experimental conditions, is efficient, and exhibits a good substrate scope. [ABSTRACT FROM AUTHOR]

Ushuaia and Punta Arenas are the southernmost cities in their belonging countries, Argentina and Chile. They have played the role of frontier towns in the process of consolidation and legitimation of these austral national territories located in the periphery of South America. During the last fifty years, these two port cities have emerged as the two main international gateways to Antárctica, and especially to Antarctic Peninsula. They participate in the structuring of a multifaceted Antarctic frontier conquest: military, scientific, ecological and tourism. The bridgehead is a key concept to rethink the territorial structuring and control of conquest frontier. If we recall proximity and connection as minimal criteria to define a gateway, political control and territorial reference tend to reinforce the gateway as the main bridgehead place of the conquest frontier. Are then Ushuaia and Punta Arenas becoming Antarctic-driven cities? How are they used in the antartic national policies of Argentina and Chile?

Abstract: Camphor- and fenchone-based bridgehead-substituted norbornan-2-ones constitute an interesting class of chiral synthons with applications in stereoselective synthesis, due to the wide variety of stereocontrolled transformations which they can undergo. Despite their synthetic value, few mass spectral studies have been realized for these compounds due to the lack of general procedures for their preparation. In this paper we have carried out an electron ionization (EI) mass spectral study of a series of heteroatomically bridgehead-substituted 7,7-dimethylnorbornan-2-ones. Their cleavage mechanisms have been rationalized on the basis of the substituent shifts, as well as on the identification of relevant peaks through accurate mass measurements and collision-induced dissociation (CID) tandem mass-spectrometric experiments. The driving force for the main fragmentation pathways of C(1)-O- and C(1)-N-substituted 7,7-dimethylnorbornan-2-ones is the α-cleavage of the C(1) directed by the bridgehead substituent. Further rearrangements and fragmentations of the distonic ions initially formed lead to the major ions in the spectra of all the compounds. A comparison with the known behaviour of their isomeric 3,3-dimethylnorbornan-2-ones reveals that the key bridgehead heteroatom effect controlling the fragmentation pathways can be modulated by the position of the gem-dimethyl group. As a consequence, the corresponding fragmentation patterns of both isomeric series are very different. In contrast, the fragmentation patterns of 7,7-dimethyl-2-oxo-1-norbornanesulphonamides, whose main fragmentation pathway involves homolytic Seavage, are almost identical to those displayed by their 3,3-dimethylated isomers. [Copyright &y& Elsevier]

Abstract: Invasion biology and agriculture are intimately related for several reasons and in particular because many agricultural pest species are recent invaders. In this article we suggest that the reconstruction of invasion routes with population genetics-based methods can address fundamental questions in ecology and practical aspects of the management of biological invasions in agricultural settings. We provide a brief description of the methods used to reconstruct invasion routes and describe their main characteristics. In particular, we focus on a scenario – the bridgehead invasion scenario –, which had been overlooked until recently. We show that this scenario, in which an invasive population is the source of other invasive populations, is evolutionarily parsimonious and may have played a crucial role in shaping the distribution of many recent agricultural pests. [Copyright &y& Elsevier]