Development of a Convergent Enantioselective Synthetic Route to (-)-Myrocin G.

Myrocins are a family of antiproliferative antibiotic fungal metabolites possessing a masked electrophilic cyclopropane. Preliminary chemical reactivity studies imputed the bioactivity of these natural products to a DNA cross-linking mechanism, but this hypothesis was not confirmed by studies with native DNA. We recently reported a total synthesis of (-)-myrocin G (4), the putative active form of the metabolite myrocin C (1), that featured a carefully orchestrated tandem fragment coupling-annulation cascade. Herein, we describe the evolution of our synthetic strategy toward 4 and report the series of discoveries that prompted the design of this cascade coupling. Efforts to convert the diosphenol (-)-myrocin G (4) to the corresponding 5-hydroxy-γ-lactone isomer myrocin C (1) are also detailed. We present a preliminary evaluation of the antiproliferative activities of (-)-myrocin G (4) and related structures, as well as DNA cross-linking studies. These studies indicate that myrocins do not cross-link DNA, suggesting an alternative mode of action potentially involving a protein target.

Synthesis of Myrocin G, the Putative Active Form of the Myrocin Antitumor Antibiotics.

The antiproliferative antimicrobial fungal metabolites known as the myrocins have been proposed to cross-link DNA by double nucleotide addition. However, the nature of the DNA-reactive species is ambiguous, as myrocins have been isolated as functionally distinct 5-hydroxy-γ-lactone and diosphenol isomers. Based on literature precedent, we hypothesized that the diosphenol 7 (assigned here the trivial name myrocin G) is the biologically active form of the representative isolate (+)-myrocin C (1). To probe this, we developed a short enantioselective route to 7. A powerful fragment-coupling reaction that forms the central ring of the target in 38% yield and in a single step was developed. In support of our hypothesis, 7 was efficiently transformed to the bis(sulfide) 6, a product previously isolated from reactions of 1 with excess benzenethiol. This work provides the first direct access to the diosphenol 7, sets the stage for elucidating the mode of interaction of the myrocins with DNA, and provides a foundation for the synthesis of other pimarane diterpenes.

Author Correction: The genetic prehistory of the Baltic Sea region.

The original version of this Article omitted references to previous work, which are detailed in the associated Author Correction. These omissions have been corrected in both the PDF and HTML versions of the Article.

Risk factors for carriage of Streptococcus pneumoniae in children.

BACKGROUND: During the past decades Streptococcus pneumoniae has developed significant resistance to many classes of antimicrobial drugs. Potential risk factors for colonization of the nasopharynx by Streptococcus pneumoniae in children and for carriage of drug resistant strains were examined. METHODS: Between 2007 and 2008 nasopharyngeal swabs were collected from 402 children 6 months to 5 years old visiting the public sector immunization centers and outpatient departments as well as offices of paediatricians from private practice in Nicosia district in Cyprus. Information on demographic characteristics and potential risk factors of participating children were collected using a standardized questionnaire distributed to parents. RESULTS: In multivariable analyses we found that attendance at day care center, having siblings in the family and having both parents originating from Cyprus, statistically increased the risk of pneumococcal colonization. Full immunization with PCV7 appears to be a protective factor against colonization by pneumococcus. Previous administration of antimicrobials during the last month prior to specimen collection appeared to be the most consistent risk factor for carrying a non susceptible strain of Streptococcus pneumoniae to either penicillin or erythromycin. Factors such as age, nationality, previous or current breastfeeding, passive exposure to cigarette smoke and attendance in a day care center do not appear as independent risk factors for colonization by non susceptible strains. CONCLUSIONS: Prudent use of antibiotics especially for upper respiratory tract infections in children as well as increased vaccination coverage by the pneumococcal conjugate vaccines could prove effective in reducing levels of colonization by drug resistant pneumococcal strains.

The genetic prehistory of the Baltic Sea region.

While the series of events that shaped the transition between foraging societies and food producers are well described for Central and Southern Europe, genetic evidence from Northern Europe surrounding the Baltic Sea is still sparse. Here, we report genome-wide DNA data from 38 ancient North Europeans ranging from ~9500 to 2200 years before present. Our analysis provides genetic evidence that hunter-gatherers settled Scandinavia via two routes. We reveal that the first Scandinavian farmers derive their ancestry from Anatolia 1000 years earlier than previously demonstrated. The range of Mesolithic Western hunter-gatherers extended to the east of the Baltic Sea, where these populations persisted without gene-flow from Central European farmers during the Early and Middle Neolithic. The arrival of steppe pastoralists in the Late Neolithic introduced a major shift in economy and mediated the spread of a new ancestry associated with the Corded Ware Complex in Northern Europe.

A convergent approach to batzelladine alkaloids. Total syntheses of (+)-batzelladine E, (-)-dehydrobatzelladine C, and (+)-batzelladine K.

We recently reported a convergent strategy to access the polycyclic guanidinium alkaloid (+)-batzelladine B via an aldol addition-retro-aldol-aza-Michael addition cascade. Here we describe the application of this approach toward the total syntheses of (+)-batzelladine E, (-)-dehydrobatzelladine C, and (+)-batzelladine K. The identification of suitable methods to functionalize a common tropane core by electrophilic alkynylation and nucleophilic 1,2-addition were essential to generalizing this approach. We provide evidence for the intermediacy of an acylallene species in the cascade reaction.

A concise synthesis of (+)-batzelladine B from simple pyrrole-based starting materials.

Alkaloids, secondary metabolites that contain basic nitrogen atoms, are some of the most well-known biologically active natural products in chemistry and medicine. Although efficient laboratory synthesis of alkaloids would enable the study and optimization of their biological properties, their preparation is often complicated by the basicity and nucleophilicity of nitrogen, its susceptibility to oxidation, and its ability to alter reaction outcomes in unexpected ways--for example, through stereochemical instability and neighbouring group participation. Efforts to address these issues have led to the invention of a large number of protecting groups that temper the reactivity of nitrogen; however, the use of protecting groups typically introduces additional steps and obstacles into the synthetic route. Alternatively, the use of aromatic nitrogen heterocycles as synthetic precursors can attenuate the reactivity of nitrogen and streamline synthetic strategies. Here we use such an approach to achieve a synthesis of the complex anti-HIV alkaloid (+)-batzelladine B in nine steps (longest linear sequence) from simple pyrrole-based starting materials. The route uses several key transformations that would be challenging or impossible to implement using saturated nitrogen heterocycles and highlights some of the advantages of beginning with aromatic reagents.

Massive migration from the steppe was a source for Indo-European languages in Europe.

We generated genome-wide data from 69 Europeans who lived between 8,000-3,000 years ago by enriching ancient DNA libraries for a target set of almost 400,000 polymorphisms. Enrichment of these positions decreases the sequencing required for genome-wide ancient DNA analysis by a median of around 250-fold, allowing us to study an order of magnitude more individuals than previous studies and to obtain new insights about the past. We show that the populations of Western and Far Eastern Europe followed opposite trajectories between 8,000-5,000 years ago. At the beginning of the Neolithic period in Europe, ∼8,000-7,000 years ago, closely related groups of early farmers appeared in Germany, Hungary and Spain, different from indigenous hunter-gatherers, whereas Russia was inhabited by a distinctive population of hunter-gatherers with high affinity to a ∼24,000-year-old Siberian. By ∼6,000-5,000 years ago, farmers throughout much of Europe had more hunter-gatherer ancestry than their predecessors, but in Russia, the Yamnaya steppe herders of this time were descended not only from the preceding eastern European hunter-gatherers, but also from a population of Near Eastern ancestry. Western and Eastern Europe came into contact ∼4,500 years ago, as the Late Neolithic Corded Ware people from Germany traced ∼75% of their ancestry to the Yamnaya, documenting a massive migration into the heartland of Europe from its eastern periphery. This steppe ancestry persisted in all sampled central Europeans until at least ∼3,000 years ago, and is ubiquitous in present-day Europeans. These results provide support for a steppe origin of at least some of the Indo-European languages of Europe.

Ancient human genomes suggest three ancestral populations for present-day Europeans.

We sequenced the genomes of a ∼7,000-year-old farmer from Germany and eight ∼8,000-year-old hunter-gatherers from Luxembourg and Sweden. We analysed these and other ancient genomes with 2,345 contemporary humans to show that most present-day Europeans derive from at least three highly differentiated populations: west European hunter-gatherers, who contributed ancestry to all Europeans but not to Near Easterners; ancient north Eurasians related to Upper Palaeolithic Siberians, who contributed to both Europeans and Near Easterners; and early European farmers, who were mainly of Near Eastern origin but also harboured west European hunter-gatherer related ancestry. We model these populations' deep relationships and show that early European farmers had ∼44% ancestry from a 'basal Eurasian' population that split before the diversification of other non-African lineages.

Genome-wide comparison of medieval and modern Mycobacterium leprae.

Leprosy was endemic in Europe until the Middle Ages. Using DNA array capture, we have obtained genome sequences of Mycobacterium leprae from skeletons of five medieval leprosy cases from the United Kingdom, Sweden, and Denmark. In one case, the DNA was so well preserved that full de novo assembly of the ancient bacterial genome could be achieved through shotgun sequencing alone. The ancient M. leprae sequences were compared with those of 11 modern strains, representing diverse genotypes and geographic origins. The comparisons revealed remarkable genomic conservation during the past 1000 years, a European origin for leprosy in the Americas, and the presence of an M. leprae genotype in medieval Europe now commonly associated with the Middle East. The exceptional preservation of M. leprae biomarkers, both DNA and mycolic acids, in ancient skeletons has major implications for palaeomicrobiology and human pathogen evolution.