A rapid and convergent synthesis of the integrastatin core

The tetracyclic core of the integrastatin natural products has been prepared in a convergent and rapidmanner. Our strategy relies upon a palladium(II)-catalyzed oxidative cyclization to form the central [3.3.1]-dioxabicycle of the natural product core. Overall, the core has been completed in only 4 linear steps from known compounds

Aryne Acyl-Alkylation in the General and Convergent Synthesis of Benzannulated Macrolactone Natural Products: An Enantioselective Synthesis of (−)-Curvularin

A general approach for the synthesis of benzannulated macrolactone natural products utilizing an aryne acyl-alkylation reaction is described. Toward this end, the total syntheses of the natural products (−)-curvularin, curvulin, and (−)-diplodialide C are reported. Furthermore, the aryne insertion technology has enabled the rapid conversion of simple diplodialide natural products to curvularin, thereby connecting these two biosynthetically distinct classes of compounds via synthetic methods

Multidecadal variability in hydro-climate of Okavango river system, southwest Africa, in the past and under future climate

The focus of this paper is to understand the multi-decadal oscillatory component of variability in the Okavango River system, in southwestern Africa, and its potential evolution through the 21st century under climate change scenarios. Statistical analyses and hydrological modelling are used to show that the observed multi-decadal wet and dry phases in the Okavango River and Delta result from multi-decadal oscillations in rainfall, which are likely to be related to processes of internal variability in the climate system, rather than external natural or anthropogenic forcing. Analyses of changes in this aspect of variability under projected climate change scenarios are based on data from a multi-model ensemble of 19 General Circulation Models, which are used to drive hydrological models of the Okavango River and Delta. Projections for the 21st century indicate a progressive shift towards drier conditions attributed to the influence of increasing temperatures on water balance. It is, however, highly likely that multi-decadal oscillations, possibly of similar magnitude to that of 20th century, will be superimposed on the overall trend. These may periodically offset or amplify the mean drying trend. This effect should be accounted for in water and catchment management and climate change adaptation strategies

Regioselective Reactions of Highly Substituted Arynes

The fully regioselective reactivity of four new highly substituted silyl aryl triflate aryne precursors in aryne acyl-alkylation, acyl-alkylation/condensation, and heteroannulation reactions is reported. The application of these more complex arynes provides access to diverse natural product scaffolds and obviates late-stage functionalization of aromatic rings

A model investigation of interannual winter rainfall variability over southwestern South Africa and associated ocean-atmosphere interaction

We have investigated the variability of inter-annual winter rainfall over the southwestern Cape region of South Africa and associated large-scale atmosphere-ocean interaction upstream over the South Atlantic using the HadAM3 atmospheric general circulation model. This model was run for the period from 1990 to 1999 using mean monthly global sea-surface temperature (SST) as surface boundary condition over the global ocean. Diagnostics of winter (May to September) model output averaged over 1990-99 suggest that the HadAM3 model represents the general circulation in the South Atlantic / African sector reasonably well for this season at least. In addition, model years with wet and dry winters over the study area tended also to be those that were observed to be anomalously wet or dry. Wet minus dry season composite fields were used to investigate the model's inter-annual variability. The composite difference fields for low- and mid-level winds, sea-level pressure, and moisture flux all indicated wet winters being associated with increased inflow from tropical South America (originating in the equatorial western Atlantic at low levels) contributing relatively moist air to the westerly flow heading towards the southwestern Cape. A stronger jet over the South Atlantic promoted the passage of storms towards the Cape. Large areas of cyclonic vorticity anomalies, enhanced eddy activity, increased thickness in the lower atmosphere and low-level convergence near and upstream of the southwestern Cape in the model composite differences all favoured increased storm systems as well as their local intensification, implying enhanced rainfall. The results presented here suggest that the model can represent the interannual variability of winter rainfall over the study region and shed light on the mechanisms potentially associated with anomalously wet winters there

No evidence for mass segregation in massive young clusters

Aims. We investigate the validity of the mass segregation indicators commonly used in analysing young stellar clusters. Methods. We simulate observations by constructing synthetic seeing-limited images of a 1000 massive clusters (10^4 Msun) with a standard IMF and a King-density distribution function. Results. We find that commonly used indicators are highly sensitive to sample incompleteness in observational data and that radial completeness determinations do not provide satisfactory corrections, rendering the studies of radial properties highly uncertain. On the other hand, we find that, under certain conditions, the global completeness can be estimated accurately, allowing for the correction of the global luminosity and mass functions of the cluster. Conclusions. We argue that there is currently no observational evidence of mass segregation in young compact clusters since there is no robust way to differentiate between true mass segregation and sample incompleteness effects. Caution should then be exercised when interpreting results from observations as evidence of mass segregation.Comment: 10 pages, 12 figures, typos corrected. Download a high-resolution version at http://www.astro.up.pt/~jascenso/mseg_v2.pdf (1 MB