Dimerization of functional pyrroloindolizines for the synthesis of complex myrmicarin alkaloids

The union of functionalized pyrroloindolizines for the synthesis of heterodimeric products relevant to myrmicarin alkaloids is described. Design and synthesis of tricyclic substrates and new methods for their union enable the investigation of late-stage cyclopentannulation strategies. The rapid assembly of dimeric structures using unique modes of pyrroloindolizine reactivity presents a concise approach to the dimeric myrmicarins and relevant derivatives.National Institutes of Health (U.S.) (NIH-NIGMS (GM074825))Alfred P. Sloan Foundation (Research Fellowship)Henry & Camille Dreyfus Foundation (Teacher-Scholar)Novartis (Firm) (Graduate Fellowship

Perspectives and challenges for the use of radar in biological conservation

Radar is at the forefront for the study of broad-scale aerial movements of birds, bats and insects and related issues in biological conservation. Radar techniques are especially useful for investigating species which fly at high altitudes, in darkness, or which are too small for applying electronic tags. Here, we present an overview of radar applications in biological conservation and highlight its future possibilities. Depending on the type of radar, information can be gathered on local- to continental-scale movements of airborne organisms and their behaviour. Such data can quantify flyway usage, biomass and nutrient transport (bioflow), population sizes, dynamics and distributions, times and dimensions of movements, areas and times of mass emergence and swarming, habitat use and activity ranges. Radar also captures behavioural responses to anthropogenic disturbances, artificial light and man-made structures. Weather surveillance and other long-range radar networks allow spatially broad overviews of important stopover areas, songbird mass roosts and emergences from bat caves. Mobile radars, including repurposed marine radars and commercially dedicated ‘bird radars’, offer the ability to track and monitor the local movements of individuals or groups of flying animals. Harmonic radar techniques have been used for tracking short-range movements of insects and other small animals of conservation interest. However, a major challenge in aeroecology is determining the taxonomic identity of the targets, which often requires ancillary data obtained from other methods. Radar data have become a global source of information on ecosystem structure, composition, services and function and will play an increasing role in the monitoring and conservation of flying animals and threatened habitats worldwide

Sequence-selective assembly of tweezer-molecules on linear templates enables frameshift-reading of sequence information

Monomer-sequence information in synthetic copolyimides can be recognised by tweezer-type molecules binding to adjacent triplet-sequences on the polymer chains. In the present paper different tweezer-molecules are found to have different sequence-selectivities, as demonstrated in solution by 1H NMR spectroscopy and in the solid state by single crystal X-ray analyses of tweezer-complexes with linear and macrocyclic oligo-imides. This work provides clear-cut confirmation of polyimide chain-folding and adjacent-tweezer-binding. It also reveals a new and entirely unexpected mechanism for sequence-recognition which, by analogy with a related process in biomolecular information processing, may be termed "frameshift-reading". The ability of one particular tweezer-molecule to detect, with exceptionally high sensitivity, long-range sequence-information in chain-folding aromatic copolyimides, is readily explained by this novel process

Computational Analysis of the Stereochemical Outcome in the Imidazolidinone-Catalyzed Enantioselective (4 + 3)-Cycloaddition Reaction.

Computations show why the catalytic, asymmetric (4 + 3)-cycloaddition reaction developed in the Harmata laboratories proceeds with facial selectivity opposite to that for models proposed for related catalyzed Diels-Alder reactions. Computations with M06-2X/6-311+G(d,p)//B3LYP/6-31G(d) show that iminium ions derived from MacMillan's chiral 2-tert-butyl-5-benzylimidazolidinone and siloxypentadienals undergo (4 + 3)-cycloadditions with furans preferentially on the more crowded face. Conformational reorganization of the benzyl group, to avoid intramolecular interaction with the silyl group, is responsible for differentiating the activation barriers of top- and bottom-face attack

Control of regioselectivity and stereoselectivity in (4 + 3) cycloadditions of chiral oxyallyls with unsymmetrically disubstituted furans

The regioselectivities and stereoselectivities of ZnCl2- catalyzed (4 + 3) cycloadditions between chiral oxazolidinone-substituted oxyallyls and unsymmetrical disubstituted furans have been determined. The substitution pattern on the furan is found to provide a valuable tool for controlling the stereochemistry (endo-I or endo-II) of the 7-membered cycloadduct. While cycloadditions with monosubstituted furans usually favor endo-I products, from addition of the furan to the more crowded face of the oxyallyl, cycloadditions with 2,3- and 2,5-disubstituted furans instead favor the endo-II stereochemistry. Density functional theory calculations are performed to account for the selectivities. For monosubstituted furans, the crowded transition state leading to the endo-I cycloadduct is stabilized by an edge-to-face interaction between the furan and the oxazolidinone 4-Ph group, but this stabilization is overcome by steric clashing if the furan bears a 2-CO2R group or is 2,3-disubstituted