Terminal substituent effects on the reactivity, thermodynamics, and stereoselectivity of the 8π-6π electrocyclization cascades of 1,3,5,7-tetraenes.

M06-2X/6-31+G(d,p) computations are reported for the 8π-6π electrocyclization cascades of 1,3,5,7-tetraenes. The rate-determining step for these cascades is typically the second (6π) ring closure. According to experiment and theory, un- and monosubstituted tetraenes readily undergo 8π electrocyclic ring closure to form 1,3,5-cyclooctatrienes; however, the 6π electrocyclizations of these cyclooctatriene intermediates are slow and reversible, and mixtures of monocyclic and bicyclic products are formed. Computations indicate that di- and trisubstituted tetraenes undergo facile but less exergonic 8π electrocyclization due to a steric clash that destabilizes the 1,3,5-cyclooctatriene intermediates. Relief of this steric clash ensures the subsequent 6π ring closures of these intermediates are both kinetically facile and thermodynamically favorable, and only the bicyclic products are observed for the cascade reactions of naturally occurring tri- and tetrasubstituted tetraenes (in agreement with computations). The 6π electrocyclization step of these cascade electrocyclizations is also potentially diastereoselective, and di- and trisubstituted tetraenes often undergo cascade reactions with high diastereoselectivities. The exo mode of ring closure is favored for these 6π electrocyclizations due to a steric interaction that destabilizes the endo transition state. Thus, theory explains both the recalcitrance of the unsubstituted 1,3,5,7-octatetraene and 1-substituted tetraenes toward formation of the bicyclo[4.2.0]octa-2,4-diene products, as well as the ease and the stereoselectivity with which terminal di- and trisubstituted tetraenes are known to react biosynthetically

How cinchona alkaloid-derived primary amines control asymmetric electrophilic fluorination of cyclic ketones.

The origin of selectivity in the α-fluorination of cyclic ketones catalyzed by cinchona alkaloid-derived primary amines is determined with density functional calculations. The chair preference of a seven-membered ring at the fluorine transfer transition state is key in determining the sense and level of enantiofacial selectivity

Effects of kelp canopy removal on young-of-the-year rockfish abundance, using two census methods

In 1976 an experimental research project was initiated to examine the possible effects of the removal of a giant kelp, Macrocystis pyrifera, canopy in a central California kelp bed on Young-of-the-Year (YOY) fish populations. A total of 7823 fishes was captured and marked using freeze branding. Pre-canopy removal population estimates were made using the Schnabel method and post-canopy removal population estimates were made using the Petersen method (Lincoln Index). A significant (p < .0004) reduction of fish occurred immediately after the canopy was removed and the population remained at a reduced level for at least 60 days. This information warranted the creation of a second expanded project. In 1977 we established three study areas consisting of a kelp bed to be harvested (C), a kelp bed not to be harvested (NC) and a control kelp bed. We captured and marked over 82,000 YOY fish in Areas C and NC. Using the same statistical methods from the 1976 study, a significant reduction was found to occur in fish populations within harvested Area C and unharvested area NC: however, the reductions were not significantly different between the two areas. The large reduction in the fish population in the harvested area occurred when fish moved into the unharvested area. The large, unexpected reduction in fish numbers in the unharvested area (NC) occurred when larger predatory YOY bocaccio moved into the control area (X) as the experimental area (C) was being harvested. The bocaccio removed in excess of 20% of the biomass of YOY blue rockfish, which was composed of resident fish and recently migrated fish from the harvested kelp bed. Predation on YOY blue rockfish was also evident in the harvested area. Population estimates using visible fish transects by divers correlated well with population estimates from the capture/recapture studies in 1976, but correlation was poorer the following year when many more fish were present. (32pp.

Cyclotella paleo-ocellata, a new centric diatom (Bacillariophyta) from Lake Kinneret (Israel)

© Czech Phycological Society (2015). Large, subfossil populations of an unknown centric, planktonic diatom were observed in a lake sediment core from Lake Kinneret (Israel), which is here described as Cyclotella paleo–ocellata sp. nov. The new taxon, which belongs to the Cyclotella ocellata species complex, is described and separated from other similar taxa (e.g., Cyclotella ocellata, Cyclotella kuetzingiana, Cyclotella polymorpha, Cyclotella paraocellata) based on a combination of the following morphological characters: valve diameter, number of orbiculi depressi (circular depressions), number of striae/10 μm, stria length, number and position of rimoportulae and the number of central and marginal fultoportulae. Cyclotella paleo–ocellata can be distinguished mainly by two prominent characteristics: (1) the number and the arrangement of the orbiculi depressi (4–8) which increase with the valve diameter and (2) the marginal fultoportulae, situated on each, every second or third costa. Since Cyclotella paleo–ocellata is at present only known from the subfossil bottom sediments of Lake Kinneret, its ecological preferences are inferred simply from the associated diatom flora

Building on Cram's legacy: stimulated gating in hemicarcerands.

CONSPECTUS: Donald Cram's pioneering Nobel Prize-winning work on host-guest molecules led eventually to his creation of the field of container molecules. Cram defined two types of container molecules: carcerands and hemicarcerands. Host-guest complexes of carcerands, called carceplexes, are formed during their synthesis; once a carceplex is formed, the trapped guest cannot exit without breaking covalent bonds. Cram defined a quantity called constrictive binding, arising from the mechanical force that prevents guest escape. The constrictive binding in carceplexes is high. In contrast, hemicarcerands have low constrictive binding and are able to release the incarcerated guests at elevated temperatures without breaking covalent bonds. We have designed molecules that can switch from carcerand to hemicarcerand through a change in structure that we call gating. The original discovery of gating in container molecules involved our computational studies of a Cram hemicarceplex that was observed to release a guest upon heating. We found that the side portals of this hemicarceplex have multiple thermally accessible conformations. An eight-membered ring that is part of a portal changes from a "chair" to a "boat" structure, leading to the enlargement of the side portal and the release of the guest. This type of gating is analogous to phenomena often observed with peptide loops in enzymes. We refer to this phenomenon as thermally controlled gating. We have also designed and synthesized redox and photochemically controlled gated hemicarceplexes. Gates are built onto host molecules so that the opening or closing of such gates is stimulated by reducing or oxidizing conditions, or by ultraviolet irradiation. In both cases, the appropriate stimuli can produce a carceplex (closed gates) or hemicarceplex (open gates). A hemicarceplex with closed gates behaves like a carceplex, due to its very high constrictive binding energy. When the gates are opened, constrictive binding is dramatically lowered, and guest entrance and exit become facile. This stimulated switching between open and closed states controls access of the guest to the binding site. The experimental and computational investigations of gated hemicarcerands and several potential applications of gated hemicarceplexes are described in this Account

IDENTIFYING COST-EFFECTIVE SOURCES FOR WATER TRANSFERS FROM AGRICULTURE TO ENDANGERED SPECIES PRESERVATION IN THE PLATTE RIVER BASIN

Resource /Energy Economics and Policy,

High Resolution Spectroscopy and Spectropolarimetry of some late F-/early G-type sun-like stars as targets for Zeeman Doppler imaging

High resolution spectroscopy and spectropolarimetry have been undertaken at the Anglo-Australian Telescope in order to identify suitable targets for magnetic studies of young sun-like stars, for the proxy study of early solar evolution. This study involved the investigation of some variable late F-/early G-type sun-like stars originally identified by the Hipparcos mission. Of the 38 stars observed for this study, HIP 31021, HIP 64732, HIP 73780 were found to be spectroscopic binary stars while HIP 19072, HIP 67651 and HIP 75636 are also likely to be binaries while HIP 33111 could even be a triple system. Magnetic fields were detected on a number of the survey stars: HIP 21632, HIP 43720, HIP 48770, HIP 62517, HIP 71933, HIP 77144, HIP 89829, HIP 90899 and HIP 105388, making these stars good candidates for follow-up Zeeman Doppler imaging studies.Comment: 16 pages, 16 figures, 4 tables Accepted for publication in PAS

Characteristics of Waterfowl Harvest at Horseshoe Lake, Madison County, Illinois

Division of Wildlife Resources Migratory Bird Section, Periodic Report No. 13Report issued on: April 21, 197