Keith A. Nelson

Virus-templated assembly of porphyrins into light-harvesting nanoantennae.

Biological molecules can be used as versatile templates for assembling nanoscale materials because of their unique structures and chemical diversities. Supramolecular organization of molecular pigments, as is found in the natural light-harvesting antenna, has drawn attention for its potential applications to sensors, photocatalytic systems, and photonic devices. Here we show the arrangement of molecular pigments into a one-dimensional light-harvesting antenna using M13 viruses as scaffolds. Chemical grafting of zinc porphyrins to M13 viruses induces distinctive spectroscopic changes, including fluorescence quenching, the extensive band broadening and small red shift of their absorption spectrum, and the shortened lifetime of the excited states. Based on these optical signatures we suggest a hypothetical model to explain the energy transfer occurring in the supramolecular porphyrin structures templated with the virus. We expect that further genetic engineering of M13 viruses can allow us to coassemble other functional materials (e.g., catalysts and electron transfer mediators) with pigments, implying potential applications to photochemical devices.

Homochiral ketals in organic synthesis. Diastereoselective cyclopropanation of α,β-unsaturated ketals derived from 1,4-D1-O-benzyl-l-threitol

Abstract 2-Cycloalken-1-one 1,4-di - O -benzyl -L-threitol ketals undergo efficient and diastereoselective cyclopropanation when treated with an excess of the Simmons-Smith reagent. For example, 2-cyclohexen-1-one 1,4-di - O -benzyl -L-threitol ketal gave in 90-98% yield a 9:1 mixture of diastereomeric cyclopropanes as established by 62.9 MHz 13C NMR spectroscopy and by hydrolysis of the mixture to (1 R ,6 S )-bicyclo[4.1.0] heptan-2-one. Sixteen other examples are presented which demonstrate the generality and predictability of the process for 2-cycloalken-1-one ketals, as wall as an unfortunate lack of diastereoselectivity for α,β-unsaturated 1,4-di- O -benzyl-L-threitol acetals.

Diastereoselective cyclopropanation via homochiral ketals. Dioxolane structural effects

Abstract A series of 2-cyclohexen-1-one ketals related to 2-cyclohexen-1-one 1,4-di- O -benzyl-L-threitol ketal but possesing different dioxolane appendages was prepared and subjected to Simmons-Smith cycloproponation. The observed diastereoselectivity decreased when oxygen was present in the appendages. In the absence of appendage oxygen, the sense of the observed diastereoselectivity was found to depend upon dioxolane chirality. The amount of diastereoselectivity observed was remarkably independent of the nature of the appendages.

Homochiral ketals in organic synthesis. Enantioselective synthesis of [m.n.1] Propellanones

Abstract Optically active [m.n.1]propellanones have been prepared by diastereoselective cyclopropanation of homochiral one ketals derived from 1,4-di- O -benzyl- L -threitol and readily available bicyclic enones.