Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

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Manganese(III)-Based Oxidative Free-Radical Cyclizations

"Economy" is referred to as the thrifty and efficient use of material resources, as the principle of "minimum effort to reach a goal." More illuminating is: "the aim to portion one's forces in order to use as little as possible of them to reach a goal." Such statements certainly apply when the goal is to synthesize a complex target molecule. Redox economy then implies the use of as few redox steps as possible in the synthetic conquest of a target compound. While any sort of economy will help to streamline the effort of total synthesis, redox economy addresses a particularly weak area in present-day total synthesis. It is not enough to point out the present deficiencies, rather the purpose of this Review is to serve as a teaching tool for all practitioners of the field by giving and illustrating guidelines to increase redox economy in multistep organic synthesis.

Redox economy in organic synthesis.

The stereoselective formation of C-C bonds is of great importance for the synthesis of enantiomerically pure natural products and pharmaceuticals. A broad repertoire of chiral auxiliaries, reagents, and catalysts can be utilized for the reliable generation of tertiary stereocenters. In contrast, the synthesis of organic compounds with quaternary stereocenters is a much more demanding and challenging task. Every enantioselective synthetic method can demonstrate its value through the generation of a fully substituted carbon center. In this Minireview examples of newer stoichiometric and catalytic methods are summarized which have proved their suitability for the enantioselective construction of quaternary stereocenters.

Enantioselective Construction of Quaternary Stereocenters

Despite a well-developed and growing body of work in copper catalysis, the potential of copper to serve as a photocatalyst remains underexplored. Here we describe a photoinduced copper-catalyzed method for coupling readily available racemic tertiary alkyl chloride electrophiles with amines to generate fully substituted stereocenters with high enantioselectivity. The reaction proceeds at –40°C under excitation by a blue light-emitting diode and benefits from the use of a single, Earth-abundant transition metal acting as both the photocatalyst and the source of asymmetric induction. An enantioconvergent mechanism transforms the racemic starting material into a single product enantiomer.

Asymmetric copper-catalyzed C-N cross-couplings induced by visible light.

A synthetic route based on a three carbon homologation of an α-aminonitrile was developed for the synthesis of unsymmetrical 1,4-diketones. The key steps were the alkylation of various aryl and heteroaryl α-aminonitriles with N-methoxy-N-methyl-3-bromopropionamide followed by the addition of a Grignard reagent to the alkylated product and then subsequent hydrolysis.

N-Methoxy-N-methyl-3-bromopropionamide: A New Three-Carbon Homologating Agent for the Synthesis of Unsymmetrical 1,4-Diketones.

Two carbon chain homolgation of enantiopure four carbon erythro configured iodo derivative 5 with N-methoxy-N-methyl-2-phenylsulfonylacetamide 6 affords for the first time a stable and storable precursor 3 of D-amicetose in the acyclic form, wherein the sensitive aldehyde is masked as a Wcinreb amide. The differential protection in this acyclic derivative offers all the potential to exclusively arrive at the pyranose form of the target dideoxy-sugar.

Synthesis of a Stable, Storable and Differentially Protected Acyclic Precursor of D-Amicetose and Its Conversion to 4-O-Benzyl-Protected D-Amicetose (V).

Unambiguous assignment of the stereochemistry at the anomeric carbon in methyl-α-D-C-aryl-glucopyranoside derivative: A representative of products from our new strategy for 2-deoxy-C-aryl-glucopyranosides

A new strategy for the synthesis of α- and β-organylseleno aryl ketones has been achieved. The strategy is based on the use of hitherto unreported 2,2'-diselenediylbis(N-methoxy-N-methylacetamide) and 3,3'-diselenediylbis(N-methoxy-N-methylpropanamide). The envisaged synthetic equivalents combine the usefulness of Weinreb amide (WA) functionality and those innate with selenium for the first time. The synthesis of the targets, α- and β-organylseleno aryl ketones could be achieved by the reductive cleavage of Se-Se bond, followed by the alkylation on selenium, and the addition of arylmagnesium halide onto the WA functionality therein.

Weinreb Amide Based BuildingBlocks for the Synthesis of α- and β-OrganylselenoAryl Ketones

A simple and efficient protocol is described for the regioselective hydrolysis of terminal isopropylidene ketal protection in carbohydrate derivatives la- 11a. It uses either CoCl 2 . 2H 2 O in acetonitrile or InCl 3 in methanol at temperatures ranging from 50 to 60 °C. The low cost of CoCl 2 . 2H 2 O along with its requirement in catalytic quantities offers a great advantage for the multi-gram scale reaction.

Indrapal Singh Aidhen

Papers

N-Methoxy-N-methyl-3-bromopropionamide: A New Three-Carbon Homologating Agent for the Synthesis of Unsymmetrical 1,4-Diketones.

Synthesis of a Stable, Storable and Differentially Protected Acyclic Precursor of D-Amicetose and Its Conversion to 4-O-Benzyl-Protected D-Amicetose (V).

Unambiguous assignment of the stereochemistry at the anomeric carbon in methyl-α-D-C-aryl-glucopyranoside derivative: A representative of products from our new strategy for 2-deoxy-C-aryl-glucopyranosides

Weinreb Amide Based BuildingBlocks for the Synthesis of α- and β-OrganylselenoAryl Ketones

Efficient and Rapid Regioselective Deprotection of Isopropylidene Ketals.