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.

Study of the Chemoselectivity of Grignard Reagent Addition to Substrates Containing Both Nitrile and Weinreb Amide Functionalities

A novel and versatile synthesis of 1-aryl-benzocyclobutenols and 1-aryl-benzocyclobutenes

Convenient Synthesis of 3‐Glycosylated Isocoumarins

A variety of α-aroylacetones 4a−g have been prepared in excellent yields following a new protocol wherein α-aminonitriles 1a−g as the aryl acyl anion equivalents readily react with propargyl bromide as the α-bromoacetone equivalent. The alkylated product undergoes one-pot unmasking of the keto functionality along with Markovnikov's hydration of the terminal alkyne with CuSO4·5H2O in aqueous methanol at 60 °C to furnish the desired target in excellent isolated yields.

Indrapal Singh Aidhen

Papers

Study of the Chemoselectivity of Grignard Reagent Addition to Substrates Containing Both Nitrile and Weinreb Amide Functionalities

A novel and versatile synthesis of 1-aryl-benzocyclobutenols and 1-aryl-benzocyclobutenes

Convenient Synthesis of 3‐Glycosylated Isocoumarins

Propargyl Bromide as an Excellent α-Bromoacetone Equivalent: Convenient and New Route to α-Aroylacetones†

Simple Synthetic Equivalents for the β-(N,N-Disubstituted)ethylamino Acyl Cation Synthon and their Applications