Proton and lithium cation affinities calculated with floating orbital geometry optimization (FOGO)

The FOGO method is used to calculate proton affinities and lithium cation affinities. The molecules of primary interest in this study are the methyl-substituted amines. In addition, the lithium cation affinity of HF, H₂O, CH₃OH, H₂CO, and HCN are calculated for comparison. Geometries of all species are fully optimized with a double-zeta (DZ) basis set, including polarization on hydrogen and the first-row elements by floating orbitals. Comparison with experimental values demonstrates that structural data and proton affinities resulting from this type of ab initio calculation are of chemical accuracy. The lithium cation affinities are also reasonably well reproduced, but the small experimental differences are not within the accuracy, which can be expected from this type of calculation.     

Geometry optimization in ab initio SCF calculations: Part IV. Energy barriers and dipole moments obtained with floating orbital geometry optimization (FOGO)

The floating orbital geometry optimization (FOGO) described previously is applied to H₂O₂, NH₃, HNC, HNO, HNCO, and CH₃OH. In the FOGO method we apply two analytically calculated energy gradients in a variable metric method. Some orbitals are no longer fixed on the corresponding nuclei, but their position is optimized simultaneously with the nuclear coordinates. It is shown that relative energies (e.g. rotational barriers) are obtained with similar accuracy to basis sets including polarization functions. Further, it is confirmed that FOGO yields excellent dipole moments. The FOGO method involves a considerable time saving compared to conventional calculations with DZ + P basis sets.     

Partial Synthesis and Characterization of the Mono-and Diepoxides of β-Cryptoxanthin

β-Cryptoxanthin ( 1 ) was acetylated and then epoxidized with monoperoxyphthalic acid. After hydrolysis, repeated chromatography, and crystallization, (3S,5R,6S)-5,6-epoxy-β-cryptoxanthin ( 3 ), (3S,5S,6R)-5,6-epoxy-β-cryptoxanthin ( 4 ), (3R,5′R,6′R)-5′,6′-epoxy-β-cryptoxanthin ( 5 ), (3S,5R,6S,5′R,6′S)-5,6:5′,6′-diepoxy-β-cryp-toxanthin ( 6 ), and (3S,5S,6R,5′S,6′R)-5,6:5′,6′-diepoxy-β-cryptoxanthin ( 7 ) were isolated as main products and characterized by their UV/VIS, CD, ¹H- and ¹³C-NMR, and mass spectra. The comparison of the carotenoid isolated from yellow, tomato-shaped paprika (Capsicum annuum var. lycopersiciforme flavum) with 3–5 strongly supports the structure of 3 for the natural product.     

A novel way to study the initial stages of soap-free emulsion polymerizations: The intercalation of polystyrene oligomers into hydrotalcite

The dominant species in the early stages of an emulsifier-free emulsion polymerization of styrene has been found to be an oligomer of two to three monomer units using a novel trapping technique. This involved the intercalation of charged primary oligomers between the layers of a hydrotalcite, [Mg₄Al₂(OH)₁₂]²⁺[A]^2- (where A = dianion). Hydrotalcites are an important class of lamellar, inorganic compounds whose interlayer spacing can be mod-ified by anion exchange. Our approach first involved preparing a hydrotalcite precursor in which the layers were propped apart by an organic dianion (terephthalate = TA). This material was then used to capture the negatively charged polystyrene oligomers from the emulsion polymerization reaction mixture. We found that TA was rapidly ion-exchanged for the charged oligomers. The resulting pillared hydrotalcite material was characterized using XRD and SEC. We found that the interlayer spacing between the hydroxide layers increased to 23.2 Å on exposure to the emulsion reaction mixture. This represents an interlayer expansion of 18.3 Å (after subtraction of the hydroxide layer contribution), which is cnsistent with intercalation of oligomers with two to three monomer units arranged in a bilayer. This size estimate was confirmed by the results of size exclusion chromatography. © 1995 John Wiley & Sons, Inc.     

Conformational Analysis of 1,2:3,4-Diepoxides: Ab Initio and semiempirical molecular-orbital calculations

Using semiempirical and ab initio procedures, the most stable conformations of meso- and rac-bioxirane and of some substituted 1,2:3,4-diepoxides were calculated. For threo-diepoxides (having the same relative configurations as rac-bioxirane, 3 ), two stable conformations with CCCC dihedral angles of ca. 90 and ca. 270° were found. For erythro-diepoxides (derivatives of meso-bioxirane, 4 ) the calculations suggest three preferred conformations with corresponding dihedral CCCC angles of ca. 90°, ca. 180°, and ca. 270°. The calculations are in fair agreement with the experimental data available for the unsubstituted compounds 3 and 4 .     

Neue Synthese von (−)-(R)-Cembren A,Synthese von (+)-(R)-Cembrenen und (+)-(S)-Cembren

Novel Synthesis of (?)-(R)-Cembrene A, Synthesis of (+)-(R)-Cembrenene and (+)-(S)-Cembrene A novel synthesis of (?)-(R)-cembrene A ((?)- 3 ) was developed using the Sharpless epoxidation for the introduction of the chiral center. Furthermore, the synthesis of (+)-(R)-cembrenene ((+)- 4 ) showed that this cembranoid must have the (R)-configuration and not, as previously reported, the (S)-configuration. Selective hydrogenation of (+)- 4 afforded (+)-(S)-cenibrene ((+)- 5 ).     

Chemie der Cumalinsäurederivate. I. Synthese und Diels-Alder-Reaktionen von 1-Arylamino-2-methoxycarbonylbutadienen

The nucleophilic attack of substituted anilines at position 6 of methyl coumalate (1) opens the α-pyrone ring to form 4-arylamino-3-(methoxycarbonyl)butadien-1-carboxylic acid 2 (Scheme 1). The latter are easily decarboxylated at room temperature in polar aprotic solvents to 1-arylamino-2-(methoxycarbonyl)butadiene 4 which smoothly undergo regio- and stereospecific Diels-Alder reactions with different dienophiles.     

C45- and C50-Carotehoids. Part 8. Synthesis of (all-E,2S,2′S)-bacterioruberin, (all-E,2S,2′S)-monoanhydrobacterioruberin, (all-E,2S,2′S)-bisanhydrobacterioruberin, (all- E,2R,2′R)-3,4,3′,4′-tetrahydrobisanhydro- bacterioruberin,and (all-E,S)-2-isopentenyl-3,4-dehydrorhodopin

Starting from (R)-3-hydroxybutyric acid ((R)- 10 ) the C₄₅- and C₅₀-carotenoids (all-E,2S,2′S)-bacterioruberm ( 1 ), (all-E,2S,2′S)-monoanhydrobacterioruberin ( 2 ), (all-E,2S,2′S)-bisanhydrobacterioruberin ( 3 ), (all-E,2R,2′R)-3,4,3′,4′-tetrahydrobisanhydrobacterioruberin ( 5 ), and (all-E,S)-2-isopentenyl-3,4-dehydrorhodopin ( 6 ) were synthesized. By comparison of the chiroptical data of the natural and the synthetic compounds, the (2S)- and (2′S)-configuration of the natural products 1–3 and 6 was established.