A concise synthesis of the octalactins

The total synthesis of octalactins A and B has been achieved in 15 steps (longest linear sequence) and 10% overall yield from commercially available materials. Key steps include the Paterson-Aldol reaction for the rapid assembly of the carbonate 46, methylenation of 46 and subsequent Claisen rearrangement of the corresponding alkenyl-substituted cyclic ketene acetal to provide the core unsaturated medium-ring lactone 47, and the use of enzyme-mediated acetate deprotection in the presence of a medium-ring lactone.     

Tetramine Aspect Ratio and Flexibility Determine Framework Symmetry for Zn8L6 Self-Assembled Structures

We derive design principles for the assembly of rectangular tetramines into Zn₈L₆ pseudo-cubic coordination cages. Because of the rectangular, as opposed to square, geometry of the ligand panels, and the possibility of either Δ or Λ handedness of each metal center at the eight corners of the pseudo-cube, many different cage diastereomers are possible. Each of the six tetra-aniline subcomponents investigated in this work assembled with zinc(II) and 2-formylpyridine in acetonitrile into a single Zn₈L₆ pseudo-cube diastereomer, however. Each product corresponded to one of four diastereomeric configurations, with T, T_h, S₆ or D₃ symmetry. The preferred diastereomer for a given tetra-aniline subcomponent was shown to be dependent on its aspect ratio and conformational flexibility. Analysis of computationally modeled individual faces or whole pseudo-cubes provided insight as to why the observed diastereomers were favored.     

A new synthesis of spiropyrrolidine-tetralones via an unexpected formal ring-contraction of 4-disubstituted piperidine to 3-disubstituted pyrrolidine

We have developed an efficient synthesis of novel racemic spiropyrrolidine-tetralones via an unexpected ring-contraction reaction of a 4-disubstituted piperidine to 3-disubstituted pyrrolidine. We suggest that intramolecular quaternization of the piperidine nitrogen of compound 7 occurs to form a bridged bicyclic quaternary ammonium salt intermediate 10. The ring opening of 10 with cyanide resulted in pyrrolidine 9. The synthesis of racemic spiropyrrolidine-tetralone 15 is described as well as the related spiropiperidine-indanone, 1b.     

Asymmetric Hydrogenation of Pyridinium Salts with an Iridium Phosphole Catalyst

Iridium‐catalyzed asymmetric hydrogenation of N‐alkyl‐2‐alkylpyridinium salts provided 2‐aryl‐substituted piperidines with high levels of enantioselectivity. Simple benzyl and other alkyl groups successfully activated the challenging pyridine substrates toward hydrogenation. The use of the unusual chiral‐phosphole‐based MP²‐SEGPHOS was the key to the success of this approach which provides a versatile and practical procedure for the synthesis of chiral piperidines.     

Diastereoselective synthesis of quaternary alpha-amino acids from diketopiperazine templates

Sequential enolate alkylations of (S)-N(1)-methyl-5-methoxy-6-isopropyl-3,6-dihydropyrazin-2-one and (S)-N(1)-p-methoxybenzyl-5-methoxy-6-isopropyl-3,6-dihydropyrazin-2-one proceed with excellent levels of diastereoselectivity (>90% de) affording quaternary alpha-amino acids in high enantiomeric excess (>98% ee) after deprotection and hydrolysis.     

Modelling and estimation of social interaction effects in new product diffusion

We present a new product choice model in which individual agents are assumed to interact with each other across spatial hierarchies under random Gaussian laws. A restricted form that allows estimation of the model and inference on the random interaction structure on available data is then derived. This is essentially a random coefficients model which we estimate on repeated cross-sectional data for the adoption of video cassette recorders in Britain. It is argued that the random coefficient approach, based on economic and social-theoretic arguments allows for more accurate inference on the structure of social interactions, and thus may provide a useful way to help alleviate the inferential problem caused by the identification issues stated in the literature. This very same problem has for long been at the core of the debate between alternative economic and marketing theories of new product diffusion.     

The solubility loop of nicotine:water

The lower consolute temperature in the nicotine:water system arises from the equilibrium formation of a covalent hydrate, revealed by its unique circular dichroism.     

Exciton polaritons in quantum wells in a transverse magnetic field

Physics of the Solid State - This paper reports on a study of the exciton polariton region in reflection spectra of wide CdTe/CdZnTe quantum wells (with well width exceeding by far the exciton Bohr...     

Photo-oxidation of proteins and its role in cataractogenesis.

Proteins comprise approximately 68% of the dry weight of cells and tissues and are therefore potentially major targets for photo-oxidation. Two major types of processes can occur with proteins. The first of these involves direct photo-oxidation arising from the absorption of UV radiation by the protein, or bound chromophore groups, thereby generating excited states (singlet or triplets) or radicals via photo-ionisation. The second major process involves indirect oxidation of the protein via the formation and subsequent reactions of singlet oxygen generated by the transfer of energy to ground state (triplet) molecular oxygen by either protein-bound, or other, chromophores. The basic principles behind these mechanisms of photo-oxidation of amino acids, peptides and proteins and the potential selectivity of damage are discussed. Emphasis is placed primarily on the intermediates that are generated on amino acids and proteins, and the subsequent reactions of these species, and not the identity or chemistry of the sensitizer itself, unless the sensitizing group is itself intrinsic to the protein. A particular system is then discussed--the cataractous lens--where UV photo-oxidation may play a role in the aetiology of the disease, and tryptophan-derived metabolites act as UV filters.