A Total Synthesis of the Antifungal Mold Metabolite (±)-LL-Z1271α

The mold metabolite (-)-LL-Z1271α (10a) was isolated from the fermentation products of an Acrostalagmus species and was found to exhibit antifungal activity toward several pathogenic fungi.¹ We wish to report a total synthesis of (±)-LL-Z1271α (10a).²     

Characterization of nonlinear optical absorption and refraction

We discuss the characterization of nonlinear optical processes that give rise to changes in the absorption coefficient and refractive index. We primarily concentrate on methods for determining the dominant nonlinearities present in condensed matter and the responsible physical mechanisms. In extensive studies of a wide variety of material, we have found that there is seldom a single nonlinear process occurring. Often several processes occur simultaneously, sometimes in unison, sometimes competing. It is necessary to experimentally distinguish and separate these processes in order to understand and model the interaction. There are a variety of methods and techniques for determining the nonlinear optical response, each with its own weaknesses and advantages. In general, it is advisable to use as many complementary techniques as possible over a broad spectral range in order to unambiguously determine the active nonlinearities. Here we concentrate on the use of nonlinear transmittance, Z-scan and degenerate four-wave mixing experiments as applied to polycrystalline and single crystal semiconductors and dielectric materials.     

Reaction of gadolinium chelates with endogenously available ions

The extent of reaction of 153Gd-radiolabeled Gd(L) chelates with 25 mM CO23- (25 mF), PO34-, Zn2+ and Cu2+ at pH 7 was determined for L = EDTA, DTPA, DOTA, HP-DO3A, and DO3A. Gd(EDTA)- and Gd(DTPA)2- reacted (greater than 20% in 10 min) with Cu2+ and Zn2+ in the presence of PO34-. These double replacement reactions yielded precipitated GdPO4 and chelated Cu(L). Gd(HP-DO3A), Gd(DO3A) and Gd(DOTA)- were inert to reaction with all four ions at room temperature (less than or equal to 1% reaction detected). The thermodynamic binding constants of the ligands for Gd3+ and Cu2+ were found to be equal (10(20) M-1) for DO3A, while DOTA and HP-DO3A favored Gd3+ over Cu2+ by greater than or equal to 10(2) M-1. The low order of reactivity of Gd(DOTA)- and Gd(HP-DO3A) was anticipated by the binding constants, but the lack of reactivity of Gd(DO3A) is attributed to kinetic inertia. This latter property, desirable in MRI contrast agents, is promoted by the conformational stability of the tetraazacyclododecane macrocycle, which forms the backbone of the ligand. It is concluded that this class of chelates is exceptionally inert in solutions of endogenously available ions, and that thermodynamics alone is an insufficient predictor of the reactivity of the highly inert Gd complexes based on the tetraazamacrocycle.     

Reaction of hydroxylamine with benzenesulfonyl chloride. X-ray crystal structure of piloty''s acid and other benzenesulfonylhydroxylamines

The X-ray crystal structures of the four stable phenylhydroxylamines PhSO₂NHOH, (PhSO₂)₂NOH,PhSO₂NHOSO₂Ph, (PhSO₂)₂NOSO₂Ph), and of PhSO₃⁻⁺H₃NHNS0₂Ph are presented. The last of these is a by-product obtained during the isolation of PhSO2NHOH (Piloty's Acid). The formation of and the bonding in these molecules are discussed.     

Stratified layer formation in particle suspensions

Initially homogeneous suspensions of colloidal particles often develop patterns during sedimentation. Commonly, the concentration profile of the particles evolves into a “staircase”: layers of nearly constant concentration, separated by sharp boundaries between successive layers, with the concentration of each successive layer increasing with depth. Siano [1] has demonstrated experimentally that uphill diffusion, diffusion against the concentration gradient, occurs during this pattern formation. Thus, these patterns appear to be the result of spinodal decomposition. We find that these staircase patterns cannot be explained by the classical spinodal decomposition theory of Cahn and Hilliard, but that they can be explained if the linear gradient-energy term of Tiller, Pound, and Hirth is added to the free energy. Such a term plays a central role in the faceting of crystals. In the present application we believe that the physical origin of this extra term may be the Rayleigh—Taylor instability.     

Chiral fluoroacetic acid: synthesis of (R)- and (S)-[2H1]-fluoroacetate in high enantiopurity

A two-step synthesis of (R)- and (S)-[²H₁]-fluoroacetate (sodium salts) in high enantioselectivity is reported. The synthesis is the development of a previous one in which the enantioselectivity has been increased from ～38% ee to >95% ee. The improvement in enantioselectivity applied Bio’s methodology, which involved a deoxyfluorination reaction with DAST on either enantiomer of [²H₁]-benzyl alcohol, adding TMS-morpholine to the reaction. The additive promotes an S_N2 inversion process, and suppresses a competing non-stereospecific S_N1 reaction course, and as a result significantly improves the stereointegrity of the C–F bond formation. The intermediate [²H₁]-benzyl alcohols, [²H₁]-benzyl fluorides and the product [²H₁]-fluoroacetates as their hexyl esters were separately assayed for their stereochemical integrity, using the Courtieu method. This method involved measuring their ²H NMR spectra in a chiral matrix of poly-γ-benzyl l-glutamate. The chiral assay demonstrated that there was no significant loss in stereointegrity during the deoxyfluorination reaction and showed that the enantiomers of [²H₁]-fluoroacetate were generated with high enantiomeric purity (95% ee).     

Extended squaraine dyes with large two-photon absorption cross-sections

Extended bis(donor)-substituted squaraine chromophores exhibit very high two-photon cross-sections (as high as 33 000 GM) in the near-IR; these can be attributed to the combination of large transition dipoles with small detuning energies. The modulus of the third-order nonlinear optical susceptibility at 1.3 mum has been found to be 7.0 x 10-11 esu for one of these chromophores.