A convenient oxazole C-2 protecting group: the synthesis of 4- and 5-substituted oxazoles via metalation of 2-triisopropylsilyloxazoles

[reaction: see text] Metalation of oxazoles at the 4 and 5 position was achieved after regioselective C-2 silyl protection. Removal of the protecting group was then accomplished under mild conditions allowing for a straightforward preparation of C-5 monosubstituted and C-4,5 disubstituted oxazoles. The first practical C-2 protecting group of oxazoles has been demonstrated.     

Niobium and tantalum tris(methimazolyl)borate complexes [M(=NC6H3iPr2-2,6)Cl2{HB(mt)3}] (M = Nb, Ta; mt = methimazolyl)

The first early transition metal tris(methimazolyl)borate com-plexes [M(=NR)Cl2{HB(mt)3}] (M = Nb, Ta; R = C6H3(i)Pr(2)-2,6; mt = methimazolyl) have been obtained from the reactions of [Nb(=NR)Cl3(DME)] or [Ta(=NR)Cl3(THF)2] (DME = dimethyl ether; THF = tetrahydrofuran) with Na[HB(mt)3] and structurally characterized, illustrating that the HB(mt)3 ligand can indeed be compatible with "hard" metals in high oxidation states.     

(+)-Phorboxazole A synthetic studies. A highly convergent, second generation total synthesis of (+)-phorboxazole A

[structure: see text] A second generation total synthesis of the potent antitumor agent (+)-phorboxazole A (1) has been achieved. The cornerstone of this approach comprises a more convergent strategy, involving late-stage Stille union of a fully elaborated C(1-28) macrocycle with a C(29-46) side chain. The second generation synthesis entails the longest linear sequence of 24 steps, with an overall yield of 4.2%.     

A general route to 5-substituted-2-furylacetic acids: a brief synthesis of plakorsin B

3,4-Dihydroxy-5-alkynylcarboxylic acids, readily obtained by the addition of lithium acetylides to α-acetoxysuccinic anhydride followed by reduction and hydrolysis, undergo smooth silver(I)-catalysed 5-endo-dig cyclisations and in situ dehydration to give excellent overall yields of 5-substituted-2-furylacetic acids, including the natural metabolite plakorsin B.     

Synthesis of alkylpyrroles by use of a vinamidinium salt

The synthesis of alkyl-substituted 2-pyrrolecarboxylate esters has been accomplished by the condensation reaction of a symmetrical vinamidinium salt and glycine ester derivatives.     

Synthesis and Crystal Structure of an Organic-Polyoxometalate (POM) Hybrid Compound Containing a Keggin Type Mo–Si Polyanion

Abstract  

Complex (1), which is comprised of a typical Keggin type anion, four protonated 3-amino-1,2,4-triazole (AmTAZ) ligands, one nitrate anion and an area of disordered unidentifiable species was synthesized via hydrothermal methods followed by slow evaporation. The structure is held together through a web of hydrogen bonding and crystallizes in the monoclinic space group P2₁/c with unit cell dimensions of a = 13.789(3) b = 15.551(3) c = 24.401(7) ?, β = 93.898(8)°, V = 5220(2) ?³ Z = 4, D = 2.896 Mg/m³.     

On surface tension modelling using the level set method

The paper describes and compares the performance of two options for numerically representing the surface tension force in combination with the level set interface‐tracking method. In both models, the surface tension is represented as a body force, concentrated near the interface, but the technical implementation is different: the first model is based on a traditional level set approach in which the force is distributed in a band around the interface using a regularized delta function, whereas in the second, the force is partly distributed in a band around the interface and partly localized to the actual computational cells containing the interface. A comparative study, involving analysis of several two‐phase flows with moving interfaces, shows that in general the two surface tension models produce results of similar accuracy. However, in the particular case of merging and pinching‐off of interfaces, the traditional level set model of surface tension produces an error that results in non‐converging solutions for film‐like interfaces (i.e. ones involving large contact areas). In contrast, the second model, based on the localized representation of the surface tension force, displays consistent first‐order convergence. Copyright © 2008 John Wiley & Sons, Ltd.     

Zum Nachweis und zur Bestimmung des Stickstoffs in organischen K?rpern

Ohne Zusammenfassung     

Gouy phase compensation in quasi-phase matching

In any focussed nonlinear interaction the focus induced phase shift, known as the Gouy phase shift, provides an imperfection in phase matching for any linearly invariant material. However, using an appropriately designed quasi-phase matched structure it is theoretically possible to compensate for the deleterious effects of the Gouy phase shift, allowing a symmetric frequency response and tighter optimal focussing than in a uniform material.