A convenient protocol for selective cleavage of 2-hydroxy acid amides. Application to semisynthesis of the cyclic heptapeptide aza HUN-7293

A two-step protocol for the first chemoselective cleavage of 2-hydroxy acid amides has been developed. Mesylation of the model substrate 2-(hydroxypropionylamino)-4-methylpentanoic acid methyl ester (11) followed by treatment with N-ethylthiourea (13) allows cleavage of 2-hydroxy acid amides under smooth conditions. Successful application of this methodology to the open-chain transesterification product 15 (methylester) of the cyclic heptadepsipeptide HUN-7293, a potent inhibitor of inducible cell adhesion molecule expression, delivered the corresponding hexapeptide 18 with unprotected N-terminus in 70-75% yield. This result demonstrates that the protocol developed even works in the presence of an ester and several methylated and unmethylated amide bonds. Finally, a sequence of ligation of methyl D-dehydroglutaminate (20) to the C-terminus of the saponification product 21, followed by the degradation protocol and ring closure, allowed chemical "point mutation" at the DGCN site affording the aza analogue of HUN-7293 (24) in 15% overall yield. To the best of our knowledge this is the first report on chemoselective cleavage of 2-hydroxy acid amides.     

The crystal and molecular structure of the cycloadduct from an activated carbodiimide and methyl isothiocyanate

The crystal and molecular structures of the cycloadduct from isopropyl[α-(dimethylthio-carbamoyl)isopropyl]carbodiimide and methyl isothiocyanate were determined by single-crystal X-ray methods. The product was established to be 2-(1-isopropyl-3,5-dimethyl-4,6-dithioxo-hexahydro-1,3,5-triazine-2-ylidenamino)-N,N-dimethylthioisoburyramide, in which contrary to expectations based on hybridisation the heterocyclic ring adopts a boat conformation.     

Über die Nullstellen homogener Differentialpolynome

In this paper it is discussed for which entire functions g the homogeneous differential polynomial gg-ag² has no zeros. It is shown that g(z)=exp (z+) are the only transcendental entire functions with this property if a1. The case a=1 is indeed exceptional. This result settles a question of Hayman.     

Über metall-alkyl- und -aryl-verbindungen: XVII. Die kristallstruktur des kaliumhydridotrimethylaluminats,K[AlH(CH3)3]

K[AlH(CH₃)₃] is formed by decomposition of K[Al(CH₃)₃SiH₃] in diethyl ether. The compound crystallizes in the orthorhombic space group Cmca with the lattice parameters a = 9.647, b = 17.332, c = 7.711 Å. There are eight formula units in the unit cell. Analogously to K[Al(CH₃)₄] the hydrido compound contains isolated anions with a tetrahedral coordination of aluminium by hydrogen and three methyl groups.     

Studies on epoxides—VI : Opening reactions of α-substituted epoxysteroids

Treatment of 1,2-epoxy-3-ketosteroids with tosylhydrazine and sodium borohydride in methanol lead to mixtures consisting of (i) ring A seco-acetylenic alcohols, (ii) methoxyhydrins formed by opening of the epoxide ring following the carbonyl reduction, and (iii) an epoxy-3,3-di- methylketal. For the corresponding 4-methylene derivative, the ring A fragmentation product was found to be the conjugated en-yne 8a. Reduction of these epoxyketones with sodium borohydride alone affords mixtures of cis- and trans epoxyalcohols; the cis derivatives were unaffected by heating with sodium borohydride in methanol, whereas the trans were converted into the corresponding diaxial methoxyhydrins.     

A constitutional diagram of the system TiC−HfC−WC

The system TiC?HfC?WC was investigated by means of melting point, differential thermoanalytical, X-ray diffraction and metallographic techniques on hot pressed and heat treated as well as melted alloy specimens and a complete constitutional diagram from 1500°C through the melting range established. According to the peritectic melting of hexagonal WC both isopleths, TiC?WC as well as HfC?WC show a class II reaction at 2760°C in Ti?W?C and at 2730°C in Hf?W?C. The phase behaviour within the TiC?HfC?WC system is characterized by the presence of a (binary) miscibility gap within TiC?HfC [T _c=1780°C, (TiC)_0.55(HfC)_0.45] which extends into the ternary forming a closed ternary miscibility gap at higher temperatures with an isolated ternary critical point:T _c=1800°C, (TiC)_0.55(HfC)_0.45(WC)_0.05. Interaction of the solvus (boundary of the cubic-B 1 monocarbide solid solution) and the ternary miscibility gap was established at 1540°C and (TiC)_0.27(HfC)_0.41(WC)_0.32: Alloys of this composition enter a decomposition reaction on cooling into two isotypic cubic B 1 phases and hexagonal WC. Isothermal sections were calculated assuming regular solutions.     

On self-repellent one dimensional random walks

Summary We consider an ordinary one dimensional recurrent random walk on. A self-repellent random walk is defined by changing the ordinary law of the random walk in the following way: A path gets a new relative weight by multiplying the old one with a factor 1– for every self intersection of the path. 0<<1 is a parameter.It is shown that if the jump distribution of the random walk has an exponential moment and if is small enough then the displacement of the endpoint is asymptotically of the order of the length of the path.Partially supported by the Deutsche Forschungsgemeinschaft and the Akademie der Wissenschaften zu Berlin     

Finite element approximation of spectral problems with Neumann boundary conditions on curved domains

This paper deals with the finite element approximation of the spectral problem for the Laplace equation with Neumann boundary conditions on a curved nonconvex domain . Convergence and optimal order error estimates are proved for standard piecewise linear continuous elements on a discrete polygonal domain in the framework of the abstract spectral approximation theory.