Enantioselective Formal C(sp2)−H Vinylation

An enantioselective formal C(sp²)?H vinylation of prochiral 2,2‐disubstituted cyclopentene‐1,3‐dione is presented. This vinylative desymmetrization is realized by using a two‐step procedure that consists of a catalytic enantioselective vinylogous Michael addition of deconjugated butenolides to cyclopentene‐1,3‐dione and a base‐mediated decarboxylation. The overall process utilizes deconjugated butenolides as the source of the highly substituted vinyl unit. Five‐membered carbocycles containing a remote all‐carbon quaternary stereogenic center are obtained in good yields and with good to high enantioselectivities.     

The molecular structure of N-hydroxyurea

Ab initio calculations were performed on the tautomers and conformers of N-hydroxyurea using a 6-31G ** basis set. The minimum-energy structures have been found and the importance of the intramolecular hydrogen bond as the stabilizing factor was pointed out. © 1996 John Wiley & Sons, Inc.     

Precise toppling balance, quenched disorder, and universality for sandpiles

A single sandpile model with quenched random toppling matrices captures the crucial features of different models of self-organized criticality. With symmetric matrices avalanche statistics falls in the multiscaling Bak-Tang-Wiesenfeld universality class. In the asymmetric case the simple scaling of the Manna model is observed. The presence or absence of a precise toppling balance between the amount of sand released by a toppling site and the total quantity the same site receives when all its neighbors topple once determines the appropriate universality class.     

Structure of 72,74Se at high spin

Lifetimes of high spin states up to { }=22⁺ in the yrast positive parity bands have been measured to investigate the shape evolution with increasing spin in ^{72, 74}Se. The Q _t values derived from these measurements indicate that prolate shape stabilizes for ⁷²Se, while a triaxial shape develops for ⁷⁴Se at higher spins. Comparison of the observed trend in Q _t with spin for ^{72, 74}Se with that of the corresponding kryptones isotones emphasizes the stability provided by N=38 prolate shell gap even at high rotational frequency.     

On the membranal monostressed profiled shells

Summary The group of profiles is found out according to which, in theory, the meridian of a shell of revolution, submitted to the action of a uniform internal pressure, should be shaped so that the hoop force be equal to zero in any point of the shell.

---

Sommario Viene individuata la schiera dei profili secondo cui occorrerebbe teoricamente sagomare il meridiano d'un involucro di rotazione che, assoggettato a pressione interna uniforme, risulti perfettamente scarico nei confronti della tensione normale al meridiano stesso.

---

---

The present note has been performed in the frame of activity of C.N.R. (Pressure Vessels Design Research Group).