2 , and the structure of this phase has been determined by Xray diffraction. The same product can be obtained by solidstate dehydration of single crystals of manganese formate dihydrate, giving crystals that, although of poor quality, still allow for a single crystal structure determination. The structural relationship between the hydrated and anhydrous
Both aromatic and aliphatic Schiff bases 1 can be cyclized asymmetrically in the presence of titanium alkoxides if N,N‐phthaloyl‐protected amino acids are employed as chiral auxiliaries. The desired heterocycles 3, which are suitable for alkaloid syntheses, are formed with diastereomer ratios of up to 99%. R nPr, iPr; R1 Me, Et, iPr, Ph, Ar; R2 Me, iPr, tBu.
Aromatic and aliphatic Schiff bases of tryptamine react with Fmoc-or phthaloyl-protected amino acid chlorides to form N-acyliminium intermediates, which, in tions to give tetrahydro-fl-carbolines with diastereomeric ratios of up to 99: 1. The chiral reduction. To rationalize the observed stereoselectivity a transition-state model is proposed in which the titanium atom coordinates both the carbonyl group of the N-acyliminium ion and the amino acid protecting group. the presence of titanium alkoxides at room temperature, undergo Pictet -Spengler reacauxiliary can be removed from the Pictet-Spengler adducts by means of a simple
X-ray powder measurements were used to investigate the phase transitions and crystal structures of K2NbCl6 and Rb2NbCl6. In both cases, the metrical changes of the unit cell are accompanied by a compressive distortion of the NbCl6
2− groups from O
h to D
4h symmetry. This static manifestation of the Jahn-Teller effect is ascribed to electro static interactions with the cations. Thus, Rb2NbCl6 trans forms from Fm-3m (Z = 4) to I4/mmm (Z = 2) due to a cation-supported compression in the c direction, whereas K2NbCl6 expands along c when converting from Fm-3m to P4/mnc (Z = 2). On further cooling a second transition leads to Cccm (Z = 4) where b as the shortest axis is approximately parallel to the direction of the octahedral contraction. The oxidation state of Nb4+ (d
1) is checked via bond-length bond-strength correlations and magnetic measurements.
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