958the r bonding in the P-N and Se-N systems is important in stabilizing the low-energy conformer. This A bonding together with the electrostatic interaction is responsible for the high stability of the tub structure.Abstract: Carboxylate and carboxamide ligands on dirhodium(I1) catalysts can provide enormous regiocontrol in carbon-hydrogen insertion reactions of diazoasetate esters. Whereas 2,3,4-trimethyl-3-pentyl diazoacetoacetate (1) forms y-lactone products from insertion into primary and tertiary C-H bonds in a statistical distribution (61:39) with dirhodium(I1) tetrakis(perfluorobutyrate), only tertiary C-H insertion is observed with dirhodium( 11) tetraacetamide. Similar results are obtained with 2-methyl-2-octyl diazoacetoacetate (3), where competition for insertion exists between secondary and primary C-H bonds and electronic factors govern regioselection. However, with 2-methyl-3-isopropyl-3-heptyl diazoacetoacetate (2) and 2-methyl-1-phenyl-2-propyl diazoacetoacetate (4), product distributions from C-H insertion are invariant with the dirhodium(I1) ligands; insertion into a secondary C-H bond is favored over tertiary C-H insertion with 2 (95:5), and insertion into a primary C-H bond is preferred to benzylic secondary C-H insertion with 4 (70:30). In such cases, which are amenable to analyses by MM2 calculations, regioselectivity is determined by conformational preferences for which C-H insertion selectivity can be as random as that found with 2 and 4. When only one C-H bond site is available for insertion to form a five-membered ring product, only one y-lactone is observed from reactions catalyzed by dirhodium(I1) tetraacetate, and that product is not necessarily the one predicted by presumed electronic preferences.
[2-(13)C]-L-lysine, [3,4-(13)C2]-L-lysine and [5,6-(13)C2]-L-lysine are prepared from simple, commercially available, highly enriched starting materials as [2-(13)C]-glycine, ethyl [1,2-(13)C2]-bromo acetate, and [1,2-(13)C2]-acetonitrile. The introduction of the chiral center is based on a general method starting from the bis-lactim ether of cyclo-(D-Val-Gly). The synthesis of (2R)-[5-(13)C]-3,6-diethoxy-2,5-dihydro-2-isopropylpyrazine is described. The availability of our method for the preparation of specifically enriched bis-lactim ethers allows the synthesis of a great variety of site specific isotopically labelled (L- and D-)α-amino acids. Moreover, intermediate 4-[(2R,5S)-3,6-diethoxy-2,5-dihydro-2-isopropyl-5-pyrazinyl]butyronitrile is a valuable precursor in the synthesis of L-α-aminoadipic acid. The synthetic scheme in this publication makes both L-lysine and L-α-aminoadipic acid(13)C- or(15)N-labelled at any position, easily available. The isotopomers of lysine are obtained on a preparative scale in good yields, with 99%(13)C and high enantiomeric purity (>97% e.e.). Three isotopomers are characterized using various spectroscopic techniques,e.g.,(1)H NMR,(13)C NMR and Mass spectrometry.
Electronic and Steric Control in Carbon-Hydrogen Insertion Reactions of Diazoacetoacetates Catalyzed by Dirhodium(II) Carboxylates and Carboxamides.-The unique capabilities of Rh(II) carboxylates and carboxamides to highly modify regiocontrol in C-H insertion reactions of diazoacetate esters is demonstrated with some examples. Whereas (I) forms γ-lactone products (II) and (III) in a statistical distribution with cat. 1, only tertiary C-H insertion is observed with cat.2. Similar results are obtained with (IV). However, with (V) and (VIII) the product distributions are invariant with the dirhodium(II) ligands giving the five-membered ring products in the shown ratio. A mechanistic model is proposed and C-H insertion selectivities are discussed in terms of molecular mechanics calculations. -(DOYLE, M. P.; WESTRUM, L. J.; WOLTHUIS, W. N. E.; SEE, M. M.; BOONE, W. P.; BAGHERI, V.; PEARSON, M. M.; J. Am. Chem. Soc. 115 (1993) 3, 958-964; Dep. Chem., Trinity Univ., San Antonio, TX 78212, USA; EN)
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