Folded metallodendrons: shell-selective metal coordination and conformational properties.

The synthesis, structure, and conformational properties of folded metallodendrons are described. The enhanced coordination ability of oxazoline-functionalized pyridyl-2,6-dicarboxamides, compared with pyridyl-2.6-diarylamides, permits unique metals to be installed at the peripheral and focal shells of the dendron. Circular dichroism (CD) studies and X-ray crystallography indicate that the intrinsic helical preference of the parent dendron is not perturbed by coordination. However, the CD spectra become insensitive to temperature due to an increase in structural rigidity that occurs upon metal coordination. [structure: see text]

Structural impact of coordinating a helical 2,6-pyridyl diamide with divalent metals.

[Structure: see text] The structural consequences of coordinating 2,6-bis[2-((4S)-4-methyl-4,5-dihydro-1,3-oxazol-2-yl)phenyl]carbamoylpyridines, 2, with divalent metals such as Cu(II), Ni(II), and Zn(II) are reported. Metal coordination occurs under mild conditions in a manner that preserves the helical bias of the parent ligand in the solid state and in solution. 1H NMR line-shape analysis indicates that metal coordination increases the helical interconversion barrier, thus rigidifying the dynamic helicity of 2.

Synthesis and selected reactions of a bicyclic sultam having sulfur at the apex position.

A practical synthesis of the bicyclic dienyl sultam 9 has been developed. The viable route involved several key steps. Of these, ring-closing metathesis represented by the conversion of 19 to 20 had to be implemented in advance of the assembly of other rings such as is present in 15. Product 20 was used as the template for more advanced framework construction, as in 16. The second double bond was best introduced by a bromination-dehydrobromination sequence, the 2-fold loss of HBr being achieved most reliably by the use of tetra-n-butylammonium fluoride in CH2Cl2 or DMSO. The direct irradiation of 9 gave rise to the endo-oriented cyclobutene derivative 30. The title diene is not a ready participant in Diels-Alder reactions. When heated with endo-bornyltriazolinedione in ethyl acetate solution, conversion to a 1:1 mixture of 33 and its diastereomer occurred as confirmed by X-ray crystallographic analysis. From the mechanistic perspective, this transformation constitutes an interesting example of a stereocontrolled and regioselective [2+2] cycloaddition followed by a vinylcyclobutane-cyclohexene rearrangement. Products 30 and 33 constitute examples of strained sulfonamides featuring a norbornyl-like structural component.

The influence of sterics on the formation of polar 1-D hydrogen-bonded networks.

Polar, noncentrosymmetric packing of directional, 1-D hydrogen-bonded networks of chiral, 4-amino-2,6-bis(oxazolinyl)pyridines (ampybox) occurs for isopropyl-substituted ampybox; in contrast, 1-D networks of methyl-substituted ampybox pack in an antiparallel arrangement.

Dynamic helical chirality of an intramolecularly hydrogen-bonded bisoxazoline.

The synthesis and conformational properties of 2,6-bis-[2-((4S)-4-methyl-4,5-dihydro-1,3-oxazol-2-yl)phenyl]carbam oylpyridines, 2, have been described. Bisoxazoline 2a was prepared in five steps from 2-nitrobenzoyl chloride in an overall yield of 71%. In contrast to related structures such as 1, bisoxazoline 2a exhibits a highly biased P-type helical conformation in solution and in the solid state. In the crystal lattice, 2a further assembles into a left-handed helical superstructure aligned along the crystallographic c axis. The barrier to helical interconversion, as measured by line-shape analysis of the temperature-dependent (1)H NMR spectra of thiobenzyl derivative 2b, was determined to be quite low ((Delta)G(++) = 12.3 kcal/mol), indicating the presence of a highly dynamic helical chirality.