Assemblies heterocyclic units

The strongly acidic character of tetramic acids and their usual low solubility prompted us to delay the assembly of this heterocyclic unit until very late in the synthesis. Accordingly, the appropriate ornithine segment was constructed next (Scheme VII). The known amino acid 57 [26b] was transformed via the fully protected derivative 58 to 59 by chemoselective unmasking of the y-amino group with formic acid. The remaining two substituents on the a-amino group are to be removed at different times, with the allyl carbamate destined to precede the 2,4-dimethoxybenzyl functionality. 

Additional combinatorial variation sites allow the heterocyclic self-assembly units. Thus, it has been shown that heterocycles 11 and 14-17 can serve as A-analogous donor-acceptor ligands self-assembling with the T-analogous acceptor-donor ligands isoquinolone 12 and 7-azaindole 18 (Scheme 30) [92]. All combinations form the heterobidentate ligands exclusively upon simple mixing in the presence of a transition metal salt (proven by X-ray, NMR). 

As in CHEC-II(1996), the syntheses of 1,3-oxazine derivatives and their carbo- or heterocycle-fused analogs are discussed according to the numbers and elemental compositions of the assembling units. Within a given type of assembling units, the sequence of discussion follows the decreasing number of double bonds in the 1,3-oxazine ring. 

Linear arrays of protonatable or hydrogen bonded sites may allow the directed long range transfer of protons, thus functioning as proton-conducting channel, i.e., as proton wire. Relevant systems would be linear polyamines or polyphenolic condensed aromatic units [8.218], self-assembled hydrogen bonded heterocyclic ribbons such as 116 (see Section 9.4.4) or polyelectrolyte membranes [8.219] in which collective proton motion may take place and lead to proton conductivity. 

Heterocycles. 3-Aminomethylisoquinohnes are obtained from o-ethynylaraldehydes by treatment with paraformaldehyde and amines, then t-BuNH2- Aminomethylation of the alkyne unit is followed by Schiff reaction and cyclization. Cyclic amidines that serve as precursors of oxindoles are assembled from o-ethynylarylamines and sulfonyl azides. ... 

Aldehydes form O-stannyl ketyls with the BujSn radical. A /3-alkoxyacrylate unit five bonds away from the ketyl is well positioned to be involved in a radical transfer process therefore, cyclization ensues. Fused oxacycles of well-defined stereochemistry are assembled by this reaction. Heterocyclic 1,2-amino alcohols are similarly procured from dialdehyde monooxime ethers. ... 

It is worth mentioning that in [Bu GaS]6 the Ga-S interatomic distances in the six-membered rings (2.316-2.323 A) are shorter than those connecting them (2.379-2.392 A) in agreement with the view of this compound as a dimer of two authentic six-membered Gas S3 heterocycles. The facile interconversions of these cages [212] support their description as self-assembled supermolecules built from RGaS units (Scheme 3.1). 

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