Homochiral side chains

Van Nostrum and co-workers first published evidence for a chiral superstructure in the discotic mesophase of chiral phthalocyanines in 1993.44 Homochiral phthalocyanine 43 was compared with its racemic analog 44. Whereas 44 showed a normal Colh<) phase and did not form helical columns, the homochiral side chains of 43 gave rise to a helical packing of the discotics... 

Peptoids based on a-chiral aliphatic side chains can form stable helices as well [43]. A crystal of a pentameric peptoid homooligomer composed of homochiral N-(1-cyclohexylethyl)glycine residues was grown by slow evaporation from methanol solution, and its structure determined by X-ray crystallographic methods. In the crystalline state, this pentamer adopts a helical conformation with repeating cis-... 

A second example of homochiral columns formed by discotics are the complexes of tetrazoles (59 and 60) with l,3,5-tris(4,5-dihydroimidazol-2-yl)benzene (61).74 Four molecules self-assemble to give a supramolecular disc and these discs subsequently form columns in nonpolar solvents. Chiral discs were obtained from the self-assembly of the chiral tetrazole (60) with 61. The chirality of the side chains was found to induce a bias in the helic-ity of the supramolecular assembly. Sergeants-and-soldiers measurements75 were performed for which chiral (60) and achiral (59) molecules were mixed. The experiments showed no amplification of chirality, thus revealing that in these systems chirality transfer from the side chains into the column is... 

Compound 72 was shown to display enantioselectivity in the extraction of chiral potassium salts from water into the organic phase.105 The supramolec-ular polymer possesses a homochiral helical architecture onto which one of the anionic enantiomers preferentially binds. Intriguingly, for some of the anions the octamer and polymer showed opposite selectivity, illustrating the difference in supramolecular chirality of the two systems. Furthermore, the polymer was capable of inducing a Cotton effect in the achiral compound potassium A-(2,4-dinitrophenyl)glycinate. Since the apolar side chains would... 

The concept (see Scheme 15) was introduced by Shue et al.,144-85-861 who used a zinc-induced reductive isomerization of diasteromeric allylic bromides which already contained the R2 side chain. Because of the lack of stereocontrol at the a-carbon, and of difficulties in obtaining the allylic bromide, this method has not been further developed instead, homochiral allylic acetates or mesylates have been used. 

The [2,3]- or [3,3]-sigmatropic rearrangements (Scheme 24) provide a means to introduce either the protected amine or the carbon atom which will become the carboxylic acid, while also positioning the double bond in the correct position for the alkene isosteres. Moreover, when starting from homochiral allyl alcohols, a very effective chirality transfer assures the stereospecific construction of the R1 and R2 side-chain stereochemistries. 

The first example of an enantioselective thiadiene cycloaddition involved the reaction of 2,-4-diphenyl- 1-thiabuta-1,3-diene with l-propenoyl-l,3-oxazolidin-2-one. Stoichiometric quantities of a copper triflate bis-imine complex catalyst 428 and 4 A molecular sieves are necessary to achieve the highest enantioselectivity and the best endojexo ratio. The absolute configuration of the major endo isomer was determined by reduction of the acyloxazolidine side chain to the known (3/ ,4/ )-5-hydroxymethyl derivative (Scheme 137) <1997J(P1)2957>. The process is improved using a homochiral Cu triflate or Ni perchlorate bis(oxazoline) complex when catalytic amounts are adequate for a range of thiabutadienes <1999CC1001>. 

The vitamin E side chain can be prepared stereoselectively by addition to an homochiral vinyl lactone (equation 62). The product is the result of selective anti addition. 

With the easy availability of many of the natural amino acids, some general methods for the synthesis of more complex structures are based on the modification of simple natural amino acids. An important benefit from this approach is the fact that homochirality at the a-carbon atom can be preserved in reactions at side-chains that are in current use. 

The use of homochiral monomers in polymerization leads to optically active polymers with chirality centers in the side chains. Additional chirality may appear in the polymer main chain if appropriate monomers are used or helical structures of the chain are built up. 

Lee D, Kim M-J (1998) Lipase-Catalyzed Transesterification as a Practical Route to Homochiral syn-l,2-Diols. The Synthesis of the Taxol Side Chain. Tetrahedron Lett 39 2163... 

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