Chiral sugar

The attachment of chiral sugar-derived auxiliaries to the uracil unit generally induced low diastereoselectivities. Only in the reaction between diyne 582 and uracil 583 was a high syn and diastereoselectivity observed, complex 584 being obtained as the major diastereomer (equation 167). 

Borrachero et al. (180) prepared a number of sugar isoxazolidines by the reaction of carbohydrate-functionalized nitrones with nitroalkenes (Scheme 1.33). They found a matched pair of chiral sugar cycloaddition reaction partners to be... 

J. A. Serrano, L. E. Cdceres, and E. Romdn, Asymmetric Diels-Alder reactions of a chiral sugar nitroalkene, J. Chem. Soc. Perkin Trans. 1 941 (1992). 

SCHEME 10.3S Reaction of chiral sugar-nitroalkanes in [4+2] cycloadditions. 

Tamura, O, Mita, N, Kusaka, N, Suzuki, H, Sakamoto, M, Intramolecular cycloaddition of a-allyloxycarbonylnitrone bearing a chiral sugar auxiliary a short-step synthesis of the A-terminal amino acid component of nikkomycin Bz, Tetrahedron Lett., 38, 428-432, 1997. 

Serrano, J A, Caceres, L E, Roman, E, Asymmetric Diels - Alder reactions of chiral sugar nitroalkene diastereofacial selectivity and regioselectivity, J. Chem. Soc. Perkin. Trans, 1, 941-942, 1992. 

Figure 22 Induced CD of complexes, achiral host 40 and chiral sugar guests 41 CD Aext data were obtained from the...

These first cycloadditions are only complete after 72 h at 60°C. When partners are more reactive and the cycloaddition can be catalysed by Lewis acids, it is possible to work at -78°C. Under these conditions, it may be possible to observe high asymmetric induction. Reaction (8.4) of cyclopentadiene with an acrylate derivative of 5-0-trimethylsilyl-a-D-xylofuranose carried out at -78°C in dichloromethane in the presence of TiCl4 gives exclusively the endo adduct ( / , 2 R). This is explained by the formation of an intermediate complex which contains both the dienophile and Lewis acid attached to the chiral sugar inductor (Kunz etal. 1987). 

This holds true for amino acids, nucleobases, and carbohydrates. Even within one class of the biogenic molecules, two or more independent routes developed. As illustrated by the formose reaction, chiral sugar molecules formed when catalyzed by an appropriate chiral amino acid (to be discussed further below). 

The formose reaction formed chiral sugar molecules when catalyzed by an appropriate chiral amino acid under credible prebiotic conditions. Alternatively, chiral amino acids likely formed via import from meteorites with subsequent transfer of chirality to other molecules. The solid chiral nucleobase cytosine arose by mechanical stirring from solution, Evaporation of ribonucleosides and subsequent hydrolysis led to chiral ribonucleosides. 

In contrast to the chiral sugar-phosphate backbone of DNA, the amino-ethylglycine backbone of PNA does not contain stereocenters. A preferred helical handedness can be induced in PNA duplexes by the attachment of a d- or L-amino acid at the C-terminus of the duplex (34). The X-ray structures of a 6 bp PNA duplex and of a partly self-complementary ssPNA dodecamer that forms a 4 bp duplex part in the crystal show that the PNA PNA duplexes adopt a distinct p-form helix, which can be either right- or left-handed (Fig. 8) (23, 24). The p-helix has a very large helical pitch and a wider diameter than that of the DNA or RNA helices (Fig. 8 and Table 1). 

Wulff et alr attached vinyl groups to a large chiral sugar based template molecule and then copolymerized this substrate wdth various monomers. With MMA and MAN they achieved some optical induction. This approach lias been extended in studies of higher molecular weight systems. "" Thus, PVA was esterilied with methacryloyl chloride to give a "multimethacrylate" (24) and... 

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