Chirons carbohydrates

A Syntheses Using Carbohydrates as Chiral Educts ( Chirons )... 

The regio- and stereo-selective functionalization of aldonolactones yields optically active lactones, which are important precursors in natural product synthesis. Concepts such as chiral templates and chirons, derived from carbohydrates, have been ingeniously and widely applied in synthesis (233). Among the commercially available aldonolactones, D-ribono-1,4-lactone is... 

The stereoselective synthesis of carbohydrates from acyclic precursors is a research topic that has attracted considerable attention over the past decadeT Efforts in this area are easily justified and have maximum impact particularly when directed toward rare sugars or other polyhydroxylated molecules that are not conveniently accessed via classical "chiron" approaches.2 An underlying theme of such efforts, of course, is the development of practical synthetic methodology that will find broad application in the enantio- and diastereoselective synthesis of natural products, their analogues, and other compounds of biological interest. 

A broader use of carbohydrates in this context has been treated by Hanessian in The Chiron Approach [9] and a recent book entitled Carbohydrate Building Blocks has just appeared [10]. 

Tor books on the synthesis of optically active compounds starting from natural products, see Coppola. Schuster Asymmetric Synthesis Wiley New York. 1987 (amino acids as starting compounds) Hanessian Total Synthesis of Natural Products The Chiron Approach-, Pergamon Elmsford. NY, 1983 (mostly carbohydrates as starting compounds). For reviews, see Jurczak Pikul Bauer Tetrahedron 1986, 42, 447-488 Hanessian Aidrichimica Acta 1989, 22, 3-15 Jurczak Gol biowski Chem. Rev. 1989, 89, 149-164. 

For reviews see (a) R. J. Fenier and S. Middleton, The conversion of carbohydrates into functionalized cyclopentanes and cyclohexanes, Chem. Rev. 95 2779 (1993) (b) K. J. Hale, Monosaccharides Use in asymmetric synthesis of natural products, Rodd s Chemistry of Carbon Compounds. Second Supplement to the 2nd ed. (M. Sainsbury, ed.), Elsevier, Amsterdam, 1993, Vol. 1E/F/G, pp. 315-435 (c) T. D. Inch, Formation of convenient chiral intermediates from carbohydrates and their use in synthesis, Tetrahedron 40 3161 (1984) (d) S. Hanessian, Total Synthesis of Natural Products The Chiron Approach Pergamon, New York, 1983. 

Corey (E.J Corey Pure Appl Chem 1969,14, 30) introduced the term synthon in 1969 when he published his innovative strategies for the construction of complex molecules by considering a retrosynthetic analysis. Later on, Hanessian s (Total Synthesis of Natural Products The Chiron Approach Pergamon Press, 1983) introduction in 1983 of the term Chiron referring to chiral synthons became the general strategy of carbohydrate like symmetry in new molecular targets of many natural products. 

The obvious approach for chiral synthesis would be to find a chiral starting material, such as a natural amino acid, carbohydrates, carboxylic acids or terpene. The major source of these chiral starting materials sometimes called chirons is nature itself. The synthesis of a complex enantiopure chemical compound from a readily available enantiopure substance such as natural amino acids is known as chiral pool synthesis. For example, chiral lithium amides 1.39 that are used for several types of enantioselective asymmetric syntheses can be prepared in both enantiomeric forms starting from the corresponding optically active amino acids, and these are often available commercially. 

This approach has been rapidly put into new concepts and reviewed. The Chiron approach concept was introduced by Hanessian in a review [4] and, in more detail, in his book [15]. The carbohydrate is considered as a chiral synthon, which has to be found retrosyntheticaUy in a... 

In another synthesis, Gallagher described a fruitful approach based on the use of a carbohydrate enolate, which takes advantage of the presence of the C7 keto group in the target compound [265-267]. In this approach, an enolate had to be formed at the anomeric position of the gluco chiron, stable toward (3-elimination. Moreover, the use of a strained system, as in 357, should force enolization to occur on the correct side. 

Like the AE, the AD has obvious utility to those contemplating the synthesis or use of carbohydrates as synthons. For one, glyceraldehyde and its acetonides have found very wide acceptance as chirons for asymmetric synthesis [85]. A clever utilization of the AD affords a building block that nicely complements the use of the naturally occurring material (Scheme 8.18) [86]. 

On the occasion of complex natural products synthesis, choice of the starting material(s) becomes a key for success of the aim in many cases. Along with well-designed retrosynthetic analysis and sophisticated synthetic schemes, availability of the starting materials, desirably in enantiomerically pure or enriched form, is an indispensable element. During these fifteen years we have extensively explored the stereoselective introduction of synthetically versatile carbon functionalities on the carbohydrate skeletons in hope of novel access to a variety of enantiomerically pure building blocks (so-called chirons ) [1-3]. For instance, we reported in 1985 that the thermal Claisen... 

K. V. Booth, F. R. da Cruz, D. J. Hotchkiss, S. F. Jenkinson, N. A. Jones, A. C. Weymouth-Wilson, R. Clarkson, T. Heinz, and G. W. J. Fleet, Carbon-branched carbohydrate chirons Practical access to both enantiomers of 2-C-methyl-ribono-1,4-lactone and 2-C-methyl-arabinonolactone, Tetrahedron Asymmetry, 19 (2008) 2417-2424. 

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