Syntheses from Acetylenes and Alkenes

From among the variety of non-carbohydrate precursors, acetylenes and alkenes have found wide application as substrates for the synthesis of monosaccharides. Although introduction of more than three chiral centers having the desired, relative stereochemistry into acyclic compounds containing multiple bonds is usually difficult, the availability of such compounds, as well as the choice of methods accessible for their functionalization, make them convenient starting-substances for the synthesis. In this Section is given an outline of all of the synthetic methods that have been utilized for the conversion of acetylenic and olefinic precursors into carbohydrates. Only reactions leading from dialkenes to hexitols are omitted, as they have already been described in this Series.7 

Acetylenic precursors employed in the syntheses of sugars may be divided into three groups (a) aldehydes (usually in the form of acetals), (b) alkyl alkynyl ethers, and (c) alkynols or alkynediols. Some of them are commercially available (for example, 2-butyne-l,4-diol), and others are prepared by Grignard-type reactions between 1-alkynylmag-nesium halides or lithium alkynes and suitable aldehydes, ketones, or epoxides. In this way, the synthesis of substrates having the desired number of carbon atoms, as well as the necessary functional groups, can be achieved. The next step consists in partial saturation of the triple bond to afford the desired cis- or trans-alkene. ct.s-Alkene systems 

In only one case was the triple bond differently functionalized a terminal acetylene group was converted by hydroboration into a for-mylmethylene (OHC-CH2-) grouping. In this way, 2-deoxy-DL-erythro-pentose (2) was obtained10 from en/t/iro-4-pentyn-l,2,3-triol (1). 

For the application of alkenes as precursors in the synthesis of carbohydrates, knowledge of their stereochemistry is essential. Fortunately, several alkenic compounds having a defined configuration of the double bond and proper functional groups are readily available, making them convenient substrates in the synthesis of sugars. 

Displacement of the bromine atom in 4-bromo-4-deoxy-DL-threonic acid (7) with silver acetate gave15 DL-threonic acid (10). 

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