Sugars alkenic precursors

Triazoline imino sugar derivatives 297 that are prospective glycosidase inhibitors have been prepared as single diastereomers in high yield via an lAOC reaction of in situ generated azido alkene 296 (Eq. 32) [78]. m-CPBA oxidation of the dithioacetal groups in the 0-acetylated 5-azido-5-deoxydibenzyl dithio-acetal of o-xylose or D-ribose 294 to the bis-sulfone 295, followed by loss of HOAc between C-1 and C-2 provided the lAOC precursor 296. 

The conversion of nitroalkanes to ketoximes can be achieved by the reduction with Zn in acetic acid,112 or Fe in acetic acid.113 Nitroalkenes are direcdy reduced into saturated ketoximes by these reagents, which are precursors for ketones (see Section 6.1.4 Nef reaction). Reduction of 3-O-ace-ty lated sugar 1 -nitro-1 -alkenes with Zn in acetic acid gives the corresponding 2,3-unsaturated sugar oximes in high yield, which is a versatile route to 2,3-unsaturated sugar derivatives (Eq. 6.58).114... 

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... 

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. 

The main interest in unsaturated sugars prepared by the Wittig reaction (and described in this Section) has concerned their synthetic utilization. The pathways of the latter depend on the structure of the unsaturated precursor. In the case of C-glycosylated alkenes, addition to the double bond (mainly hydration and hydrogenation) leads to a branched-chain or long-chain sugar, although correct choice of the reactant to be added may provide a variety of derivatives. 

The synthesis of lincosamine employs a slightly different strategy than that used for NeuAc and KDO. By using aldehyde (130) a precursor to the sugar side chain was installed directly in the cyclocondensation reaction (Scheme 37). Reduction of the carbonyl group of pyrone (131) under Luche conditions, epoxidation of the glycal, and a series of manipulations on the alkene side chain to install the amino alcohol portion gives lincosamine (132). 

Aldono-1,4-lactone iV-tosyl- or iV-naphthalene-2-sulfonylhydrazone derivatives such as 77 were prepared from the corresponding free sugars (e.g. 76), and shown to act as glycofuranosylidene carbene precursors on deprotonation and photolysis (Scheme 14). The carbenes could be trapped with alkenes to give adducts such as 78, or phenols to give glycosides e.g. 79. ... 

Schmidt and coworkers, on the other hand, have developed an approach to C-linked 3-disaccharides employing C4-branched carbanions which can be prepared by lithium-halogen exchange as exemplified with the 1,6-anhydro sugar 85 in Scheme 20 [45]. This precursor was prepared via a stereoselective hydroboration-oxidation of the exocyclic alkene in 84 obtained in two steps from alcohol 83. Addition of the anion generated from 85 to gluconolactone 3 gave a hemiketal which was stereoselectively reduced to the C-disaccharide 86. Further standard manipulations provided the octaacetate derivative of P-D-Glc(l-4)-D-Gal 87. The per-acetylated derivative of 3-D-Gal(l-4)-D-Gal 88 was prepared in a similar manner. 

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