Carbonyl compounds unsaturated carbohydrates

Camphor, 14, 139 Cannabinoid, 198 Cannabinol, 197 Carbodiimide, 266 Carbohydrate pseudoglycal, 98 p-Carbomethoxynitrosobenzene, 74 Carbon dioxide, 193 synthetic equivalent of, 101 Carbon monoxide, 193 o,/3-Carbonyl compounds, unsaturated, 169-187... 

A simple approach for the formation of 2-substituted 3,4-dihydro-2H-pyrans, which are useful precursors for natural products such as optically active carbohydrates, is the catalytic enantioselective cycloaddition reaction of a,/ -unsaturated carbonyl compounds with electron-rich alkenes. This is an inverse electron-demand cycloaddition reaction which is controlled by a dominant interaction between the LUMO of the 1-oxa-1,3-butadiene and the HOMO of the alkene (Scheme 4.2, right). This is usually a concerted non-synchronous reaction with retention of the configuration of the die-nophile and results in normally high regioselectivity, which in the presence of Lewis acids is improved and, furthermore, also increases the reaction rate. 

Similarly, the P-phosphono-a,P-unsaturated carbonyl compounds thus synthesized have been used as dienes in a Diels-Alder approach toward the preparation of analogues of myo-inositol 1,4,5-triphos-phate,45 as well as other carbohydrate-related materials.46-48... 

Photochemical Cycloaddition Reactions between Unsaturated Carbohydrate Derivatives and Carbonyl Compounds... 

An attractive entry to the carbohydrate synthesis is provided by the cycloaddition reaction. Hetero-Diels-Alder reaction, either between an oxa-diene (a,[3-unsaturated aldehyde) and an nucleophilic dienophile, or between activated diene and carbonyl compound (usually an aldehyde), leads to dihydropyrans, which can be subsequently functionalized to sugars in the desired manner (Scheme 3). 

Tributyltin hydride, 316 Tributyltinlithium, 319 Trichloroacetonitrile, 321 Other carbohydrates (Diethylamino)sulfur trifluoride, 110 Triethyloxonium tetrafluoroborate, 44 Carbonates (see also Enol carbonates) Carbon dioxide, 65 Di-/-butyl dicarbonate, 94 Carboxylic acids (see also Dicarbonyl compounds, Unsaturated carbonyl compounds)... 

Several diastereoselective HDA (hetero-Diels-Alder) reactions of a,/3-unsaturated carbonyl compounds and electron-rich alkenes have been exploited to gain carbohydrate derivatives with good diastereomeric excess. In HDA reaction, up to three chiral centers are formed with high stereoselectivity at each chiral carbon [78]. 

Lewis acid catalysis can lower the energy of LUMOdiene in heterodienes even further, as in the cycloaddition of enol ethers to a,j8-unsaturated carbonyl compounds. This reaction provides an important method for the synthesis of glucals, dehydrated carbohydrates that are important building blocks in the synthesis of polysaccharides. 

One of the most common examples of such a sequence is reductive amination of aldehydes or ketones, followed by acylation of the newly formed secondary amine (Scheme 3.6, see [26, 30, 31]). Such a sequence was also applied in the synthesis of carbohydrate-based libraries [110]. The carbonyl compounds were formed by reacting protected 1-thiol-carbohydrates with an a-halo or a,(3-unsaturated ketone. 

Carbonyl compounds are quantitatively reduced regio- and stereo-selectively by NaBH4 at room temperature in aqueous solutions containing glycosidic amphiphiles such as methyl-p-D-galactoside, dodecanoyl-p-D-maltoside, L-arabinose and sucrose [63]. a,P-Unsaturated ketones give 1,2-reduction to corresponding allylic alcohols and cyclohexanones furnish the thermodynamically more stable alcohol. The observed stereodifferentiation can be attributed to hydrophobic interactions between amphiphilic carbohydrates and lipophilic substrates. 

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