Cycloaddition, photochemical, carbohydrate

The most important photochemical reaction of carbon to carbon unsaturated carbohydrates is addition to the unsaturated system. Two types of addition reaction are readily recognized. The first consists of those in which the molecule adding to the carbohydrate does so by involving a 77-bond of its own. Processes of this type, listed in Table I, are those which lead to formation of a new ring-system (cycloaddition). The second class of addition reaction is one in which a cr-bond is broken in the molecule adding to the unsaturated carbohydrate. The reactions that belong to the latter category (see Tables II and III) follow two basic patterns, and comprise the majority of the addition processes reported. 

Cycloaddition reactions (see Table I) involving unsaturated carbohydrates are regio- and stereo-selective. These selectivities can be understood by assuming that the photochemical interaction between the two 7r-systems results in formation of the more stable 1,4-diradical. The reaction between 3,4,6-tri-O-acetyl-D-glucal (1) and acetone pro-... 

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

The relative proportions of unsaturated carbohydrate, sensitizer (usually acetone), and solvent may have a decided effect upon a photochemical addition reaction, as at least three competing processes (cycloaddition, radical addition, and energy transfer) are possible. The irradiation of 1 in the presence of 2-propanol and acetone provides an illustration (see Scheme 4). When a small proportion of sensitizer... 

Several other types of photochemical reactions involving unsaturated carbohydrates have been reported. One of these is38 photochemical, E -Z isomerization of the groups attached to a double bond (see Scheme 5). A second is the internal cycloaddition between two double bonds connected by a carbohydrate chain.39-41 Although the carbohydrate portion of the molecule is not directly involved in this cycloaddition, its presence induces optical activity in the cyclobutane derivatives produced photochemically. Finally, a group of acid-catalyzed addition-reactions has been observed for which the catalyst appears to arise from photochemical decomposition of a noncarbohydrate reactant.42-44... 

The double bond in glycals such as (35) contribute two atoms in a photochemical [4 + 2] cycloaddition reaction with dibenzyl azodicarboxylate (Equation (9)). The adducts so formed are useful intermediates in the preparation of carbohydrates with an amino function at the C-2 position... 

Photochemical cycloadditions have been among the least widely studied cycloaddition reactions of carbohydrate derivatives, however, some noteworthy examples have been described in the earlier as well as more recent lierature.. Photoannulations of pyranoid enones with both vinyl acetate and ethylene have been described by Fraser-Reid, Hicks, and Primeau 49). Cycloaddition of vinyl acetate to 86 gave the cyclobutano pyranoside 87 as a mixture of diastereomers. This reaction was explored as an approach to the asymmetric synthesis of the insect pheromone grandisol. 

In addition to the bicyclic products formed by photochemical (2 + 2) cycloadditions between tri-O-acetyl-D-glucal and an acetone-propan-2-ol mixture Carbohydrate Res., 1973, 29, 459), compound (46) and minor proportions of (47) and (48) have now been shown to be produced [(46) presumably arises by hydrolysis of the main bicyclic adduct]. On acetylation of compound (46) the secondary hydroxy-group was substituted specifically. The INDOR n.m.r. technique was used in the product analysis. ... 

Araki, Y., Nagasawa, J.-l. and Ishido, Y, Photochemical cycloaddition of l,3-diacetoxy-2-propanone to (trimethylsilyloxy)ethylene. Carbohydrate Res., 91,77, 1981. 

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