Subject oxidative cleavage

Meinwald and coworkers71 studied the chemistry of naphtho[l, 8-bc]thiete and its S-oxides. The reaction of the sulphone 2 with LAH (equation 29) is of particular and direct relevance to this section since it is different from the reductions that have been discussed thus far, because the major reaction pathway is now cleavage of an S—C bond, rather than a deoxygenation of the sulphur atom. The major product (equation 29) was isolated in 65% yield two minor products accounted for a further 15% yield. One of the minor products is 1-methylthionaphthalene and this was most probably produced by an initial reduction of the strained 1,8-naphthosulphone, 2, to the thiete, which was then cleaved to the thiol and subsequently methylated. Meinwald also showed71 that the thiete was subject to cleavage by LAH as well as that both molecules were susceptible to attack and cleavage by other nucleophiles, notably methyllithium. These reactions are in fact very useful in attempts to assess a probable mechanism for the reduction of sulphones by LAH and this will be discussed at the end of this section. 

A convenient procedure for the lactonization of alkenols has been recently revealed by Borhan and coworkers [19]. This methodology was successfully applied in the total synthesis of (+)-tanikolide (7-37), a natural product of marine origin which exhibits antifungal activities. Thus, when alkenol 7-36 is subjected to soluble oxone and a catalytic amount of 0s04, a smooth domino oxidative cleavage/lactonization process takes place which leads, after debenzylation, to the desired product in good overall yield (Scheme 7.12). 

Reaction of nitromethane and monosaccharide-derived dialdehydes is a useful tool that has been broadly used for the preparation of nitro and amino sugars, and carbocycles.30 Dialdehydes can easily be obtained by oxidative cleavage of conveniently protected monosaccharides with sodium periodate. Their subsequent Henry reaction with a nitroalkene, commonly nitromethane, usually gives isomeric mixtures that require the isolation of the major isomer.31 Thus, treatment of the D-ribose derivative 27 with sodium periodate gave dialdehyde 28, which was subjected to a Henry reaction with nitromethane, to afford nitrosugar 29 as an epimeric mixture (Scheme 11).32... 

Oxidative cleavage of the carbon-silicon bond has been the subject of a recent and comprehensive review119, in which emphasis has been placed on the compatibility of the oxidation conditions with various functional groups, with the inclusion of very useful compatibility tables. 

Finally, two tetrahydrofurofuran lignans (the stereoisomers asarinin and sesamin, Scheme 12) have been subjected to MTO-catalyzed oxidation. In both cases, the major product originates from oxidative cleavage of one of the phenyl-tetrahydrofuran bonds yielding a lactone. In a CH2Cl2/MeCN mixture using UHP as the oxidant, 65% conversion and 87% selectivity for the lactone is obtained. With immobilized MTO/PVP-2% as catalyst in CH2Cl2/EtOH, similar results are obtained [95]. 

An advanced approach used a silyloxyethylene linker as in complex 53 (Scheme 22) [31]. Benzannulation in the presence of an excess of diphenylethyne ("xenochemical effect ) gave naphthoquinone 54, which was subjected to oxidative cleavage and demetalation to afford regioisomer 52b exclusively. The complementary intermolecular benzannulation protocol... 

Lewis acids readily isomerize both 1,3-dioxolanes and 1,3-oxathiolanes in ether solution. The reaction proceeds by coordination with the oxygen atom in the latter case since 1,3-dithiolanes do not isomerize under the same conditions. With trityl carbonium ion, an oxidative cleavage reaction takes place as shown in Scheme 6. Hydride extraction from the 4-position of 2,2-disubstituted 1,3-dioxolanes leads to an a-ketol in a preparatively useful reaction. 1,3-Oxathiolanes are reported to undergo similar cleavage but no mention of products other than regeneration of the ketone has been made (71CC861). Cationic polymerization of 1,3-dioxolane has been initiated by a wide variety of proton acids, Lewis acids and complex catalytic systems. The exact mechanism of the polymerization is still the subject of controversy, as is the structure of the polymer itself. It is unclear if polymerization... 

Conagenin (39) is a promising anticancer natural product isolated in 1991 from the culture broths of Streptomyces roseosporus20 (Scheme 4.1n). The chelation-controlled Zn(B 114)2 reduction of the p-hydroxy ketone 40 produced the 1,3-syn-diol 41 in 30 1 diastereoselectivity. Protection of the hydroxyl groups gave the bis-acetate 42, which was then subjected to oxidative cleavage to afford the acid 43. 

The details of the reaction conditions used in this study have been described elsewhere (Dershem, S. M., et al., Holzforschung Fisher, T. H., et al., J. Org. CAem., in press). To test the importance of a p-hydroxyl substituent, the kinetics of oxidation of three benzyl alcohols p-hydroxybenzyl alcohol, (1), m-hydroxybenzyl alcohol, (2), and 4-hydroxy-3-methoxybenzyl alcohol, (3), were examined under alkaline nitrobenzene oxidation conditions. Some l-(4-hydroxyphenyl)-2-(4 -substituted phenyl)ethanols, (4), were synthesized as / -l lignin model compounds and subjected to alkaline nitrobenzene oxidation at 120 °C to study substituent effects. For controls, some of these compounds were reacted with or without nitrobenzene, alkaline catalyst, or water. In an effort to determine the effects of substituents on the oxidative-cleavage reaction of 4-hydroxystilbenes (5), a series of competitive rate experiments using both nitrobenzene and copper(II) as the oxidants in 2N NaOH was performed (Dershem, S. M., et al., Holzforschung, in press). 

There are three known metabolic pathways to 3-deoxyulosonic acids. In the first, aldonic acids, formed by oxidation of aldopyranoses to the corresponding lactones, are dehydrated to the ulosonic acids (see Fig. 6). Thus, D-arabinose is converted by way of D-arabinonic acid to 3-deoxy-o-ffZj/cero-pentulosonic acid. The latter is then subjected to oxidative cleavage, yielding pyruvic and glycolic acids. L-Arabinose is oxidized and dehydrated to the 3-deoxy-L-g Z2/cero-pentulosonic acid, which is further oxidized to... 

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