Branched-chain sugars oxiranes

The racemic form of methyl 3,4-anhydro-/3-mycaroside (82) has been synthesized 35 it is an epoxide of a branched-chain sugar having the oxirane ring fused to the pyranose ring. Hydrolysis of the oxirane ring with hydrochloric acid in aqueous p-dioxane occurs with substitution at C-4, the (more accessible) secondary position, as would be expected from its favored °H, conformation (82) anticipated, because it is an analog of the 3,4-anhydro-D-allopyranoside derivatives considered previously (see p. 148). 

With the spiro-epoxides of branched-chain sugars, the oxirane protons appear somewhat farther upfield, between r 6.8 and 7.5, the two protons showing a variable difference in chemical shift.224,227... 

Epoxy sugars are frequently used as starting compounds in the synthesis of sugar derivatives (compare Section IV) such as halo, amino, azido, thio, deoxy, and branched-chain derivatives. The oxirane ring is in general more reactive than the oxetane or oxolane ring. It is opened with nucleophiles under base or acid catalysis. On the other hand, the oxirane ring remains unattacked under the conditions of catalytic debenzylation on palladium,... 

J. Yoshimura, N. Kawauchi, T. Yasumori, K. Sato, and H. Hashimoto, Branched-chain sugars. 37. Synthesis of 2,3-anhydro-hexopyranosides and 3,4-anhydro-hexopyranosides having a methyl branch on the oxirane ring, and their reactions with some lithium methylcuprate reagents, Carbohydr. Res., 133 (1984) 255-274. 

The diol 59 was used for an ingenious synthesis of L-mycarose (62, 2,6-dideoxy-3-C-methyl-L-n bo-hexose) and olivomycose (63, 2,6-dideoxy-3-C-methyl-L-ara /no-hexose). The sequence of reactions consisted in 1 epoxidation of the double bond and separation of both stereoisomeric epoxides formed, 2° reduction of the oxirane ring to form two triols 60 and 61, and 3° ozonolytic degradation of the aromatic ring to a carboxyl group. Each trihydroxyacid was converted into a lactone and the lactones were eventually reduced to the required branched-chain sugars 62 and 63. 

Oxirans - The use of pyranose epoxides in the preparation of sugar amino acids and peptides, aminodeoxy, halodeoxy, branched-chain, cyclopropanated and aziridino sugars has been reviewed. ... 

The most common methods for the synthesis of branched-chain sugars involve the reduction of aUcyUdene glycosides, opening of the sugar oxiranes by carbon nucleophiles, and the additicm of organometallic reagents to glycopyranosiduloses [11]. 

The preferential reaction of benzylidene acetals in compounds also containing oxiran units is mentioned in Chapter 5. Other hydroxy branched-chain sugars are mentioned below as hydration products of unsaturated analogues. 

Oxirans.—A convenient synthesis of methyl 2,3-anhydro-)8-D-ribofuranoside is shown in Scheme Several oxirans of branched-chain sugars have been... 

Branched-Chain Alditols. - Reaction of sugar oxiran derivatives with excess of trimethylaluminium yield branched-chain deoxy-alditol derivatives, e.g.. (8)—>(9), in an unusual ring opening, chain extension process. The conversion of the benzyl glycoside (10) to... 

Oxirans derived from branched-chain sugars are mentioned in Chapter 4, while the m.s. of some 2,6-dideoxy-3-C-methylhexopyranose derivatives is referred to in Chapter 24. 

Lukac s group use D-galactose to prepare the branched-chain sugar (26), introducing the branch methyl groups using lithium dimethyl cuprate on oxiran intermediates, followed by 3-deoxygenation via the xanthate ester. 

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