Carbohydrates, 2,3-anhydro rearranging

A simple and efficient method for rearranging 2,3-anhydro carbohydrates to unsaturated sugars under mild conditions has been reported (Scheme 127).191 High stereoselectivity provides a method for the preparation of unsaturated carbohydrate derivatives. 

L. Hoffmeyer, S. Jacobsen, O. Mols, and C. Pedersen, Dioxolanylium ions derived from carbohydrates. V. Rearrangement of derivatives of 1,6-anhydro-P-D-glycopyranoses and their reaction with nucleophiles, Acta Chem. Scand, B33 (1979) 175-186. 

A further example of the complexity of the structures of derivatives of hydroxy-carbonyl carbohydrates is provided by l,6-anhydro-j3-D-crabmo-hexopyranos-3-ulose (8) which exists as the dimer (9) (see Vol. 5, p. 107), but gives the iso-propylidene compounds (10) (with a rearranged structure) as well as the diacetal... 

In continuing their studies on the rearrangements of acyloxonium ions in carbohydrate chemistry, Paulsen s group has examined the ions formed from 2,3,4-tri-O-acetyl-l,5-anhydro-D-arabinitol. The acetoxonium ion (110) can undergo a 1,2-rearrangement to give the chiral ion (111) or a competitive 1,3-rearrangement... 

Oxirans. - Treatment of methyl 4,6-0-benzylidene-a-D-glucopyranoside with a mixture of diethyl carbonate and potassium carbonate in DMF has afforded directly a new simple synthesis of methyl 2,3-anhydro-4,6-0-benzylidene-a-D-allopyranoside. The corresponding 2,3-cyclic carbonate is postulated as an intomediate. Some carbohydrate examples have been published of the use of sodium hydride and p-toluenesulfonyl chloride for the conversion of 1,2-diols to epoxides. Simple routes to the Cemy epoxides 6 and 7 finom D-glucal have been develc red (Scheme 1). In this scheme the flrst step involves a dehydration elimination with allylic rearrangement Syntheses of methyl 2,3-anhydro-a- and L-OTthiofuranoside have been reported. Some nitro-sugar epoxides are mentioned in Chapter 10. 

---