Acyloxonium Ions and Orthoesters

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 

2-Orthoacetates have been obtained in good yields from 2,3,4,6-tetra-O-acetyl-a-D-glucopyranosyl bromide and alcohols (e.g, cholesterol) in THF in the presence of silver salicylate. The presence of an intermediate oxonium ion (116), which loses THF in forming the acetoxonium ion intermediate, was inferred from 

Peracylated 1,2-thio-orthoesters derived from a-n-glucopyranose [e.g. (121)], methyl o-glucopyranuronate, and a-lactose gave the l,2-(ethyl or isopropyl orthoester) derivatives when treated with Raney nickel in ethanol or propan-2-ol, respectively, although complex alcohols e.g. t-butanol) failed to yield ortho- 

The use of 1,2-thio-orthoacetates as glycosylating agents and the isomerization of (121), for example, into the l,2-trans-l-thioglycopyranoside are mentioned in Chapter 3. 

Syntheses of 1,2-orthoesters of 4,6-0-benzylidene-a-D-galactopyranose have been reported. 1,2-Orthoesters of D-glucopyranose and several steroid or triterpenoid alcohols have been prepared via acetylated glycosyl nitrates e.g, 3,4,6-tri-O-acetyl-a-D-glucopyranose l,2-(cholesteryl orthoacetate) was obtained when a mixture of cholesterol, 2,3,4,6-tetra-O-acetyl-a-D-glucopyranosyl bromide, 

2- orthoesters of steroid and triterpenoid alcohols were also obtained from 

Examples of the use of 1,2-orthoesters in the synthesis of glycosides, glycans, acetals, and anhydrides are noted in Chapters 3 and 5. 

Yamakawa, Bull. Chem. Soc. Japan, 1976, 49, 1335. 

Steroidal orthoesters (72) have been prepared in good yield from the l,2-(t-butyl orthoester) (73), which also reacted with carboxylic acids in 

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There are several ways to prepare cyclic acyloxonium ions. We will only discuss the reaction of antimony pentachloiide with vicinal diacetates and vicinal chloro- or bromo-acetates because this technique has allowed a most interesting reaction in carbohydrate chemistry (reaction 7.9). The acyloxonium cation can react in two ways the so-called kinetic reaction takes place at the most charged site as, for example with H2O and OH, to give the unstable orthoester 7.20 which opens to the cis-hydroxyl ester 7.21. The so-called thermodynamic reaction, with acetate in an acidic medium, choride, and bromide leads to the formation of the trans product 7.22 (X = AcO, Cl, Br). 

In a detailed study of the hydrolysis of a-D-gluco- and P-D-manno-pyranose 1,2-orthoesters, various intermediates such as hemiorthoesters, tricyclic 1,2,6-orthoesters, and acyloxonium ions have been detected. Unusual regiochemistry was observed in the opening of a-D-xylopyranose 1,2-ethylorthoacetates. Whereas the protected derivative (79) gave the expected 2-hydroxy-1-acetate on exposure to aqueous acedc acid, the more basic 0-2 being preferentially protonated, its... 

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