Orthoester synthesis

A reaction which is analogous to the orthoester synthesis but which utilises glycosyl oxazolines affords means of synthesising, 2-trans-2 amino-2-deoxy-glycoside derivatives. Treatment of the gluco compomd (17) with phenol in the presence of toluene- -sulphonic acid thus gave the glycoside (18), and it was also utilised in a nucleoside synthesis 48). 

O-Acetyl-1,6-anhydro-3,4-di-0-benzyl-/f-D-glucopyranose results from the reaction of 3,4,6-tri-0-benzyl-l,2-0-(l-ethoxyethylidene)-a-D-glucopyranose with mercuric chloride in nitromethane, and it may be formed as a by-product in the orthoester synthesis of oligosaccharides.318... 

In general, it is possible to obtain higher degrees of polymerization by ring-opening polymerization than with polycondensation. Two types are used in oligo and polysaccharide syntheses orthoester synthesis and cationic anhydrosugar polymerization. 

In the orthoester synthesis, using HgBr2 as catalyst, a cyclic orthoester with no free hydroxyl groups is converted into products with degrees of polymerization of up to 50 and yields of up to 50% ... 

From a historic perspective glycosyl chlorides and bromides introduced, respectively, by Michael [2] and Koenigs and Knorr [3] were the most widely used donors in the saccharide synthesis for a very long time. The introduction of 1,2-orthoesters in 1964 [4] was the first important attempt to find an alternative to the Koenigs-Knorr method. However, Paulsen in his 1990 review of reliable donors for glycosyl... 

Scheme 4.22 Asymmetric synthesis of allene 89 by the orthoester Claisen rearrangement.

---