Trichloroacetimidate chemistry

TentaGel-N, linked via the succinoyl linker to a monosaccharide acceptor was utilized using trichloroacetimidate chemistry to prepare disaccharides [34] and trisaccharides [78]. It was observed that TentaGel is slightly unstable under the acidic conditions required by trichloroacetimidate chemistry [78], most likely due to the acid sensitivity of PEG [79]. No similar sensitivity was observed in similar applications employing Argogel [78]. 

This review covers the recent advances in the use of O-glycosyl imidates in oligosaccharide and glycoconjugate synthesis, with emphasis on literature published between 1999 and 2006. However, because of the large volume of work in this area, only the most representative applications will be presented. One can refer to the similar preceding review [381] published in 2000 and another quite comprehensive review [375] on trichloroacetimidate method published in 1994 for earlier application of glycosyl trichloroacetimidates. Trifluoroacetimidate method will be discussed separately in this review in the light of its less popularity in carbohydrate chemistry. 

Anhydrous hydrogen fluoride, application of for the structural analysis of polysaccharides, 47, 167-202 Anomeric and exo-anomeric effects in carbohydrate chemistry, 47, 45-123 Anomeric-oxygen activation for glycoside synthesis, trichloroacetimidate method, 50,21-123... 

After developing procedures for the activation and coupling of anomeric trichloroacetimidate and glycosyl phosphate donors as well as for the deprotection of acetyl and levulinoyl esters, we designed a synthesis utilizing all aspects of our automated chemistry. Trisaccharide 24, composed of three... 

Schmidt RR, Jung K-H. Oligosaccharide synthesis with trichloroacetimidates. In Preparative Carbohydrate Chemistry. Hanessian S, ed. 1997. Marcel Dekker, New York. pp. 283-312. 

In one of the earliest demonstrations of C-arylation chemistry, Schmidt and Holfman [109] utilized trichloroacetimidates to prepare the products shown in Scheme 7.24. In all cases, the (3 anomer was the preferred product. An interesting observation is the demonstrated ability to utilize both alkyl and silyl phenolic protecting groups. This chemistry was later expanded to include couplings with heterocyclic substrates [110,111]. 

Nicolaou s total synthesis of the enediyne antibiotic, calicheamicin 71 [19], (Scheme 12.15 to Scheme 12.19) involves several new aspects in carbohydrate chemistry. The calicheamicin 71 oligosaccharide synthesis began with the construction of the D-ring intermediate from L-rhamnose. Thus, L-rhamnose 112 was converted into the glycosyl donor, trichloroacetimidate 118, through the... 

Schmidt RR, Jung KH (1997) Oligosaccharide Synthesis via Trichloroacetimidates. In Hannes-sian S (ed) Preparative Carbohydrate Chemistry. Marcel Dekker, p 283... 

Schmidt RR, Jung KH (2000) Trichloroacetimidates. In Ernst B, Hart GW, Sinay P (eds) Carbohydrates in Chemistry and Biology, Part I Chemistry of Saccharides, vol 1. Wiley-VCH, Weinheim, p 5... 

Trichloroacetimidates function well as donors also with saccharide thiol acceptors. A number of thiodisaccharides have accordingly been prepared using this method [169,170,171,172,173], Possible by-products are various elimination products and orthoester formation. The stereochemical outcome is not easy to predict or control and, as so often in carbohydrate chemistry, an optimization of conditions is usually necessary for each glycosylation. For example, the use of an acetylated o -trichloroacetimidate glucose donor can yield not only an almost exclusively -linked disaccharide product but also preferentially an a-linked product, depending on the acceptor and conditions used ( Scheme 53) (see also Sect. 3.3). 

In amino acid chemistry r-butyl ethers are also used successfully for the protection of the serine and threonine hydroxy groups. They are formed by the addition of the alcohols to isobutene and are cleaved with TFA and HBr/AcOH. t-Butyl trichloroacetimidate is also a convenient reagent for their introduction.However, the preparation of O-f-butyl serine and threonine themselves requires several steps including methyl orp-nitrobenzyl esters as intermediates. 

R. R. Schmidt and K. H. Jung, Oligosaccharide synthesis with trichloroacetimidates, in S. Hanessian, (Ed.) Preparative Carbohydrate Chemistry, Marcel Dekker, Inc, New York, 1997,. ... 

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