SYNTHESIS OF MULTI-EPITOPIC GLYCOPEPTIDE-BASED CANCER VACCINES - Biologically-Responsive Hybrid Biomaterials - page 142

3.1.2. Synthesis of linear peptide linker bearing clustered carbohydrate antigens

While being developed for the conjugation to carrier proteins rather than as
complete immunoactive structures, the clustered peptide linkers developed by
Danishefsky have to be undeniably mentioned in this part. Denoted as the
“Cassette Approach” by Danishefsky, the concept discussed above was explored
to insert natural or unnatural amino acid functionality into carbohydrate motifs.
The resulting glycosyl amino acid building blocks were then incorporated into
peptide linkers displaying either single or clusters of carbohydrate antigens. It
has to be noted that the final conjugation of glycopeptide linkers to the protein
carrier is ensured by reductive amination or thio-maleimide chemoselective
ligation but this aspect won't be further discussed in this part.

Strategy A

O

1.

O

O O O

O CCl 3

PO O

PO

O

2.

glycal HO CO 2 H

NH

HO

ZnCl 2 NHFmoc

TMSOTf

O

1. O 3

Strategy B

P CO 2 TSE

(MeO) 2

2.

O

O CO 2 H

PO

O NHBo c

O

PO

NHFmoc

3. H 2 , asymetric catalyst

n-pentenyl glycoside

glycosylamino building block
("Ca ssette")

PCy 3

CO 2 Bn

Strategy C

Cl

Ru
PCy 3

1.

NHFmoc Cl Ph

O

O

PO

2. H 2 /Pd-C

allyl glycoside

Fig. 6. Several “Cassette” strategies developed by Danishefsky and co-workers.

The major advancements of the Cassette Approach relies on previous
development of the modular glycal approach which revealed extremely efficient
to prepare the full carbohydrate domain both in solution [27] and on solid phase
[28] before the amino acid coupling. It relies on the use of 1,2-anhydrosugars,
namely glycals, as glycosyl donors and acceptors (Figure 6, strategy A). By
treatment with dimethyldioxirane, the resulting highly reactive epoxide was
proved to be excellent Ȳ -glycosyl donor in the presence of Lewis acid catalysts
such as zinc chloride. If the reaction is performed with other suitably
differentiated glycal acceptors, the procedure can be repeated iteratively to obtain
the full carbohydrate antigen. This strategy allowed the preparation of sialylated
structures such as sialylTF epitope [29] or the first total synthesis of the Globo-H