Protecting coupling efficiency

In a darkened lab, dissolve FITC (Thermo Fisher) in dry DMSO at a concentration of 1 mg/ml. Do not use old FITC, as breakdown of the isothiocyanate group over time may decrease coupling efficiency. Protect from light by wrapping in aluminum foil or using amber vials. 

To decide whether to use Ddz or Trt protection, the following considerations apply In general, Ddz-protected derivatives couple more efficiently that the corresponding Trt derivatives. Thus, Trt-Gly-OH and Trt-Ala-OH couple very well, but more sterically crowded amino acids with Trt protection couple slowly and Ddz is preferred. However, because Ddz removal conditions require a somewhat higher acid concentration, low-level premature cleavage (1-3%) of dipeptide from the resin can occur as a side reaction. 

Postsynthetic intramolecular alkylation of a cysteine thiol group with a bromo amino acid has also been proposed/431 but this approach is limited by the low coupling efficiency of the bromo amino acid in the peptide synthesis due to a competing intramolecular cyclization reaction. A promising new approach consists of using suitably protected cysteine and homoserine derivatives for the peptide synthesis, whereby the side-chain hydroxy group of homoserine is protected as the TBDMS derivative which is converted on resin into the chloro derivative with freshly prepared triphenylphosphine dichloride. Upon cleavage from the resin, cyclization is performed in solution as shown in Scheme 11144]... 

The N-Dde protected Laas can be coupled to a growing peptide on Rink amide resin using standard coupling techniques. When the desired coupling efficiency is reached the protecting group is removed by treatment with 2% hydrazine hydrate in DMF (three washes of 5 min) followed by efficient rinsing of the resin with DMF as usual. 

Condensation of the D-galactopyranosyl-derived phosphonium salt 60 and aldehyde 61 provides the unsaturated dimer 62 after desilylation and oxidation. Iteration of the sequence twice with phosphonium salt 60 leads to tetramer 63 with, however, low coupling efficiencies (36% for the trimer and 11% for the tetramer) mostly due to the base-induced a,/3-unsaturation of the aldehydic substrates. Protecting group removal and hydrogenation of the carbon-carbon double bonds furnish tetramer 64, a C-mimetic of the /3-(1 6) tetragalactoside similar to the one shown in O Scheme 3. 

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