Phosphopeptides deprotection

Cleavage of the phosphonate allyl esters from Abu[PO(OAl)2]-peptide-resins is accomplished by treatment with Pd(PPh3)4 (50 °C, 5 h)[103l followed by acidolytic deprotection of the Abu[PO(OH)2]-peptide from the resin support. The viability of employing allyl phosphonate protection for phosphopeptide synthesis has been demonstrated by the Fmoc solid-phase synthesis (DIC/HOBt couplings) of several 6-9-mer Abu[PO(OH)2]-containing peptides and has been extended to the synthesis of a 19-mer caldneurin-related Abu[PO(OH)2]-peptide.[104 ... 

N -Protection as allyl carbamate is used generally only for specific synthetic purposes where acid- or base-mediated deprotection procedures must be avoided or reduced to a minimal extent as in the case in the synthesis of glyco- and phosphopeptides (see Sections 6.3 and 6.5). Otherwise the orthogonality of this type of amino protection has been mainly exploited in side-chain protection to allow for selective chemistry such as selective acylations, e.g. cyclizations, to be performed on protected peptides in solution or in the resin-bound state (see Section 4.3.2.3.4). An additional interesting application of the Aloe group is the one-pot deprotection and acylation with carboxy components resistant to the paUadium(0)-catalyzed allyl cleavage by hydrostannolysis as reported in Section 4.3.2.3.4.P 1... 

Enzymatic Deprotection of Phenylacetyl-Protected Phosphopeptides General Procedure ti ... 

Lipase Aspergillus niger)-mediated saponification of heptyl esters is employed for the C-terminal deprotection of several phosphopeptides. For example this method has been used in the synthesis (Scheme 24) of a consensus sequence of the Raf- kinase. 

Enzymatic Deprotection of Phosphopeptide Heptyl Esters General Procedure ... 

The full capacity of the lipase mediated technique for C-terminal deprotection was demonstrated by the synthesis of complex base-labile phosphopeptides,441 and O-glycopeptides, which are sensitive to both acids and bases[los 107). To this end, e. g. the serine glycoside 24 was selectively deprotected at the C-terminus by lipase from the fungus Mucorjavanicus (Fig. 18-12). 

II.B.l), Otaka et al. [101,120] have proposed a two-step deprotection protocol consisting of high-acid (Sn1/Sn2) and low-acid (Sn2) reagent systems for the deprotection of 0,0-dimethyloxyphosphinyl amino acid derivatives. Using these optimized deprotection conditions, several phosphopeptides in 69-81% yields, based on the loading of the protected peptide resins, were obtained [101]. 

Iodine (1 M) in THF-2,6-dimethylpyridine-water (40 10 1 v/v/v) (25-fold molar excess) is added and the reaction vessel is mixed for 30 min. After this time, the vessel is drained and the peptide resin is washed with DMF and diethyl ether (three times each) and dried in vacuum. The peptide resin is ready for final cleavage and deprotection. As usual, the scavenger cocktail is dictated by the linker used and the protecting groups present in the sequence. The phosphopeptide is purified by ion-exchange chromatography or preparative HPLC on a reversed-phase colunm. The separation of phosphopeptide from nonphosphorylated impurities can be carried out by affinity chromatography on Fe " Chelex gel (iminodiacetic acid epoxy activated Sepharose 6B) [118]. 

Otaka A, Miyoshi K, Kaneko M, Tamamura H, Fuji N, Nomizu M, Burke TR, Roller PP (1995) Development of efficient 2-step deprotection methodology for dimethyl-protected phosphoamino acid-containing peptide resins and its application to the practical synthesis of phosphopeptides. J Org Chem 60 3967-3974... 

Figure 3 The PGA-mediated selective N-terminal deprotection of phosphopeptides.

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