80.8 C, low, good for casting

In a standard coupling procedure the HOBt ester is generated by the reaction between the protected amino acid and HOBt. The reaction is mediated by DCC or DIC.

Chemically and thermally stable

  Low sensitivity to impact and friction

1863 TNT first prepared by a German named Wilbrand.

Capping is realized through a short treatment of the peptide resin with a large excess of a highly reactive unhindered acid derivative and abase, usually acetic anhydride or benzoyl chloride and pyridine. At the end of the capping step the reagents are filtered off and the resin is carefully washed before proceeding to the next deprotection step.

1891 Manufacture of TNT started in Germany

For that purpose a deficient amount of the C-terminal or penultimate Fmoc amino acid is coupled to the unloaded or preloaded resin. The resulting load is determined and when the desired level of substitution has been reached the remaining free amino groups are blocked by acetylation.

Then the mono-nitroluene is nitrated again to form Dinitrotoluene.

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Also small scale manual SPPS as well as multiple peptide synthesis and the synthesis of pep- tide libraries can be performed very rapidly and conveniently with preformed active esters. Fmoc-AA-ONp and Fmoc-AA-OSu have found only restricted application in SPPS.

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CAUTION: DCC is an aggressive allergen. Inhalation and contact must be avoided; adequate protection must be worn when working with DCC and efficient ventilation is also required!

However the activation by carbodiimides presents several drawbacks:

Fmoc amino acid chlorides can also be used but their applicability is more limited [33]; also acid-sensitive groups may not be present when treating Fmoc amino acids with oxalyl chloride or thionyl chloride to obtain the acid chlorides [34,35].

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A recent publication has described the in situ preparation of Fmoc amino acid chlorides by reaction with bis(trichloromethyl) carbonate and their use in difficult couplings [36].

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For the coupling of very bulky amino acids such as N-alkylamino acids or α-dialkylamino acids, the moderately reactive TBTU should be replaced by more potent reagents such as HATU [16], TATU, or PyBOP [25].

RECOMMENDED STANDARD PROCEDURE

It is necessary to carry out the preactivation step when working with uronium derivatives such as TBTU or TATU as they can react with the free amino group of the peptide-resin to yield substituted guanidines [15].

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The preactivation time must be kept at a minimum when generating the Fmoc-Arg(Pmc,Pbf)-OBt derivative as the activated Arg derivatives may cyclize yielding an unreactive lactam.