Synthesis and use of amino acid fluorides as peptide coupling reagents

A process for preparing a protected amino acid fluoride of the formula:

• ##STR48## or the hydrogen fluoride salts thereof wherein BLK is an N-amino protecting group;
• AA is an amino acid residue;
• and
• X is a--or a protecting group,
• comprising reacting a protected amino acid with a fluoroformamidinium salt of the formula ##STR49## wherein R.sub.15, R.sub.16, R.sub.17 and R.sub.18 are independently lower alkyl, aryl, aryl lower alkyl, cycloalkyl, cycloalkyl lower alkyl or
• R.sub.15 and R.sub.16, taken together with the nitrogen atom to which they are attached form a 5- or 6- membered ring containing a nitrogen ring atom and 4 or 5 carbon ring atoms and up to a total of 10 carbon atoms, or
• R.sub.17 and R.sub.18, taken together with the nitrogen atom to which they are attached form a 5- or 6- membered ring containing a nitrogen ring atom and 4-5 ring carbon atoms and up to a total of 10 carbon atoms or
• R.sub.16 and R.sub.17 and taken together with the nitrogen atoms to which they are attached and the carbon atom attached to said nitrogen atoms form a 5- or 6- membered ring containing 2 nitrogen ring atoms and 3-4 ring carbon atoms and up to a total of 10 carbon atoms and A is a counterion.