Patent ID: 8153815

Claim:
A process for the manufacture of a compound of formula wherein R 1 is a side chain of an alpha amino acid, R 2 is an amino protecting group and R 3 and R 4 are independently selected from hydrogen or C 1-4 -alkyl, with the proviso that not both R 3 and R 4 are hydrogen, R 5 is hydrogen or methyl, comprising a) converting an amino acid derivative of the formula wherein R 1 and R 2 are as above, with serine or threonine into the dipeptide of formula using a water soluble carbodiimide as activating agent, without isolating or crystallizing the dipeptide of formula III and b) effecting the ring closure of the dipeptide of formula III with a compound of formula wherein R 3 and R 4 are independently selected from hydrogen or C 1-4 -alkyl, with the proviso that not both R 3 and R 4 are hydrogen, and R 9a and R 9b independently is C 1-4 -alkyl, in the presence of an acidic catalyst.