A process for the preparation of a compound of formula XI or a salt thereof comprising the steps of: (i) contacting a compound of formula VI or a salt thereof with a compound of formula VII or VII' to obtain a compound of formula VIII or VIII', respectively (ii) optionally, if a compound of formula VIII' is obtained, reducing the compound of formula VIII' to a compound of formula VIII, (iii) converting a compound of formula VIII by nitrosation to a compound of formula IX or a compound of formula IX' and (iv) coupling the compound of formula IX or IX' with a compound of formula X or a salt thereof in a presence of a base to provide a compound of formula XI or a salt thereof, wherein the compound of formula VI or a salt thereof is prepared by comprising the steps of either providing a compound of formula III or a salt thereof wherein R 1 is Boc, Cbz, TFA, Tr, TCA, CHO, Ac, Bz, Fmoc, C 4 -C 5 - tert -alkyl, or mono, di- or triphenyl substituted methyl, and R 2 is Bn, t -Bu, TBDMS, MOM, Tr, Ac or Bz; directly converting a compound of formula III to a compound of formula VI, and optionally converting a compound of formula VI to a salt thereof; or (0-1') providing a compound of formula III or a salt thereof wherein R 1 and R 2 is as defined above, (0-2') converting a compound of formula III to a compound of formula IV wherein R 1 ' is hydrogen, Boc, Cbz, TFA, Tr, TCA, CHO, Ac, Bz, Fmoc, C 4 -C 5 - tert -alkyl, or mono, di- or triphenyl substituted methyl and R 2 is as defined above, and (0-3') converting a compound of formula IV to a compound of formula VI, (0-4') optionally converting a compound of formula VI to a salt thereof; or (0-1") providing a compound of formula III or a salt thereof wherein R 1 and R 2 is as defined above, (0-2") converting a compound of formula III to a compound of formula V wherein R 2 is as defined above, and (0-3") converting a compound of formula V to a compound of formula VI, (0-4") optionally converting a compound of formula VI to a salt thereof; or (0-1"') providing a compound of formula J wherein R 1 " is Boc, Cbz, TFA, TCA, CHO, Ac, Bz, Fmoc, (0-2'") reacting the compound of formula J with the compound of formula Z wherein R 2 is Bn, t -Bu, TBDMS, MOM, Tr, Ac or Bz, to yield a compound of formula K wherein R 1 " and R 2 are as defined above, (0-3"') oxidising the compound of formula K to obtain a compound of formula L wherein R 1 " and R 2 are as defined above, (0-4'") converting the compound of formula L to the compound of formula VI, and (0-5'") optionally converting a compound of formula VI to a salt thereof. or (0-1"") providing a compound of formula AMALA (0-2"") directly converting a compound by acid hydrolysis from AMALA to a compound of formula VI (i) contacting a compound of formula VI or a salt thereof with a compound of formula VII or VII' to obtain a compound of formula VIII or VIII', respectively (ii) optionally, if a compound of formula VIII' is obtained, reducing the compound of formula VIII' to a compound of formula VIII, (iii) converting a compound of formula VIII by nitrosation to a compound of formula IX or a compound of formula IX' and (iv) coupling the compound of formula IX or IX' with a compound of formula X or a salt thereof in a presence of a base to provide a compound of formula XI or a salt thereof, providing a compound of formula III or a salt thereof wherein R 1 is Boc, Cbz, TFA, Tr, TCA, CHO, Ac, Bz, Fmoc, C 4 -C 5 - tert -alkyl, or mono, di- or triphenyl substituted methyl, and R 2 is Bn, t -Bu, TBDMS, MOM, Tr, Ac or Bz; directly converting a compound of formula III to a compound of formula VI, and optionally converting a compound of formula VI to a salt thereof; (0-1') providing a compound of formula III or a salt thereof wherein R 1 and R 2 is as defined above, (0-2') converting a compound of formula III to a compound of formula IV wherein R 1 ' is hydrogen, Boc, Cbz, TFA, Tr, TCA, CHO, Ac, Bz, Fmoc, C 4 -C 5 - tert -alkyl, or mono, di- or triphenyl substituted methyl and R 2 is as defined above, and (0-3') converting a compound of formula IV to a compound of formula VI, (0-4') optionally converting a compound of formula VI to a salt thereof; (0-1") providing a compound of formula III or a salt thereof wherein R 1 and R 2 is as defined above, (0-2") converting a compound of formula III to a compound of formula V wherein R 2 is as defined above, and (0-3") converting a compound of formula V to a compound of formula VI, (0-4") optionally converting a compound of formula VI to a salt thereof; (0-1"') providing a compound of formula J wherein R 1 " is Boc, Cbz, TFA, TCA, CHO, Ac, Bz, Fmoc, (0-2'") reacting the compound of formula J with the compound of formula Z wherein R 2 is Bn, t -Bu, TBDMS, MOM, Tr, Ac or Bz, to yield a compound of formula K wherein R 1 " and R 2 are as defined above, (0-3"') oxidising the compound of formula K to obtain a compound of formula L wherein R 1 " and R 2 are as defined above, (0-4'") converting the compound of formula L to the compound of formula VI, and (0-5'") optionally converting a compound of formula VI to a salt thereof. or (0-1"") providing a compound of formula AMALA (0-2"") directly converting a compound by acid hydrolysis from AMALA to a compound of formula VI