[Racemization in peptide synthesis]

Efficient microwave assisted access to chiral O-(alpha-protected-aminoacyl)steroids

Chiral O-(alpha-protected-aminoacyl)steroids 4a-f, 6a-b, 8 and 4a+4d and O-(alpha-protected-dipeptidoyl)steroids 12a,b are conveniently prepared under microwave irradiation in isolated yields of 65-96%, with complete chirality retention. The reaction utilized readily available N-(Z-alpha-aminoacyl)benzotriazoles 2a-f and Z-dipeptidoylbenzotriazole 11, with naturally occurring steroidal alcohols 3,5,7,9.

Carbonyldiimidazole/1-hydroxybenzotriazole activation in polymer phase synthesis of the arginine rich proalbumin hexapeptide extension

The synthesis on different polymer phases of Arg-Gly-Val-Phe-Arg-Arg, the proalbumin extension, is reported. The peptide was prepared both on a 0.5% cross-linked polystyrene gel containing 2-oxoethyl bromide anchor functions and on a 1% cross-linked chloromethyl polystyrene. For temporary blocking of the amino groups we utilized the 2-(3,5-dimethoxyphenyl) propyl-(2)-oxycarbonyl (Ddz). The guanidino groups of the three arginine moieties were protected by the 4-toluenesulfonyl (Tos) group. As coupling procedure we used 1 equiv. carbonyldiimidazole (CDI)/2 equiv. 1-hydroxybenzotriazole (HOBt). The C-terminal activation of the Ddz-amino acids with CDI/HOBt made it possible to recover the excess Ddz-amino acids in 60-80% yield. We also investigated different procedures to cleave the 2-oxoethyl ester bond between the peptide and the polymer. This bond was completely stable against trifluoromethanesulfonic acid and was split by 1 N triethylamine in methanol/dioxane 1:1 (v/v) + 1 vol% 1 N NaOH. The optical rotation and HPLC properties of Arg-Gly-Val-Phe-Arg-Arg from this synthesis are identical to the product from a different synthesis published earlier.