Synthesis of S‐alkyl and C‐terminal analogs of the Saccharomyces cerevisiae a‐factor: Influence of temperature on the stability of Fmoc and OFm groups toward HF

Abstract

The a‐mating factor of Saccharomyces cerevisiae, Tyr‐Ile‐Ile‐Lys‐Gly‐Val‐Phe‐Trp‐Asp‐Pro‐Ala‐Cys(far‐nesyl)OCH3, and 10 analogs modified at the cysteine side chain and/or the terminal carboxyl were synthesized using a combination of solid phase and solution phase methodologies. The strategy of synthesis involved the condensation of an amine terminal protected decapeptide with a carboxyl terminal S‐alkylated dipeptide ester or amide using benzotriazoI‐1‐yloxy‐tris(dimethylamino)‐phosphonium hexafluoro‐phosphate as the coupling agent. The protected decapeptide was assembled on a PAM‐resin using 9‐fluorenylmethoxycarbonyl (Fmoc) for the protection of the Tyr α‐amine and Lys ω‐amine and 9‐fluorenyl‐methyl ester (OFm) for the protection of the Asp β‐carboxyl. Premature loss of the OFm group from the HF cleavage was observed at 0‐2°, whereas no loss occurred when the cleavage reaction was conducted at – 5°. In contrast to these results, the OFm group in Asp(OFm) was partially removed by HF at – 5° and was completely stable to HF only at – 20°. The S‐alkylated dipeptide esters were prepared, in yields from 64% to 88%, via thioalkylation of amine protected or unprotected dipeptide esters using potassium fluoride dihydrate as the base. The use of a tertiary amine as the base for thiohexadecanylation resulted in low reactivity. Copyright © 1991, Wiley Blackwell. All rights reserved