Syntheses and properties of silicon- and germanium-containing alpha-amino acids and peptides: A study on C/Si/Ge bioisosterism

The unnatural silicon-containing alpha-amino acids (R)- and (S)-H2NCH(CH2Si Me3)COOH [(R)-2 and (S)-2], (R)-H2NCH(CH2SiMe2Ph)COOH [(R)-4], and (R)-H2NC H(CH2SiMe2CH=CH2)COOH [(R)-6] as well as the unnatural germanium-containing alpha-amino acids (R)- and (S)-H2NCH(CH2GeMe3)COOH [(R)-3 and (S)-3] and ( R)-H2NCH(CH2GeMe2Ph)COOH [(R)-5] were prepare d in three-step syntheses, st arting from (R)-3,6-diethoxy-2-isopropyl-2,5-dihydropyrazine [(R)-10]. All amino acids were isolated as enantiomerically pure (greater than or equal t o 99% ee) compounds. The (R)- and (S)-enantiomers of beta-(trimethylsilyl)a lanine [(R)-2 and (S)-2] and beta-(trimethylgermyl)alanine [(R)-3 and (S)-3 ] are sila-analogues and germa-analogues, respectively, of the (S)- and (R) -enantiomers of the nonproteinogenic amino acid beta-tert-butylalanine [(S) - and (R)-H2NCH(CH2CMe3)COOH; (S)-1 and (R)-1]. The C/Si/Ge-analogous (L-co nfigurated) amino acids (S)-1, (R)-2, and (R)-3 mere treated with (fluoren- 9-yl)methyl chloroformate to give the corresponding N-Fmoc derivatives (S)- 26, (R)-27, and (R)-28. These N-Fmoc-protected amino acids were used as bui lding blocks for the solid-phase syntheses of the C/Si/Ge-analogous decapep tides 7-9 [Ac-D-Nal(1)-4-Cl-D-Phe(2)-D-Pal(3)-Ser(4)-Me(3)El-Ala(5)-D-Cit(6 )-Leu(7)-Arg(8)-Pro(9)-D-Ala(10)-NH2 (7, El = C; 8, El = Si; 9, El = Ge)]. The C/Si/Ge analogues 7-9 are derivatives of the GnRH antagonist Cetrorelix (TM), which bears an (S)-tyrosine residue [instead of the (S)-Me3C-Ala, (R) -Me3Si-Ala, or (R)-Me3Ge-Ala residue] in position 5 of its decapeptide back bone. The decapeptides 7-9 were studied in vitro in receptor binding and fu nctional assays using recombinant cell lines expressing the human GnRH rece ptor. All compounds behaved as potent GnRH antagonists, the binding affinit ies and antagonistic potencies of the three C/Si/Ge analogues being quite s imilar. Compounds 7-9 were also studied for their in vivo activities in the male rat after s.c. administration. They produced both a strong testostero ne suppression (single-dose treatment, 1.5 mg/kg) and a strong LH suppressi on (castrated male rat; single-dose treatment, 0.05 mg/kg). For the silicon - and germanium-containing decapeptides 8 and 9 the testosterone and LH sup pression lasted for a significantly longer period of time compared with the effects of the carbon analogue 7.