Strong bases (lithium diisopropylamide (LDA) or BuLi) convert cyclosporin A (CS) to a hexalithio derivative containing a Li alkoxide, four Li azaenolate, and one Li enolate units. The Li, compound is solubilized in tetrahydrofuran (THF) by addition of excess LDA or LiC1. Reactions with electrophiles (alkyl halides, aldehydes, CICO,R, C02, (RS),, D,O) at low temperatures give products containing new side chains in amino-acid residue 3 of the cyclic undecapeptide (see 1-13, Schemes I and 2, and Figx. I and 2) in moderate to high yields and, with Reor Si-selectivities, depending upon the conditions of lithiation of up to 7: 1. Pure CS derivatives (Scheme 2, Table I

The tripeptide and hexapeptide derivatives Boc-Gly-Sar-MeLeu-OH (5b), Boc-Ala-Sar-Sar-OH (6b), BocAla-Sar-MeLeu-OH (7b), and Boc-Abu-Sar-MeLeu-Val-MeLeu-Ala-OH (12b) can be poly-deprotonated (tri-and pentalithio derivatives K and P, respectively), and thus C-alkylated on sarcosine (Sar) moieties with Me1 and ally1 or PhCH2Br. The polylithiated species are solubilized in THF, and their reactivity modified by excess base (lithium diisopropylamide (LDA)), by added LEI, and/or the cosolvent N,N'-dimethylpropyleneurea (DMPU). Optimization of the reaction conditions for methylation in the cases of 7b (Table 3) and 12b (Scheme 8 ) gave products in which the Sar residue of the educt has been transformed into a Me-D-Ala unit in yields of 80 (9c/8c) and 67% (14c/13c), respectively, and with a diastereoselectivity of ca. 4 : 1. Less selective methylations and benzylations were observed with the tripeptides 5b and 6b containing only one stereogenic center; also, excess base and alkyl halide may lead to double alkylations in those latter two cases (Tables 1 and 2 ) . No epimerization of stereogenic centers was detected under the strong-base conditions. The analysis of the products was accomplished by a combination of NMR and FAB-MS spectroscopy, as well as by hydrolysis to the parent amino acids, subsequent formation of derivatives with isopropyl isocyanate, and GC analysis on the chiral column Chirasil-Val". For instance b-amino acids, amino thioacids, (+endothiopeptides), ethylenediamines (-+reduced peptides), 5-amino-4-0x0-amino acids (+oxomethylene peptides), ct -hydroxy acids (+depsipeptides), phospho-amino or -hydroxy acids (+phosphopeptides, phosphono-depsipeptides). Other modifications are called thiomethylene-, hydroxymethylene-, carboretro-or inverso-, and alkene-peptides. For a review about such modifications, see [6]. A) 3, 4,

(1 2.VII. 93) Solutions (2 ml) of small linear and cyclic peptides (411), of a peptolide containing nine amino acids and a lactate moiety (12), of the cyclic undecapeptide cyclosporin A (CS, l), and of the macrolides ascomycin, fujimycin, and rapamycin (1S-15) in THF were added to excess LiCI, LiBr, or LiCIO, (up to 3000 equiv. in 40 ml THF) in a calorimeter (calorimetric titration). The enthalpies of interaction measured are in the range of AH = -8 to -37 kcal/mol. A similar experiment was carried out with one of the binding proteins of cyclosporin, the human cyclophilin A, to give the thermodynamic parameters for the complexation AHo = -16, AG" = -10 kcal/mol, and AS' = -20 cal/mol .deg. at 25' which corresponds to an equilibrium constant K = 2. lo7 l/mol, in good agreement with the result of independent measurements using different methods. NMR Measurements of the macrolides in (D8)THF containing LiCl show strong down-field shifts of signals of the H-atoms next to C=O and C-OH groups in these molecules.

The Boc‐protected tripeptides Boc‐Val‐Gly‐Leu‐OH (1), Boc‐Leu‐Sar‐Leu‐OH (2), Boc‐Leu‐Gly‐MeLeu‐OH (3), and Boc‐Val‐BzlGly‐Leu‐OMe (64), tetrapeptide Boc‐Leu‐Gly‐Pro‐Leu‐OH (9), and pentapeptides Boc‐Val‐Leu‐Gly‐Abu‐Ile‐OH (4), Boc‐Val‐Leu‐Sar‐MeAbu‐Ile‐OH (5), Boc‐Val‐Leu‐Gly‐MeAbu‐Ile‐OH (6), Boc‐Val‐Leu‐BzlGly‐BzlAbu‐Ile‐OH (7), and Boc‐Val‐Leu‐Gly‐BzlAbu‐Ile‐OH (8) are prepared by conventional methods (Schemes 4–7) or by direct benzylation of the corresponding precursors (Scheme 8). Polylithiations in THF give up to Li6 derivatives containing glycine, sarcosine or N‐benzylglycine Li enolate moieties (A–H). The polylithiated systems with a dilithium azadienediolate unit (C, F–H) are best generated by treatment with t‐BuLi. The yields of alkylation of the glycine or sarcosine residues are up to 90%, with diastereoselectivities from nil to 9:1. Normally, the newly formed stereogenic center has (R)‐configuration (i.e. a D‐amino‐acid residue is incorporated in the peptide chain). Electrophiles which can be employed with the highly reactive azadienediolate moiety are: MeI, EtI, i‐PrI, allyl and benzyl bromide, ethyl bromoacetate, CO2, and Me2S2 (Schemes 11–13). No epimerizations of the starting materials (racemization of the amino‐acid residues) are observed under the strongly basic conditions. Selected conformations of the peptide precursors, generated by shock‐freezing or by very slow cooling from room temperature to −75° before lithiation, give rise to different stereoselectivities (Scheme 11). The latter and the yields can also be influenced by tempering the lithiated species before (Scheme 9) or after addition of the electrophiles (Scheme 12). Besides the desired products, starting peptides are recovered in the chromatographic purification and isolation procedures (material balance 80–95%). The results described are yet another demonstration that peptides may be backbone‐modified through Li enolates, and that whole series of analogous peptide derivatives with various side chains may thus be produced from a given precursor.

The preparation of a whole series of peptides—starting from a single precursor—can be achieved with readily available tripeptides such as 1 (R = H). They contain aminomalonate building blocks which can be alkylated with alkyl halides and Michael acceptors under basic conditions in good to excellent yields. By cleavage of the benzyloxycarbonyl group, peptide derivatives 1, R = alkyl, etc. are obtained, which can then be further modified. Such peptide series are of particular interest, for example, for screenings (“peptide libraries”).