Solubilization of Peptides in Non‐polar Organic Solvents by the Addition of Inorganic Salts: Facts and Implications

Self Cite

(2 1. V. 9 1)A mixture of the amidine base 1,8-diazabicyclo[5,4.0]undec-7-ene (DBU) and LiBr (preferably 0.5 and 5 equiv., resp.) turns out to be a highly efficient catalyst (at 0-25O) for saponifications (in THF/H20) and transesterifications (in ROH). The scope and limitations of the method are determined using ca. two dozens of different ester/alcohol combinations (Schemes 2 and 3 ) . The investigation is focused on peptides as substrates. Under carefully controlled conditions, no epimerization occurs with N-Boc-and N-Z-protected peptide esters, when methyl, ethyl, isopropyl, or ally1 esters are the products, as shown for peptides containing up to six amino acids, with Ala, Leu, MeLeu, Asp(OEt), or Tyr at the C-terminus (Scheme 3 and Tables 1 and 2). Hydrolytic and transesterifying detachments of Boc-Leu-Ala-Gly-Val-OR and Boc-Leu-Ala-Gly-Phe-OR (R = H, Me) from PAM and Wang resins (1-8 h at 0-25", 2 equiv. of DBU, 5 equiv. of LiBr) can be achieved by this method without epimerization of the C-terminal stereogenic center; a comparison with other methods (HF, Ti(OR),) is given (Schemes 4 and 5 ) . Possible protecting-group strategies involving the DBU/LiBr method are discussed (Table 3 ) . Extensive experimental details are given.In the course of our work on olefinations 'A la Horner-Emmons-Wadsworth' [l] with the phosphonate [2] and the highly efficient base system DBU')/LiBr [3] shown in Scheme 1, we noticed that traces of moisture led to rapid hydrolysis of the phosphonate ester at room temperature. Since such esters are normally not readily hydrolyzed, we embarked on a systematic investigation to test, whether saponifications and transesterifications could be generally effected under these conditions.There are numerous methods of achieving transesterifications and ester hydrolyses4) under acidic and basic conditions, with ester-cleaving enzymes'), with titanate@), with ion-exchange resins [8], with KF/crown ether [9], and with distannoxane [lo] to mention

mentioning

confidence: 87%

Section: Conditions Bmentioning

confidence: 99%

Transesterifications with 1,8‐Diazabicyclo[5.4.0]undec‐7‐ene/Lithium Bromide (DBU/LiBr) – Also Applicable to Cleavage of Peptides from Resins in Merrifleld Syntheses

Scebach¹,

Thaler

Blaser

et al. 1991

Self Cite

“…LiCliDMPU was the best solvent especially for PDMAA-KG resin. In THF -the solvent it all began with [24] -coupling reactions were very incomplete on all resins, and salt-additives had a negative effect. This was also true for PEO-PS resins where one might have expected a favourable interaction between the ether solvent, the polyether moiety of the support, and Li-salts [24].…”

Section: Dmf Was Used Instead Of Dmf/chci For Swelling Experimentsmentioning

confidence: 98%

Lithium‐Salt Effects in Peptide Synthesis. Part II. Improvement of Degree of Resin Swelling and of Efficiency of Coupling in Solid‐Phase Synthesis

Thaler

Seebàch²,

Cardinaux³

1991

Self Cite

The course of solid-phase peptide-coupling reactions as well as the swelling properties of a peptide-resin are influenced by the addition of inorganic salts (LiCI, LiBr, LiCIO,, KSCN). Used as additives, these salts can i ) improve coupling yields (e.g., for Fmoc-(Ala),-Phe-resin+ Fmoc-(Ala)6-Phe-resin in DMF/CH2C12 1 : 1 from 89.4 to 97.1 % (for poly(ethy1ene oxide) on polystyrene ( = PEO-PS) resin) or from 77.5 to 93.8% (for poly-( N , N'-dimethylacrylamide) on 'Kieselgur' ( = PDMAA-KG) resin) without and with 0 . 4~ LiCl, respectively), i i ) increase resin swelling (e.g. for Fmoc-(Ah),-Phe-(polystyrene resin) from 2.42-to 5.71-fold in l-methylpyrrolidin-2-one ( = NMP) without and with LiC1, respectively), and iii) change coupling rates. Examples of coupling reactions and swelling behaviour (degree and rate) in different solvents (DMF, DMF/CH2CI2 1 : 1, THF, NMP, N,N-dimethylpropyleneurea ( = DMPU) with and without salts) using different resins (polystyrene (PS); PEO-PS, and PDMAA-KG) and an improved analysis of alanine oligomers up to Ala,,-Phe by HPLC and FAB-MS are reported.Introduction. -The equivalent to the problem of poor solubility of peptide intermediates in solution synthesis is insufficient solvation of peptide chains in solid-phase peptide synthesis2). Its symptoms are a sharp decrease in reactivity towards coupling reagents and often a macroscopic change in the swelling properties of the peptide-resin [3-81. It is frequently observed at a length of between five and about twelve amino acid residues and has been associated with the onset of intra-or interchain aggregation and the concomitant decrease of solvation of the peptide chains on the resin support. Another important factor in solid-phase peptide synthesis is the extent of peptide-resin swelling in the reaction medium; only solvents which are able to swell the peptide-resin sufficiently will allow for rapid and complete coupling reactions [9].

“…The excess of LiCl is 10-30 equiv. per mole of peptide in these experiments, an amount of salt found to be sufficient for the maximum conformational change by NMR analysis [I] [10][11][12].…”

Section: Calorimetric Measurements Of Enthalpies Of Interaction Of LImentioning

confidence: 99%

Calorimetric Measurements of the Complexation of Cyclosporin A, Ascomycin, Fujimycin, and Rapamycin with Lithium Chloride and with an Immunophilin

Seebàch

Bossler

Flowers

et al. 1994

Self Cite

(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.