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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
Author(s) -
Scebach Dieter,
Thaler Adrian,
Blaser Denis,
Ko Soo Y.
Publication year - 1991
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19910740520
Subject(s) - chemistry , epimer , bromide , lithium bromide , hydrolysis , isopropyl , stereocenter , cleavage (geology) , medicinal chemistry , organic chemistry , stereochemistry , deamination , racemization , catalysis , enzyme , enantioselective synthesis , physics , heat exchanger , thermodynamics , geotechnical engineering , fracture (geology) , engineering
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 0–25°) for saponifications (in THF/H 2 O) 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 epirnerization occurs with N ‐Boc‐ and N ‐Z‐protected peptide esters, when methyl, ethyl, isopropyl, or allyl esters are the products, as shown for peptides containing up to six amino acids, with Ala, Len 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‐terrninal stereogenic center; a comparison with other methods (HF, Ti(OR) 4 ) is given ( Schemes 4 and 5 ). Possible protecting‐group strategies involving the DBU/LiBr method are discussed ( Table 3 ), Extensive experimental details are given.