Pergamon Tetrahedron Letters 40 (1999) 1865-1868 TETRAHEDRON LETTERS Preparation of Amino Acid-Appended Cholic Acid Derivatives as Sensitizers of Gram-Negative Bacteria Atiq-ur-Rehman, Chunhong Li, Loren P. Budge, Sarah E. Street and Paul B. Savage* Department of Chemistry and Biochemistry, Brigham Young University. Provo, Utah 84602 Received 16 December 1998; accepted 5 January 1999 Abstract. Attachment of aminoacidsto cholic acid derivatives yielded compounds that sensitize Gram- negativebacteriato hydropbobic antibiotics. Incorporation of three aminoacid ontoa cholicacid derivativevia ester bondswas onlyachievedwith non-a-branched aminoacids. Threenon-a-branched or three a-branched amino acids werecoupledto the steroidbackbone via amidebonds. © 1999ElsevierScienceLtd. All rights reserved. The outer membranes of Gram-negative bacteria provide a formidable permeability barrier to many types of hydrophobic antibiotics.1 Consequently, Gram-negative bacteria are resistant to many antibiotics that are active against Gram-positive organisms.2 A number of peptides have been identified that increase the permeability of the outer membranes of Gram-negative bacteria and sensitize these organisms to hydrophobic antibiotics that ineffectively traverse the outer membranes.3 The best studied of these peptides are the polymyxin B derivatives.3a,b We have modeled polymyxin B derivatives to determine potential active conformations and determined functionality conserved among antibiotics related to polymyxin B. This conserved functionality was incorporated onto a steroid scaffolding yielding compounds that sensitize Gram- negative bacteria to hydrophobic antibiotics. 4 Compounds we have previously reported are comprised of amine-bearing groups linked to a cholic acid scaffolding via ether linkages. We now report preparation of cholic acid derivatives in which amine-bearing groups are attached to the steroid via ester or amide bonds. Preparation of these compounds results from our effort to increase the affinity of the cholic acid derivatives for components of the outer membranes of Gram-negative bacteria by taking advantage of associative interactions that might be offered by the side chains of amino acids coupled to a cholic acid scaffolding. As part of this effort, we have developed efficient means of appending cholic acid derivatives with three amino acids. A number of the resulting compounds effectively sensitize Gram-negative bacteria to erythromycin and novobiocin. We first attempted to attach three amino acids to a cholic acid scaffolding using ester bonds. Amino acids attached to steroids have been reported; 5 however, we are unaware of any report of three amino acids linked to the 3, 7 and 12 positions of cholic acid. We reacted BOC-glycine with DCC, DMAP and cholic acid derivative 14b (Scheme 1) to give triester 2a in good yield. A similar reaction incorporating BOC-13-alanine was also successful giving 2b. However, attempts to form triesters with m-branched amino acids failed; only diesters were isolated. Use of excess reagents, heating and long reaction times did not yield triester products. Reactions using CBZ-protected amino acids also failed. Molecular modeling of a triester of BOC-alanine suggested that unfavorable steric interactions may have prevented triester formation. Alternatively, steric interactions may have made the triester unstable and caused it to be readily hydrolyzed. Deprotection of 2a and 2b with HC1 in dioxane and purification gave triesters 3a and 3b in good yield. However, 3a and 3b proved to be marginally unstable under the acidic conditions used in the deprotection. At least one of the ester bonds (presumably the least sterically hindered at C3) was susceptible to hydrolysis. 0040-4039/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII: S0040-4039(99)00075-1