a-Amino acid Tro ¨ger base derivatives, possible conformationally restricted scaffolds? {{ Sean P. Bew,* Laurent Legentil, Vincent Scholier and Sunil V. Sharma Received (in Cambridge, UK) 4th October 2006, Accepted 7th December 2006 First published as an Advance Article on the web 15th December 2006 DOI: 10.1039/b614371g The first synthesis of innovative a-amino acid conjugates of Tro ¨ ger base is reported; their potential application as con- formationally restricted scaffolds is proposed and has been investigated using high level ab initio calculations. We report studies on 2,8-bis-a-amino acid Tro ¨ ger base adducts (1, Fig. 1) and propose them as scaffolds for use as peptide chain directors i.e. 2. Tro ¨ ger base 1 is a C 2 -symmetric heterocycle with a relatively rigid backbone, hydrophobic cavity and concave conformation. Due to its sharply folded geometry the aryl rings reside in a near perpendicular arrangement (generally 90–100u). 2 Gaining insights into the biological mode of action of natural/ non-natural proteins, polypeptides and enzymes is critical if a comprehensive understanding of protein action is to be acquired. In this respect many studies have been undertaken on the application/development of innovative mimics of b-turns and hairpins. 3 The synthesis of an a-amino acid derived Tro ¨ ger base scaffold 4 and its appendage with additional a-amino acid derivatives affords a new opportunity to investigate the loop and hinge regions of proteins. a-Amino acid Tro ¨ ger base derivatives similar to 3 (Fig. 2) appear not only to be able to act as scaffolds, but also to have the capacity to direct appended peptides within a 90–100u range. Small peptide chains have been identified that contain 90–100u bends. For example, the polypeptide TNYLFSPNGPIARAW that binds to EphB4 (IC 50 15 nM) embodies a 90u turn induced by the GP dipeptide. This turn within the pentadecapeptide is critical for high affinity binding of the polypeptide into the hydrophobic upper convex portion of the active site within EphB4. 5 The transition-metal mediated synthesis of adducts based on 3 requires a practical, efficient and convenient synthesis of building block 4 (Scheme 1). 6 The bis-2,8-dibromo analog of 4 was not considered due to its recalcitrant nature towards Sonogashira couplings. 8 However, the synthesis of milligram quantities of racemic bis-2,8-diiodo 4 has been reported by Wa ¨ rnmark et al. 7 Translating this protocol directly to the production of multigram quantities of 4 was problematic with low yields (y10%) resulting. After careful optimisation of the experimental and purification procedures we were able to accrue multigram quantities of racemic 4 in y46% yield. Employing a single enantiomer of 4 would significantly simplify the analysis of the resulting diastereomeric a-amino acid Tro ¨ ger base conjugates. That said, all our attempts at resolving 4 using literature protocols failed. 8 Undeterred, we transformed racemic 4 into bis-2,8-dialkyne 5 via a Sonogashira coupling with TMSA. Desilylation of 5 using TBAF afforded 6 in a poor 23% yield. Switching to sodium hydroxide in a methanol– THF mix negated this problem; an excellent 91% yield of bis-2,8-ethynyl 6 resulted. Coupling 6 with 4-iodo-N-Boc- (S)-phenylalanine methyl ester 7 was attempted. Utilising standard Sonogashira coupling conditions either no reaction took place or poor yields of 9 resulted (y15%). Gratifyingly, when DMF was employed, and freshly synthesised tetrakis(triphenylphosphine)pal- ladium(0) and 2.2 equiv. of 7 were utilised the desired Tro ¨ ger base adduct 9 was returned in an unoptimised 60% yield (Scheme 1). School of Chemical Sciences & Pharmacy, University of East Anglia, Norwich, Norfolk, UK NR4 7TJ. E-mail: s.bew@uea.ac.uk; Fax: 44 (0)1603 592003; Tel: 44(0)1603 593142 { Electronic supplementary information (ESI) available: Experimental details. See DOI: 10.1039/b614371g { The HTML version of this article has been enhanced with colour images. Fig. 1 a-Amino acid Tro ¨ ger base scaffolds as peptide chain directors. Fig. 2 Chem 3D representation of bis-2,8-(N,C-protected a-amino acid) Tro ¨ ger base adduct 3. Scheme 1 Synthesis of 2,8-difunctionalised Tro ¨ ger base adducts. COMMUNICATION www.rsc.org/chemcomm | ChemComm This journal is ß The Royal Society of Chemistry 2007 Chem. Commun., 2007, 389–391 | 389