The synthesis of 3,4-dihydroxyprolines and the incorporation of hydroxylated prolines into peptides and glycopeptides

Abstract

L-2,3-cis-3,4-trans-Dihydroxyproline (1), L-2,3-trans-3,4-trans-dihydroxyproline (2) and L-2,3-trans-3,4-cis-dihydroxyproline (3) have been isolated from nature. More specifically DHP (3) was identified as the sixth residue in the repeating decapeptide unit of Mytilus edulis foot protein 1 (Mefp1); the adhesive protein believed to be involved in the attachment of mussels to solid surfaces of the rocky shore. A proven adhesive in an aqueous environment, Mefp1 has potential as an adhesive with biomedical applications. In order to assess the conformation of the Mefp1 decapeptide and to analyse how conformation dictates its adhesive behaviour, we required access to dihydroxyproline which is not commercially available. As a prelude to the synthesis of the Mefp1 decapeptide, we developed a synthesis of orthogonally protected L-2,3-trans-3,4-cis-dihydroxyproline 78 from D-gulonolactone (62). Dihydroxyprolines 78 and 95 were investigated as building blocks for the incorporation of dihydroxyprolines into peptides. Their utility was demonstrated by the synthesis of dipeptides 89 and 96. Although the synthesis of DHP 78 was efficient, in order to investigate the conformation of the Mefp1 decapeptide, we required a synthesis which would allow access to all eight stereoisomers of dihydroxyproline. A generalised synthesis of orthogonally protected dihydroxyprolines, suitable for direct incorporation into peptides was developed utilising the pentose series of sugars as precursors. Thus, each of the eight pentoses could be transferred into a specific stereoisomer of dihydroxyproline, via the same chemistry. This methodology was initially applied to the synthesis of D-2,3-cis-3,4-cis-dihydroxyproline 166 from D-ribonolactone (41). The generality of the synthesis was investigated in the synthesis of dihydroxyprolines 177 and, 185. Glycosylated hydroxyprolines have been extensively reported in plants. The presence of the post-translationally modified amino acids, hydroxyproline and dihydroxyproline, in the Mefp1 decapeptide was intriguing and were interested in the effects of glycosylation on peptide conformation. Glycosidic linkages via these amino acids are possible, however they have simply not been identified due to the harsh acid conditions applied to their isolation. Utilising the sulfoxide glycosylation methodology the synthesis of both α- and β-galactosides of hydroxyproline are reported. Progress towards the synthesis of glycosides of dihydroxyproline are reported.