Synthesis of C-11 linked active ester derivatives of vitamin D3 and their conjugations to 42-residue helix–loop–helix peptides

Derivatives of vitamin D3 carrying an 8-carbon linker at C-11 terminating in an active ester were synthesized from commercial vitamin D3 using a disassembly–reassembly strategy. Vitamin D3 was cleaved at the C6–C7 double bond and the ‘upper’ fragment was converted, via a series of reactions, to derivatives substituted at C-11 with an 8-carbon linker terminating in an ethyl ester. Reassembly with modified ‘lower’ fragments using Horner–Wittig olefination followed by linker ester hydrolysis and re-esterification with p-nitrophenol gave C-11 substituted p-nitrophenyl esters. These vitamin D derivatives were conjugated to 42-amino acid helix–loop–helix peptides by reaction of their p-nitrophenyl esters with lysyl side-chain amino groups on the peptides. The vitamin D—peptide conjugates, being potential specific binder candidates for vitamin D-binding protein, were characterized by mass spectroscopy and CD measurements.