Custom Synthesis (Case 2)

A pharma client requested a product compound used as a standard in an in vitro assay, in order to ‘calibrate’ the compounds obtained in a drug discovery project during the hit-to-lead phase. The synthesis of the compound was deceptively simple, just the coupling of two fragments, but each fragment had to be a specific enantiomer, which were not commercially available and had to be prepared: eight steps for one and seven steps for the other. A failure in assessing the enantiomeric excess (ee) of each fragment would lead to a final compound which was not the expected estereoisomer, resulting in improper activity.

The 8-step fragment required an enantiospecific synthesis. In order to get a value for the ee, the compound was synthesized first in racemic form and a chiral HPLC method was developed. The enantiomerically pure compound was then prepared; DoE was used extensively in order to achieve the best yield and ee.

The 7-step fragment was troublesome. As with the other fragment, in order to get a value for the ee, the compound was synthetized in racemic form and a chiral HPLC method was developed. The synthesis of the chiral product was accomplished easily, but ee was low. A method of enantiomeric enrichment was developed, but stability issues arose during the salt releasing step. A battery of tests was run to isolate the problem, and finally it was determined that a resin exchange step was needed to avoid racemization of the product.

The final step, coupling of the fragments was also troublesome, but with new optimisation, the compound was delivered.