Onyx is now licensed to manufacture commercial API from our UK facility

Chiral Resolution Screening

Chiral resolution screening

Classical resolution screening is a well-established method for the delivery of enantiopure API. It provides a truly robust alternative when chiral synthesis is either synthetically challenging or financially restrictive and to preparative HPLC purification that, whilst effective, can be a costly and time-consuming activity.
Ideally, chiral resolution screening takes place as early as possible within a synthetic route, thereby minimising the impact of loss of material in the form of the unwanted enantiomer.

The work-flow is effectively a salt screen and as such, collaboration between chemistry and crystallisation groups is beneficial and the approach taken in-house.

Defining the crystallization

At the start of any resolution screen an investigation into the stability and approximate solubility of the target compound is performed, alongside considerations such as pKa values and solubility of the selected agents. The stability component and composition of the input should not be under-estimated as impurity entrainment can provide a significant risk of failure due to inhibition of crystallisation.

Having identified a solvent range, the candidate is screened against a range of appropriate chiral agents for the ability to form a crystalline salt in a range of organic, aqueous and mixed solvents.Ideally, a cooling process is identified to help with the critical nucleation event, although multiple alternative modes of operation are available.

The chiral agents are generally selected based upon availability of both isomers in bulk quantities such that the process has applicability for scale-up. A selection are listed below.

  • (+) and (-) tartaric acid
  • (+) and (-) dibenzoyltartaric acid
  • (+) and (-) malic acid
  • (+) and (-) mandelic acid
  • (+) and (-) camphor sulphonic acid
  • (+) and (-) N-Boc phenylalanine
  • (+) and (-) N-Boc Valine
  • (S)-(-)-1-Phenylethylamine
  • (S)-(-)-1-(4-Bromophenyl)ethylamine
  • (S)-(-)-1-(4-methoxyphenyl)ethylamine
  • d-Valinol
  • (-)-Bis-[(S)-1-phenylethyl]amine
  • Quinine
  • (S)-(-)-1-(1-Naphthyl)ethylamine
  • (R)-(-)-2-Phenylglycinol
  • (S)-(-)-N,N-Dimethyl-1-phenylethylamine
  • L-Phenylalaninol

Other more expensive agents (or examples where only single enantiomers can be sourced) are available for use where required.

In the absence of an analytical method for the HPLC separation of enantiomers in-house method development is available to provide a short method to support rather than limit screening activity.

At this stage it can also be very beneficial to secure a chiral seed of the pure enantiomer for use in seeding experiments. This approach has been used many times in-house and shown to have a profound effect on the efficiency of crystallizations and resolutions.

Salt screen methodology in more detail:

During the first pass, stoichiometry is limited to 0.5 molar equivalents of chiral agent as experience has shown that this is generally the most efficient method for a rapid evaluation of enantiomeric enrichment via salt formation.

Any resulting salts are systematically analysed by HPLC to observe chiral enrichment.  The most successful candidates are also formed with a full equivalent of chiral agent on a larger scale and solubility tests performed identify the most suitable solvents for the resolution process and/or formal recrystallization.

Assessments of viability from this initial phase are made based not only upon enantiomeric purity but also efficiency (combination of yield and ee % ).

Our approach to such crystallizations/resolutions allows us to account for isolated yields on a low mg scale and so with this information we can reliably assess the efficiency of each resolution of interest.

The successful candidate is then systematically scaled-up with the aim of enhancing and controlling yield and throughput, in order to deliver a controllable crystallization with reproducible chiral purity.

Key to the success of any high-throughput investigation is experimental design and implementation coupled with a keen eye for material behaviour.  This allows for a large area of chemical space to be covered with excellent throughput without losing touch with the performance of the API in process.

Our teams have collaborated to complete numerous challenging chiral resolution screens that have significantly impacted upon the cost effectiveness of production and quality of products delivered to our customers.


  • “We were delighted with the service provided by Onyx Scientific in conjunction with Reach Separations in carrying out full characterisation and identification. The joint effort allowed us to undertake impurity synthesis to get a positive identification of impurity followed by the purification of a sample by prep LC. We found the whole process to be quick, efficient and cost-effective.”

    Sam Hubbard, Analytical Laboratory Manager at Oxford Analytical Ltd
  • “We have found Onyx Scientific to be a cost-effective CMO for API scale-up as well as related analytical and solid state development related to ESN364, Euroscreen’s clinical candidate currently in clinical trials. In fact, we found Onyx Scientific to be cost competitive even when compared against certain Indian CMOs. In addition to this, Onyx Scientific offers the added advantage of proximity in terms of location (and time zone), ease of communication as well as their experience and know-how.”

    Hamid Hoveyda, Director of Chemistry at Euroscreen SA
  • “Onyx Scientific has proven to be technically very capable when it comes to carrying out analytical projects. The team took their time to understand our specific requirements and came up with solutions to overcome any challenges. Communication with the team has been particularly excellent. Importantly for us, the speed of delivery was also exceptional.”

    David Childs, Director CMC at Shield Therapeutics