Patent Description:
Neurofibrillary tangles (NFTs) are deposits in the brain that are believed to be a hallmark of several neuropathologies, including Alzheimer's disease (AD). NFT plaques are comprised of aggregated hyperphosphorylated tau protein. The tau protein is associated with cytoskeleton and is involved in the transport of vesicles along microtubules in neurons. Under pathological conditions, tau is hyperphosphorylated and forms beta-sheet aggregates with fibrillar appearances similar to Aβ in senile plaques. Some tau-targeted therapies aim to slow disease progression by interfering with cell-to-cell transfer of soluble tau oligomers. If a patient's tau burden can be reliably monitored, currently and over time, then disease progression can be better understood and such therapeutic approaches can be improved. Tau-specific positron emission tomography (PET) imaging biomarkers can non-invasively monitor disease progression as well as provide a direct measure of tau-targeted agent efficacy and confirmation of its mechanism of action in clinical trials (see, e.g., <NPL>; and <NPL>).

One promising tau-specific PET imaging biomarker is a substituted benzo[<NUM>,<NUM>]imidazo[<NUM>,<NUM>-a]pyrimidin-<NUM>-amine that is both deuterated as well as labeled by a radio-isotope. For clinical applications, the radio-isotope <NUM>F is introduced into a precursor molecule immediately prior to administration of the compound to a patient. Correspondingly, an efficient synthesis of the precursor is particularly desirable because the precursor must be commercially available in large quantities that are ready for labeling.

The benzo[<NUM>,<NUM>]imidazo[<NUM>,<NUM>-a]pyrimidin-<NUM>-amine structural element has also found utility in a number of other PET imaging agents that target tau protein plaques, and has also been incorporated into investigational tau degrader molecules. Additionally, this scaffold has been used as a PET imaging agent for the identification of other proteins associated with neurodegenerative conditions such as Huntington's disease. More generally, the benzo[<NUM>,<NUM>]imidazo[<NUM>,<NUM>-a]pyrimidine motif has been incorporated into molecules which have shown anti-neurodegenerative, anti-hypertensive, anti-microbial, and antiviral activity.

Traditionally, benzo[<NUM>,<NUM>]imidazo[<NUM>,<NUM>-a]pyrimidine derivatives have been accessed via the condensation of amino-imidazoles with enones or enals bearing a leaving group in the β-position. While this approach generally provides imidazo-pyrimidine products in high yield and good isomeric selectivity, the scope of available substrates is limited to α,β-unsaturated aldehydes and ketones. To enable more efficient access to functionalized imidazo[<NUM>,<NUM>-a]pyrimidin-amines, other methods are needed. <CIT> is directed to deuterated and optionally detectably labeled heterocyclic compounds and their use as imaging agents.

The discussion of the background herein is included to explain the context of the technology. This is not to be taken as an admission that any of the material referred to was published, known, or part of the common general knowledge as at the priority date of any of the claims found appended hereto.

Throughout the description and claims of the instant application the word "comprise" and variations thereof, such as "comprising" and "comprises", is not intended to exclude other additives, components, integers or steps.

The instant disclosure addresses the synthesis of substituted imidazopyrimidine molecules, particularly a number that are deuterated at specific positions.

In particular, the disclosure comprises a coupling step that can make a substituted imidazo-pyrimidine in high yield.

Selective N-phosphorylation of aminoimidazoles results in a key steering element that controls isomeric selectivity in the condensation of β-ethoxy acrylamides and amino-imidazoles to furnish imidazo[<NUM>,<NUM>-a]pyrimidines. Conditions that provide highly selective (<NUM>:<NUM>) phosphorylation at the endo- or exocyclic nitrogen are disclosed. Either the <NUM>-amino or <NUM>-amino isomer of the (benzo)imidazo[<NUM>,<NUM>-a]pyrimidine products can be isolated in <NUM> - <NUM>% yield.

The disclosure further includes a compound of formula (V):
<CHM>.

In particular, compound (V) has a deuterium enrichment factor of <NUM>,<NUM> or greater at each of the positions shown as occupied by a deuterium atom.

The disclosure further includes a compound of formula (VI):
<CHM>.

In particular, compound (VI) has a deuterium enrichment factor of <NUM>,<NUM> or greater at each of the positions shown as occupied by a deuterium atom.

The disclosure further comprises a method of synthesizing the compound of formula (I),
<CHM>
the method comprising:
at least a step of coupling a compound of formula (II) with a compound of formula (III)
<CHM>
in the presence of POCl<NUM> and EtsN in a non-aqueous solvent to give a first precursor (I-P1).

The synthesis further comprises removing the benzoate group in I-P1 to give a second precursor (I-P2); and
<CHM>.

Reference will now be made in detail to certain embodiments of the invention, examples of which are illustrated in the accompanying structures and formulas. While the invention will be described in conjunction with the enumerated embodiments, it will be understood that they are not intended to limit the invention to those embodiments.

The nomenclature used in this application is based on the ACS Style Guide and the <NPL> list of "<NPL>. ), as well as on IUPAC systematic nomenclature, unless indicated otherwise.

Chemical elements are identified in accordance with the Periodic Table of the Elements, CAS version, <NPL>.

Unless otherwise stated, the compounds include enantiomeric, diastereomeric and geometric (or conformational) isomeric forms of a given structure unless the structure is otherwise intended to be limited. For example, the R and S configurations for each asymmetric center, Z and E double bond isomers, Z and E conformational isomers, single stereochemical isomers, as well as enantiomeric, diastereomeric, and geometric (or conformational) mixtures are included. Unless otherwise stated, all tautomeric forms of structures depicted herein are also included.

It is to be understood that when a compound or Example herein is shown as a specific salt, the corresponding free-base, as well as other salts of the corresponding free-base (including pharmaceutically acceptable salts of the corresponding free-base) are contemplated.

The term "tautomer" or "tautomeric form" refers to structural isomers of different energies which are interconvertible via a low energy barrier. For example, proton tautomers (also known as prototropic tautomers) include interconversions via migration of a proton, such as keto-enol and imine-enamine isomerizations. Valence tautomers include interconversions by reorganization of some of the bonding electrons.

Unless otherwise stated, structures depicted herein are also meant to include compounds that differ only in the presence of one or more isotopically enriched atoms in addition to any isotopically enriched atom that has been explicitly identified. For example, compounds, wherein the independent replacement or enrichment of one or more hydrogen by deuterium or tritium, carbon by <NUM>C- or <NUM>C carbon, nitrogen by a <NUM>N nitrogen, sulfur by a <NUM>S, <NUM>S or <NUM>S sulfur, or oxygen by a <NUM>O or <NUM>O oxygen are included. Such compounds are useful, for example, as analytical tools, as probes in biological assays, or as therapeutic agents.

The term "deuterated" means that a hydrogen atom is replaced by a deuterium atom at a level above its natural abundance at one or more positions of a molecule. When a particular position is deuterated, it is understood that the abundance of deuterium at that position is substantially greater than the natural abundance of deuterium, which is <NUM>%. A deuterated position typically has a minimum isotopic enrichment factor of at least <NUM> (<NUM>% deuterium incorporation).

The term "isotopic enrichment factor" as used herein means the ratio between the isotopic abundance and the natural abundance of a specified isotope. In certain embodiments, a given position in a molecule has an isotopic enrichment factor of at least <NUM> (<NUM>% deuterium incorporation), at least <NUM> (<NUM>% deuterium incorporation), at least <NUM> (<NUM>% deuterium incorporation), at least <NUM> (<NUM>% deuterium incorporation), at least <NUM> (<NUM>% deuterium incorporation), at least <NUM> (<NUM>% deuterium incorporation), at least <NUM> (<NUM>% deuterium incorporation), at least <NUM> (<NUM>% deuterium incorporation), at least <NUM> (<NUM>% deuterium incorporation), or at least <NUM> (<NUM>% deuterium incorporation). In some embodiments, <NUM>% deuterium incorporation is achieved.

Deuterium can be incorporated into a compound of the present invention using a variety of known reagents and synthetic techniques. For example, deuterium can be incorporated into a compound of formula (I) using LiAlD<NUM> at an earlier stage in the synthesis that leads to compound (I). It can also be incorporated into a compound of formula (I) such as through reduction, catalytic hydrogenation or isotopic exchange using appropriate deuterated reagents such as deuterium gas D<NUM>, and deuterated (or "heavy") forms of water such as HDO or D<NUM>O.

As between chemical names and structures shown, if there are any discrepancies, the structure prevails.

As used herein, "a" or "an" means one or more, unless clearly indicated otherwise.

Described herein are compounds that are deuterated at four positions (often referred to by "d4" as a prefix to the compound name), as follows:
<CHM>.

As described elsewhere herein, the invention further comprises methods of synthesis of such compounds.

It should be understood that "d4" means that each of the deuterated positions has an enrichment factor of at least <NUM>, and preferably at least <NUM>, and still more preferably at least <NUM>, and even more preferably at least <NUM>.

It should further be understood that, after synthesis, a d4-compound can be further purified to ensure that the enrichment factor at each of the deuterated positions is effectively at least <NUM> or more.

As described by way of specific synthetic schemes in the Examples that follow, the invention comprises a method of synthesizing a compound of formula (I):
<CHM>
wherein the carbon atom labeled with a star (*) is optionally doubly-deuterated. In the instance that the carbon atom indicated (*) is doubly-deuterated, compound (I) is a d4-compound as described elsewhere herein.

The methods herein can obtain a di-deuterated ("d2") form at a purity of ><NUM>% (i.e., D2/(D0+D1+D2) > <NUM>%, where Dn denotes a molecule having n of the designated hydrogens substituted by deuterium atoms). In preferred embodiments, the purity is at least <NUM>% of the d2 form. The quality (i.e., isotopic enrichment factor) of lithium aluminum deuterate (LiAlD<NUM>) is the principal driving force behind the actual purity of the compound obtained. A synthetic approach in which catalytic D<NUM> gas is used to introduce deuterium atoms into the molecule typically cannot produce more than <NUM>% purity of the d2 form. Consequently, the deuterium enrichment factor for each substituted position is typically in excess of <NUM>, and preferably in excess of <NUM>.

A method of synthesizing compounds of formula (I) comprises, in a first step, coupling an acrylamide compound of formula (II) with an amino-imidazole compound of formula (III)
<CHM>
in the presence of POCl<NUM> and EtsN in a non-aqueous solvent to give a first precursor (I-P1)
<CHM>.

This reaction provides the benzo[<NUM>,<NUM>]imidazo[<NUM>,<NUM>-a]pyrimidin-<NUM>-amine or (I-P1) with excellent selectivity over the alternative isomer, a benzo[<NUM>,<NUM>]imidazo[<NUM>,<NUM>-a]pyrimidin-<NUM>-amine.

It would be understood by those of skill in the art that a number of suitable choices of non-aqueous solvent for this step exist, and also that other reagents may be suitably identified to achieve the coupling. The group Bz in (II) and (I-P1) denotes carboxyphenyl (-C(=O)Ph).

In a second step, the benzoate group in (I-P1) is removed to give a second precursor (I-P2):
<CHM>.

This second step may occur in the presence of a base such as NaOH.

In a third step, the hydroxyl group in I-P2 is replaced by a tosyl group to give compound (I). Tosylation may optionally involve an acidification step after reacting (I-P2) with tosylate.

It is to be assumed that in formulae (II), (I-P1) and (I-P2), the carbon atom labeled (*) is also optionally doubly-deuterated. The reagents deployed to achieve the second and third steps may differ according to whether the carbon atom labeled (*) is doubly-deuterated.

In some embodiments, the compound of formula (II) is synthesized by a method comprising: reacting
<CHM>
with a first deuterating agent to produce
<CHM>
P1). The first deuterating agent can be LiAlD<NUM> or can be D<NUM> gas.

In a subsequent step, a benzoyl group is added to the hydroxyl group of (II-P1) to produce a further intermediate
<CHM>.

In a further step, the Boc protecting group is removed from (II-P2) to produce a hydrochloride salt (II-P3)
<CHM>.

A compound of formula (II) can be formed by reacting (II-P3) with
<CHM>.

In some embodiments, compound (IVa) is formed by reacting (IV) with SOCl<NUM>.

In some embodiments, wherein the carbon atom labeled (*) in (II) is doubly deuterated, compound (II-P1) is created by reacting
<CHM>
with a second deuterating agent to produce intermediate (II-P0)
<CHM>
and reacting the first deuterating agent with (II-P0) to form (II-P1).

In some embodiments, the compound of formula (III) is synthesized by a method comprising: reacting
<CHM>
with PCl(O)(OEt)<NUM>, N-methylimidazole, and MeCN.

In some embodiments, the phosphorylating agent, PCl(O)(OEt)<NUM> is formed in situ by reacting diethyl hydrogen phosphate with a chlorinating agent such as <NUM>,<NUM>,<NUM>-trichloro-<NUM>,<NUM>,<NUM>-triazinane-<NUM>,<NUM>,<NUM>-trione.

One of skill in the art would appreciate that there are various ways to introduce a radio-isotope into a compound of formula (I) in order to make a tracer compound for use with imaging technology such as positron emission tomographay. For example, compounds of formula (I) can be tritiated (labeled with one or more tritium atoms), or the tosylate function can be replaced by a <NUM>F isotope, both according to methods known in the art.

Example <NUM> describes a representative synthesis of a tracer molecule deuterated at two positions including use of a step of conjugating an acrylamide with a phosphorylated imidazole. <FIG> shows the synthetic pathway of Example <NUM> in overview, including steps to prepare starting materials.

To a reactor under nitrogen equipped with overhead agitation was charged tert-butyl <NUM>-(<NUM>-methoxy-<NUM>-oxoethyl)piperidine-<NUM>-carboxylate (<NUM>, <NUM> mol, <NUM> equiv) and MTBE (<NUM>). Once full dissolution of the oil was observed, Norit™ A Supra charcoal (<NUM>, <NUM> w%) was charged to the reactor and the jacket was warmed to <NUM>-<NUM>. The mixture was held at this temperature for <NUM>, cooled to <NUM>, then filtered through a Büchner funnel containing <NUM> of Celite®. The funnel was then washed with MTBE (<NUM>).

This procedure was repeated on another <NUM> batch of tert-butyl <NUM>-(<NUM>-methoxy-<NUM>-oxoethyl)piperidine-<NUM>-carboxylate and after filtration through Celite® and washing with MTBE (<NUM>), both solutions were combined then concentrated to dryness to obtain tert-butyl <NUM>-(<NUM>-methoxy-<NUM>-oxoethyl)piperidine-<NUM>-carboxylate as a clear yellow oil (<NUM>, <NUM>%) in <NUM> A% HPLC purity.

To a pressure reactor was charged tert-butyl <NUM>-(<NUM>-methoxy-<NUM>-oxoethyl)piperidine-<NUM>-carboxylate (<NUM>, <NUM> mmol, <NUM> equiv), NaOMe (<NUM>, <NUM> mmol, <NUM> equiv), RuMACHO® (<NUM>, <NUM> mmol, <NUM> equiv), then PhMe (<NUM>). The reactor was then sealed, the atmosphere evacuated under vacuum then backfilled with deuterium gas (<NUM> atm) three times. The pressure reactor was then heated to <NUM> and stirred for <NUM>. The reactor was then cooled to <NUM>, filtered through short pad of silica gel, rinsed with PhMe, and concentrated under vacuum to afford tert-butyl <NUM>-(<NUM>-hydroxyethyl-<NUM>,<NUM>-d<NUM>)piperidine-<NUM>-carboxylate as a yellow oil (<NUM>, <NUM>% corrected yield) in <NUM> A% HPLC purity.

To a reactor under nitrogen equipped with overhead agitation was charged anhydrous THF (<NUM>) and tert-butyl <NUM>-(<NUM>-methoxy-<NUM>-oxoethyl)piperidine-<NUM>-carboxylate (<NUM>, <NUM> mol, <NUM> equiv). The reactor was then cooled to <NUM>-<NUM> over <NUM>-<NUM> then LiAlD<NUM> (<NUM>, <NUM> mol, <NUM> equiv) was slowly charged in a portion-wise fashion. (Caution was appropriate as there was significant gas evolution. ) Upon complete charging of the reagent, the reaction was stirred at <NUM>-<NUM> for <NUM>. After reaction completion was confirmed, the reaction was quenched by slow addition of H<NUM>O (<NUM>) while maintaining the internal temperature to ≤ <NUM> (Caution was appropriate as there was significant gas evolution. ) The reactor was then warmed to <NUM>-<NUM> and stirred for <NUM>. An aqueous 4N NaOH solution (<NUM>, <NUM>) was charged to the reactor and the mixture was again stirred at <NUM>-<NUM> for <NUM>. Lastly, H<NUM>O (<NUM>) was charged to the reactor and the mixture was again stirred at <NUM>-<NUM> for <NUM>. The layers from the filtered mixture were then separated and the aqueous layer was discarded to waste. Na<NUM>SO<NUM> (<NUM>) was charged to the reactor containing the organic layer and the mixture was stirred at <NUM>-<NUM> for <NUM>. The suspension was then filtered through a short pad of Celite (<NUM>) then the pad washed with three separate portions of THF (<NUM>). The combined organic layer and washes were then concentrated under vacuum distillation (jacket temperature set to <NUM>). tert-butyl <NUM>-(<NUM>-hydroxyethyl-<NUM>,<NUM>-d<NUM>)piperidine-<NUM>-carboxylate was then collected as a yellow oil (<NUM>, <NUM>%) in <NUM> A% HPLC purity and used directly in the next step.

To a reactor under nitrogen equipped with overhead agitation was charged tert-butyl <NUM>-(<NUM>-hydroxyethyl-<NUM>,<NUM>-d<NUM>)piperidine-<NUM>-carboxylate (<NUM>, <NUM> mol, <NUM> equiv), toluene (<NUM>), EtsN (<NUM>, <NUM> mol, <NUM> equiv), and DMAP (<NUM>, <NUM> mol, <NUM> mol%). This mixture was stirred then the jacket was cooled to between -<NUM> and <NUM> until the internal temperature achieved < <NUM>. Benzoyl chloride (<NUM>, <NUM> mol, <NUM> equiv) was charged slowly over <NUM> while maintaining the internal temperature < <NUM>. Upon complete addition of the reagent, the reactor was warmed to <NUM>-<NUM> then held for <NUM> after reaching the desired jacket temperature. After completion of the reaction was confirmed, a half-saturated NaHCO<NUM> solution (<NUM>) was charged to the reactor and stirred for <NUM>. The layers were then separated and the aqueous layer drained to waste. Water (<NUM>) was then charged to the reactor and again stirred for <NUM>. The layers were then separated and the aqueous layer drained to waste. The organic layer was then azeotropically dried via vacuum distillation with toluene (<NUM> V) with the jacket temperature set to <NUM>. The resulting tert-butyl <NUM>-(<NUM>-(benzoyloxy)ethyl-<NUM>,<NUM>-d<NUM>)piperidine-<NUM>-carboxylate was obtained as a <NUM> V toluene solution in <NUM> A% HPLC purity and then used directly in the next step.

To a reactor under nitrogen equipped with overhead agitation was charged <NUM>-PrOH (<NUM>). The jacket was cooled to <NUM> to <NUM> then AcCl (<NUM>, <NUM> mol, <NUM> equiv) was charged slowly while maintaining the internal temperature < <NUM>. (Caution was appropriate as the reaction was exothermic. ) Following complete addition of the reagent, the contents were held between <NUM>-<NUM> for <NUM> mins then tert-butyl <NUM>-(<NUM>-(benzoyloxy)ethyl-<NUM>,<NUM>-d<NUM>)piperidine-<NUM>-carboxylate (<NUM>, <NUM> mol, <NUM> equiv) was charged as a toluene solution (<NUM>) then the retaining flask and transfer line were washed forward with toluene (<NUM>). The reactor jacket was then heated to <NUM> and stirred at temperature for <NUM>. Upon confirmation of reaction completion, heptane was charged at temperature (jacket <NUM>-<NUM>) over <NUM> to the self-seeded mixture. Following complete addition of the anti-solvent, the contents were aged for <NUM>, the jacket then cooled to <NUM> over <NUM>, and held at temperature for <NUM>. The slurry was then filtered cold and washed with a pre-chilled <NUM>% <NUM>-PrOH: <NUM>% heptane (<NUM>) solution at <NUM>. The solids were then dried under vacuum at <NUM> for <NUM> to afford <NUM>-(piperidin-<NUM>-yl)ethyl-<NUM>,<NUM>-d<NUM> benzoate hydrochloride as an off-white solid (<NUM>, <NUM>% corrected yield over two steps, in <NUM> A% HPLC purity): mp <NUM>-<NUM>.

<NUM>H NMR (<NUM>, DMSO-d<NUM>) δ = <NUM> -<NUM> (m, <NUM>), <NUM> -<NUM> (m, <NUM>), <NUM> -<NUM> (m, <NUM>), <NUM> -<NUM> (m, <NUM>), <NUM> (br d, J=<NUM>, <NUM>), <NUM> -<NUM> (m, <NUM>), <NUM> (br d, J=<NUM>, <NUM>), <NUM> -<NUM> (m, <NUM>), <NUM> -<NUM> (m, <NUM>), <NUM> -<NUM> (m, <NUM>). <NUM>C NMR (<NUM>, DMSO-d<NUM>) δ = ppm <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

To a reactor under nitrogen equipped with overhead agitation was charged <NUM>-(piperidin-<NUM>-yl)ethyl-<NUM>,<NUM>-d<NUM> benzoate hydrochloride (<NUM>, <NUM> mol, <NUM> equiv), (E)-<NUM>-ethoxyacrylic acid (<NUM>, <NUM> mol, <NUM> equiv), IPA (<NUM>, <NUM> V), and EtsN (<NUM>, <NUM> mol, <NUM> equiv). The contents were then stirred at <NUM>-<NUM> for <NUM> then T3P® (<NUM>% solution in EtOAc, <NUM>, <NUM> mol, <NUM> equiv) was charged slowly over <NUM> while maintaining the temperature below <NUM>. The reactor jacket was then heated to <NUM> and held at temperature for <NUM>. Upon confirmation of reaction completion by HPLC, water (<NUM>) was charged to the reactor over <NUM>, then the reactor jacket was cooled to <NUM>-<NUM> over <NUM> then aged for <NUM>. The reaction mixture was filtered and the resulting cake was washed with two portions of a <NUM>:<NUM> mixture of water: IPA (<NUM>). The washed cake was then dried at <NUM> for <NUM> and combined with another <NUM> reaction conducted identically to the first batch, to afford (E)-<NUM>-(<NUM>-(<NUM>-ethoxyacryloyl)piperidin-<NUM>-yl)ethyl-<NUM>,<NUM>-d<NUM> benzoate as a white solid (<NUM>, <NUM> % corrected yield, in <NUM> A% HPLC purity): mp <NUM>-<NUM>.

<NUM>H NMR (<NUM>, DMSO-d<NUM>, <NUM>) δ = <NUM> (d, J=<NUM>, <NUM>), <NUM> -<NUM> (m, <NUM>), <NUM> -<NUM> (m, <NUM>), <NUM> (d, J=<NUM>, <NUM>), <NUM> (d, J=<NUM>, <NUM>), <NUM> (br s, <NUM>), <NUM> (q, J=<NUM>, <NUM>), <NUM> (br s, <NUM>), <NUM>-<NUM> (m, <NUM>), <NUM> (t, J=<NUM>, <NUM>), <NUM> - <NUM> (m, <NUM>). <NUM>C NMR (<NUM>, DMSO-d<NUM>, <NUM>) δ = <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

To a reactor under nitrogen was charged <NUM>H-benzo[d]imidazol-<NUM>-amine (<NUM>, <NUM> mol, <NUM> equiv) followed by MeCN (<NUM>). To this mixture was then added n-methylimidazole (<NUM>, <NUM> mol, <NUM> equiv) followed by MeCN (<NUM>) to rinse the reaction vessel. This slurry was then stirred at <NUM> for a minimum of <NUM>. Next, CIP(O)(OEt)<NUM> (<NUM>, <NUM> mol, <NUM> equiv) was added dropwise over <NUM> keeping the internal temperature below <NUM>. Upon the completion of the CIP(O)(OEt)<NUM> addition, MeCN (<NUM>) was added to rinse the reaction vessel. This slurry was then stirred at <NUM> for a minimum of <NUM>. Upon completion of reaction, the vessel was cooled to <NUM> over a minimum of <NUM> and then stirred at this temperature for a minimum of <NUM>. This mixture was then filtered and the wetcake was washed with two portions of MeCN (<NUM>). The solids were dried under vacuum at <NUM> for a minimum of <NUM> to afford diethyl (<NUM>-benzo[d]imidazol-<NUM>-yl)phosphoramidate as an off-white solid (<NUM>, corrected yield: <NUM>%, <NUM>% HPLC purity): mp <NUM>.

<NUM>H NMR (<NUM>, Methanol-d<NUM>) δ <NUM> (dt, J = <NUM>, <NUM>, <NUM>), <NUM> (dd, J = <NUM>, <NUM>, <NUM>), <NUM> (p, J = <NUM>, <NUM>), <NUM> (t, J = <NUM>, <NUM>); <NUM>C NMR (<NUM>, Methanol-d<NUM>) δ <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>; <NUM>P NMR (<NUM>, Methanol-d4) δ <NUM>.

To a reactor under nitrogen was charged MeCN (<NUM>) followed by (E)-<NUM>-(<NUM>-(<NUM>-ethoxyacryloyl)piperidin-<NUM>-yl)ethyl-<NUM>,<NUM>-d<NUM> benzoate (<NUM>, <NUM> mol, <NUM> equiv) and diethyl (<NUM>-benzo[d]imidazol-<NUM>-yl)phosphoramidate (<NUM>, <NUM> mol, <NUM> equiv). MeTHF (<NUM>) was charged followed by EtsN (<NUM>, <NUM> mol, <NUM> equiv). Next, the reactor was cooled between -<NUM> and <NUM> and POCl<NUM>(<NUM>, <NUM> mol, <NUM> equiv) was added over a minimum <NUM> while maintaining the internal temperature between -<NUM> to <NUM>. Upon completion of the addition, the contents were then heated to <NUM>-<NUM> over a minimum of <NUM> and the reaction held at an internal temperature of <NUM> for a minimum of <NUM>. After completion of reaction was confirmed, the reactor was cooled to <NUM> - <NUM> then added to a <NUM> w/w% solution of K<NUM>CO<NUM> (premade by mixing water (<NUM>) with K<NUM>CO<NUM> (<NUM>) and stirring at <NUM> for a minimum of <NUM>) over a minimum of <NUM> while keeping the internal temperature below <NUM>. This was then stirred for a minimum of <NUM> after which the layers were separated and the aqueous drained to waste. This reaction solution was then used directly in the next step.

To a reactor under nitrogen containing the crude solution from the previous step was charged a <NUM> w/w% solution of NaOEt in EtOH (<NUM>, <NUM> mol). The reactor was then heated to <NUM>-<NUM> and stirred for <NUM>-<NUM>. After reaction completion was confirmed, the mixture was concentrated to <NUM> via vacuum distillation. EtOH (<NUM>) was added and a constant-volume distillation was performed while maintaining the internal temperature below <NUM>. Water (<NUM>) was then added and the constant-volume distillation was maintained while maintaining the temperature below <NUM>. Vacuum was disabled and the solution was cooled to <NUM> over <NUM>. An additional charge of a <NUM> w/w% solution of NaOEt in NaOH was added (<NUM>, <NUM> mol, <NUM> equiv) and the reaction was heated to <NUM>-<NUM> for <NUM>. After which, the reactor was cooled to <NUM> over the course of <NUM> then held at this temperature for a minimum of <NUM>. The mixture was filtered and rinsed with a pre-cooled <NUM>:<NUM> water:EtOH solution (<NUM>) at <NUM>-<NUM> followed by a wash with water (<NUM>) at <NUM>-<NUM>. The solids were then dried under vacuum at <NUM> for a minimum of <NUM> to afford <NUM>-(<NUM>-(benzo[<NUM>,<NUM>]imidazo[<NUM>,<NUM>-a]pyrimidin-<NUM>-yl)piperidin-<NUM>-yl)ethan-<NUM>,<NUM>-d<NUM>-<NUM>-ol as a light-yellow solid (<NUM>, corrected yield: <NUM>%, <NUM>% HPLC purity): mp <NUM>.

<NUM>H NMR (<NUM>, DMSO-d<NUM>) δ <NUM> (d, J = <NUM>, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (t, J = <NUM>, <NUM>), <NUM> (t, J = <NUM>, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (s, <NUM>), <NUM> (s, <NUM>), <NUM> (t, J = <NUM>, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (ddt, J = <NUM>, <NUM>, <NUM>, <NUM>); <NUM>C NMR (<NUM>, DMSO-d<NUM>) δ <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

To a reactor under nitrogen was charged <NUM>-(<NUM>-(benzo[<NUM>,<NUM>]imidazo[<NUM>,<NUM>-a]pyrimidin-<NUM>-yl)piperidin-<NUM>-yl)ethan-<NUM>,<NUM>-d<NUM>-<NUM>-ol (<NUM>, <NUM> mol, <NUM> equiv) followed by TsOH·H<NUM>O (<NUM>, <NUM> mmol. <NUM> equiv). MeCN (<NUM>) was then added and the reaction was stirred for a minimum of <NUM>. Next, Ts<NUM>O (<NUM>, <NUM> mmol, <NUM> equiv) was added followed by MeCN (<NUM>) to rinse the reaction vessel. This slurry was then stirred at <NUM> for <NUM>. After reaction completion was confirmed, H<NUM>O (<NUM>) was added. Next, NMP (<NUM>) was added while keeping the internal temperature below <NUM> and following complete addition, the reaction mixture was then stirred for a minimum of <NUM>. Meanwhile, in a separate reactor was added a <NUM> wt% aqueous solution of K<NUM>PO<NUM> (<NUM>). Then the crude reaction mixture was added to the basic solution over <NUM>-<NUM> then the contents aged for a minimum of <NUM>. The mixture was then filtered and the wetcake was washed with two portions of water (<NUM>). The solids were dried under vacuum at <NUM> for a minimum of <NUM> followed by drying under vacuum at <NUM> for a minimum of <NUM> to afford <NUM>-(<NUM>-(benzo[<NUM>,<NUM>]imidazo[<NUM>,<NUM>-a]pyrimidin-<NUM>-yl)piperidin-<NUM>-yl)ethyl-<NUM>,<NUM>-d<NUM> <NUM>-methylbenzenesulfonate as an off-white solid (<NUM>, corrected yield: <NUM>%, <NUM>% HPLC purity): mp <NUM>.

<NUM>H NMR (<NUM>, Chloroform-d) δ <NUM> (d, J = <NUM>, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (dd, J = <NUM>, <NUM>, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (s, <NUM>), <NUM> (t, J = <NUM>, <NUM>), <NUM> (s, <NUM>), <NUM> (s, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (qd, J = <NUM>, <NUM>, <NUM>); <NUM>C NMR (<NUM>, Chloroform-d) δ <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

Example <NUM> describes a representative synthesis of a tracer molecule that is deuterated at four positions wherein the method uses a step of conjugating an acrylamide with a phosphorylated imidazole. <FIG> shows the synthetic pathway in overview, including steps to prepare various starting materials.

To a flask at <NUM>-<NUM> under nitrogen was charged <NUM>,<NUM>,<NUM>-trichloro-<NUM>,<NUM>,<NUM>-triazinane-<NUM>,<NUM>,<NUM>-trione (<NUM>, <NUM> mmol, <NUM> equiv) and MeCN (<NUM>). The mixture was stirred until homogenous. Diethyl hydrogen phosphate (<NUM>, <NUM> mmol, <NUM>, <NUM> eq) and Et<NUM>N (<NUM>, <NUM> mmol, <NUM>, <NUM> eq) were then added and the flask was heated to <NUM> and stirred for <NUM>, at which point precipitation had occurred. The jacket was then cooled to <NUM> and <NUM>-benzo[d]imidazol-<NUM>-amine (<NUM>, <NUM> mmol, <NUM> equiv), dissolved in THF (<NUM>), was charged to the flask. The reaction mixture was then warmed to <NUM>-<NUM> and stirred for <NUM>. Upon confirmation of reaction completion, the reaction was quenched with water (<NUM>) then extracted with two portions of ethyl acetate (<NUM>). The combined organic layers were dried over Na<NUM>SO<NUM>, filtered, and then concentrated under reduced pressure to afford compound diethyl (<NUM>-benzo[d]imidazol-<NUM>-yl)phosphoramidate as a brown solid (<NUM>, <NUM> mmol, <NUM>% yield).

<NUM>H NMR (<NUM>, CDCl<NUM>) δ ppm <NUM> (dd, J = <NUM>, <NUM>, <NUM>), <NUM> (td, J = <NUM>, <NUM>, <NUM>), <NUM> (td, J = <NUM>, <NUM>, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> (td, J = <NUM>, <NUM>, <NUM>).

To a solution of (E)-<NUM>-ethoxyacrylic acid (<NUM>, <NUM> mmol, <NUM> equiv) in DCM (<NUM>) was added SOCI<NUM> (<NUM>, <NUM> mmol, <NUM>, <NUM> equiv) at <NUM>. The mixture was stirred at <NUM> for <NUM>. After reaction completion was confirmed, the mixture was evaporated to dryness to afford (E)-<NUM>-ethoxyacryloyl chloride as a yellow oil (<NUM>, <NUM> mmol, <NUM>% yield). This was used directly in the next step without purification.

To a solution of tert-butyl <NUM>-(<NUM>-methoxy-<NUM>-oxoethyl)piperidine-<NUM>-carboxylate (<NUM>, <NUM> mmol, <NUM> equiv) in CDsOD (<NUM>) at <NUM> under nitrogen was added CD<NUM>ONa (<NUM>, <NUM> mmol, <NUM> equiv) then the reaction was heated to <NUM> for <NUM>. After reaction completion was confirmed, the mixture was evaporated to dryness then 1N HCl was added to adjust the pH to a range of <NUM>-<NUM>. The resulting mixture was then extracted with three portions of EtOAc (<NUM>). The organic phases were combined, dried over Na<NUM>SO<NUM>, and then evaporated to dryness. The crude mixture was then purified via flash chromatography (<NUM>:<NUM> heptane:ethyl acetate) to afford <NUM>-(<NUM>-(tert-butoxycarbonyl)piperidin-<NUM>-yl)acetic-<NUM>,<NUM>-d<NUM> acid as an off-white solid (<NUM>, <NUM>% yield over two steps).

<NUM>H NMR (<NUM>, CDCl<NUM>) δ ppm <NUM> - <NUM> (m, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> (tt, J = <NUM>, <NUM>, <NUM>), <NUM> (br d, J = <NUM>, <NUM>), <NUM> (s, <NUM>), <NUM> - <NUM> (m, <NUM>).

To a solution of LiAlD<NUM> (<NUM>, <NUM> mmol, <NUM>, <NUM> equiv) in THF (<NUM>) at -<NUM> was then slowly added <NUM>-(<NUM>-(tert-butoxycarbonyl)piperidin-<NUM>-yl)acetic-<NUM>,<NUM>-d<NUM> acid (<NUM>, <NUM> mmol, <NUM> equiv) and upon complete addition of the starting material, the mixture was stirred for <NUM>. After confirmation of reaction completion, H<NUM>O (<NUM>) and <NUM>% NaOH (<NUM>) were added to the reaction. The mixture was then filtered and extracted with three portions of EtOAc (<NUM>). The organic phases were combined, dried over Na<NUM>SO<NUM>, and evaporated to dryness to afford tert-butyl <NUM>-(<NUM>-hydroxyethyl-<NUM>,<NUM>,<NUM>,<NUM>-d<NUM>)piperidine-<NUM>-carboxylate as a yellow oil (<NUM>, <NUM> mmol, <NUM>% yield).

<NUM>H NMR (<NUM>, CDCl<NUM>) δ ppm <NUM> - <NUM> (m, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> (s, <NUM>), <NUM> - <NUM> (m, <NUM>).

To a solution of tert-butyl <NUM>-(<NUM>-hydroxyethyl-<NUM>,<NUM>,<NUM>,<NUM>-d<NUM>)piperidine-<NUM>-carboxylate (<NUM>, <NUM> mmol, <NUM> equiv), Et<NUM>N (<NUM>, <NUM> mmol, <NUM>, <NUM> equiv), and THF (<NUM>) at <NUM>-<NUM> was added BzCl (<NUM>, <NUM> mmol, <NUM>, <NUM> equiv) in a dropwise fashion. The mixture was then stirred at <NUM>-<NUM> for <NUM>. Upon confirmation of reaction completion, H<NUM>O (<NUM>) was added and the aqueous layer was extracted with three portions of EtOAc (<NUM>). The organic phases were combined, dried over Na<NUM>SO<NUM>, and evaporated to dryness. The crude was purified by flash chromatography (<NUM>:<NUM> petroleum ether: ethyl acetate mobile phase) to afford tert-butyl <NUM>-(<NUM>-(benzoyloxy)ethyl-<NUM>,<NUM>,<NUM>,<NUM>-d<NUM>)piperidine-<NUM>-carboxylate as a yellow oil (<NUM>, <NUM>% yield).

<NUM>H NMR (<NUM>, CDCl<NUM>) δ ppm <NUM> (d, J = <NUM>, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> -<NUM> (m, <NUM>), <NUM> (q, J = <NUM>, <NUM>), <NUM> (br t, J = <NUM>, <NUM>), <NUM> (br d, J = <NUM>, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> (s, <NUM>), <NUM> - <NUM> (m, <NUM>).

To a solution of tert-butyl <NUM>-(<NUM>-(benzoyloxy)ethyl-<NUM>,<NUM>,<NUM>,<NUM>-d<NUM>)piperidine-<NUM>-carboxylate (<NUM>, <NUM> mmol, <NUM> equiv) in EtOAc (<NUM>) was added HCl in EtOAc (<NUM>, <NUM>) at <NUM>. The mixture was stirred at <NUM> for <NUM> and after confirmation of reaction completion, the reaction was evaporated to dryness and the crude was used directly in the next step.

To a solution of <NUM>-(piperidin-<NUM>-yl)ethyl-<NUM>,<NUM>,<NUM>,<NUM>-d<NUM> benzoate hydrochloride (<NUM>, <NUM> mmol, <NUM> equiv) and EtsN (<NUM>, <NUM> mmol, <NUM>, <NUM> equiv) in THF (<NUM>) at <NUM>-<NUM> was added the crude (E)-<NUM>-ethoxyacryloyl chloride (<NUM>, <NUM> mmol, <NUM> equiv). The reaction was stirred at <NUM>-<NUM> for <NUM>. Upon confirmation of reaction completion, H<NUM>O (<NUM>) was added and resulting aqueous was extracted with three portions of EtOAc (<NUM>). The organic phases were then combined, died over Na<NUM>SO<NUM>, and evaporated to dryness. The residue was then slurried in MTBE (<NUM>) then filtered to afford (E)-<NUM>-(<NUM>-(<NUM>-ethoxyacryloyl)piperidin-<NUM>-yl)ethyl-<NUM>,<NUM>,<NUM>,<NUM>-d<NUM> benzoate (<NUM>, <NUM> mmol, <NUM>% yield) as a yellow solid.

<NUM>H NMR (<NUM>, DMSO-d6) δ ppm <NUM> (br d, J=<NUM>, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> - <NUM>(m, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> (t, J = <NUM>, <NUM>), <NUM> - <NUM> (m, <NUM>).

To a solution of diethyl (<NUM>-benzo[d]imidazol-<NUM>-yl)phosphoramidate (<NUM>, <NUM> mmol, <NUM> equiv) and (E)-<NUM>-(<NUM>-(<NUM>-ethoxyacryloyl)piperidin-<NUM>-yl)ethyl-<NUM>,<NUM>,<NUM>,<NUM>-d<NUM> benzoate (<NUM>, <NUM> mmol, <NUM> equiv) in <NUM>-MeTHF (<NUM>) and MeCN (<NUM>) at <NUM> was added Et<NUM>N (<NUM>, <NUM> mmol, <NUM>µL, <NUM> equiv. ) followed by POCl<NUM> (<NUM>, <NUM> mmol, <NUM>, <NUM> equiv). The reaction was then heated to <NUM> and stirred for <NUM>. Upon confirmation of reaction completion, saturated aqueous NaHCO<NUM> (<NUM>) was charged and the resulting aqueous layer was extracted with three portions of DCM (<NUM>). The organic phases were then combined, died over Na<NUM>SO<NUM>, and evaporated to dryness. The crude was then slurried in EtOAc (<NUM>) then filtered. This slurry process was repeated twice more. The resulting solids were then purified by prep-HPLC to afford <NUM>-(<NUM>-(benzo[<NUM>,<NUM>]imidazo[<NUM>,<NUM>-a]pyrimidin-<NUM>-yl)piperidin-<NUM>-yl)ethyl-<NUM>,<NUM>,<NUM>,<NUM>-d<NUM> benzoate (<NUM>, <NUM> mmol, <NUM>% yield) as a yellow solid.

<NUM>H NMR (<NUM>, DMSO-d6) δ ppm <NUM> (d, J = <NUM>, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> (br t, J = <NUM>, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> - <NUM> (m, <NUM>).

<NUM>C NMR (<NUM>, DMSO-d6) δ ppm <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

To a solution of <NUM>-(<NUM>-(benzo[<NUM>,<NUM>]imidazo[<NUM>,<NUM>-a]pyrimidin-<NUM>-yl)piperidin-<NUM>-yl)ethyl-<NUM>,<NUM>,<NUM>,<NUM>-d<NUM> benzoate (<NUM>, <NUM> mmol, <NUM> equiv) in EtOH (<NUM>) at <NUM> was added 5N NaOH (<NUM>, <NUM> mmol, <NUM> equiv). The reaction was heated at <NUM> for <NUM>. Upon confirmation of reaction completion, H<NUM>O (<NUM>) was added and the aqueous layer was extracted with three portions of DCM (<NUM>). The organic phases were then combined, died over Na<NUM>SO<NUM>, and evaporated to dryness. The crude was then slurried in MTBE (<NUM>) and filtered to afford <NUM>-(<NUM>-(benzo[<NUM>,<NUM>]imidazo[<NUM>,<NUM>-a]pyrimidin-<NUM>-yl)piperidin-<NUM>-yl)ethan-<NUM>,<NUM>,<NUM>,<NUM>-d<NUM>-<NUM>-ol (<NUM>, <NUM> mmol, <NUM>% yield) as a yellow solid.

<NUM>H NMR (<NUM>, DMSO-d6) δ ppm <NUM> (d, J = <NUM>, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (t, J = <NUM>, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> (br s, <NUM>), <NUM> (br t, J = <NUM>, <NUM>), <NUM> - <NUM> (m, <NUM>), <NUM> - <NUM> (m, <NUM>).

13C NMR (<NUM>, DMSO-d6) δ ppm <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

To a solution of <NUM>-(<NUM>-(benzo[<NUM>,<NUM>]imidazo[<NUM>,<NUM>-a]pyrimidin-<NUM>-yl)piperidin-<NUM>-yl)ethan-<NUM>,<NUM>,<NUM>,<NUM>-d<NUM>-<NUM>-ol (<NUM>, <NUM> mmol, <NUM> equiv) and TsOH•H<NUM>O (<NUM>, <NUM> mmol, <NUM> equiv) in MeCN (<NUM>) at <NUM> was charged <NUM>-methylbenzenesulfonic anhydride (<NUM>, <NUM> mmol, <NUM> equiv). The reaction was heated to <NUM> and stirred for <NUM>. Upon confirmation of reaction completion, H<NUM>O (<NUM>) and NMP (<NUM>) were added. A <NUM> w% solution of aqueous K<NUM>PO<NUM> (<NUM>) was then added and the resulting mixture was stirred at <NUM> for <NUM>. After aging, the mixture was filtered and the cake washed with three portions of H<NUM>O (<NUM>). The cake was then slurried in MTBE (<NUM>) and filtered to give <NUM>-(<NUM>-(benzo[<NUM>,<NUM>]imidazo[<NUM>,<NUM>-a]pyrimidin-<NUM>-yl)piperidin-<NUM>-yl)ethyl-<NUM>,<NUM>,<NUM>,<NUM>-d<NUM> <NUM>-methylbenzenesulfonate (<NUM>, <NUM> mmol, <NUM>% yield, <NUM>% purity) as an off-white solid.

<NUM>H NMR (<NUM>, DMSO-d<NUM>) δ <NUM> (d, J = <NUM>, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (t, J = <NUM>, <NUM>), <NUM> (m, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (t, J = <NUM>, <NUM>), <NUM> (t, J = <NUM>, <NUM>), <NUM> (s, <NUM>), <NUM> (m, <NUM>), <NUM> (m, <NUM>), <NUM>(m, <NUM>); <NUM>C NMR (<NUM>, DMSO-d<NUM>) δ <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. HRMS m/z ([M+H]+) calculated for C<NUM>H<NUM>D<NUM>N<NUM>O<NUM>S <NUM>; found <NUM>.

Claim 1:
A method of synthesizing the compound of formula (I), the method comprising:
<CHM>
coupling a compound of formula (II) with a compound of formula (III)
<CHM>
in the presence of POCl<NUM> and Et<NUM>N in a non-aqueous solvent to give a first precursor (I-P1);
<CHM>
removing the benzoate group in I-P1 to give second precursor (I-P2); and
<CHM>
replacing the hydroxyl group in I-P2 by a tosyl group to give compound (I),
wherein, in formulae (I), (II), (I-P1) and (I-P2), the carbon atom labeled (*) is optionally doubly-deuterated,
wherein a deuterium enrichment factor is <NUM>,<NUM> or greater at each deuterium atom.