Highlights of aliphatic C(sp3)-H hydrogen isotope exchange reactions

This review summarizes the highlights of aliphatic C (sp³)-H carbon hydrogen isotope exchange (HIE) methods developed in the last 10 years. In particular, new highly selective and reactive protocols in the areas of nanoparticle and metal-catalyzed homogeneous catalysis are reported.     

Burgess iridium(I)‐catalyst for selective hydrogen isotope exchange

We have evaluated the commercially available Burgess catalyst in hydrogen isotope exchange reactions with several substrates bearing different directing group functionalities and have obtained moderate to high (50%‐97%D) deuterium incorporations. The broad applicability in hydrogen isotope exchange reactions makes the Burgess catalyst a possible alternative compared to other commercially available iridium(I)‐catalysts.     

Proton exchange reactions in isotope chemistry (I)

Direct H–D exchange reactions were applied to the preparations of stable isotope‐labeled TKI258 and two TKI258 metabolites. Each compound was made in one single H–D exchange reaction with excellent isotope incorporation. The number of deuterium incorporation and deuterium distribution in the molecules was similar in all three compounds. Stable isotope‐labeled TKI258 was also prepared from d₈‐methylpiperazine in a multistep synthesis.     

Recent advances in iridium(I) catalysis towards directed hydrogen isotope exchange

The initial discovery and establishment of a family of novel iridium catalysts possessing N-heterocyclic carbene units alongside bulky phosphine ligands allowed selected substrates to be labelled using deuterium or tritium gas at desirably low catalyst loadings via an ortho-directed C―H insertion process. Such a method has broad applicability and offers distinct advantages within the pharmaceutical industry, directly facilitating the ability to carefully monitor a potential drug molecule's biological fate. Over the past decade since these initial protocols were divulged, many additional advances have been made in terms of catalyst design and substrate scope. This review describes the broadened array of new iridium catalysts and associated protocols for direct and selective C―H activation and hydrogen isotope insertion within a number of new chemical entities of direct relevance to the pharmaceutical industry.     

An improved bidentate complex of iridium as a catalyst for hydrogen isotope exchange

Iridium(I) complexes 1 , containing bidentate phosphines, and 3 , with arsine ligands, are generated in situ. These species mediate hydrogen isotope exchange in a variety of aromatic substrates including benzyl ketones. Although the catalytic activities of complexes 1 and 3 are generally unexceptional, a logical step leads to the use of [ethylene‐1,2‐bis(diphenylarsine)](cyclooctadiene)iridium(I) tetrafluoroborate ( 5 ), which is an efficient catalyst for both aryl and benzyl ketones, and mediates exchange to a substantial extent in other substrates also. Copyright © 2005 John Wiley & Sons, Ltd.     

Evaluation of a P,N‐ligated iridium(I) catalyst in hydrogen isotope exchange reactions of aryl and heteroaryl compounds

We have developed a novel and efficient iridium‐catalyzed hydrogen isotope exchange reaction method with secondary and tertiary sulfonamides at ambient temperatures. Furthermore N‐oxides and phosphonamides have been successfully applied in hydrogen isotope exchange reactions with moderate to excellent deuterium introduction.     

The effect of adding Crabtree's catalyst to rhodium black in direct hydrogen isotope exchange reactions

A new catalytic system based on rhodium black using Crabtree's catalyst as an additive for direct hydrogen isotope exchange in aromatic compounds has been investigated. The level of deuterium incorporation can be improved from for example 16 to 93%. The new catalyst mixture tolerates a variety of solvents. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis of [3H] and [2H6]AZD6642, an inhibitor of 5‐lipoxygenase activating protein (FLAP)

An AstraZeneca effort to identify a 5‐lipoxygenase activating protein inhibitor with good drug‐like properties resulted in the identification of AZD6642. To further understand its drug metabolism and pharmacokinetic properties, it was required labeled with tritium. The tritiation of AZD6642 was effected by Ir‐catalyzed exchange chemistry to give an average of one tritium per molecule. Additionally, a stable isotope labeled version of AZD6642 was required to support bioanalytical studies. The synthesis originated from [²H₆]acetone which was converted to the trimethylsilyl cyanide adduct and subsequently reduced to give 2‐(aminomethyl)‐[1,1,1,3,3,3‐²H₆]propan‐2‐ol in good yield. Carbonylation to give an amide adduct resulted in an intermediate that was converted to the final compound in four steps.     

Proton exchange reactions in isotope chemistry (II) synthesis of stable isotope‐labeled LCQ908

The proton exchange reaction was applied to the preparation of stable isotope‐labeled LCQ908. For this synthesis, a suitable intermediate with protons alpha to a carbonyl group was first subjected to the H–D exchange reaction; subsequent coupling of a carbonyl group with [¹³C₂]triethyl phosphonoacetate, followed by hydrogenation and hydrolysis, led to the stable labeled compound. Incorporation of two carbon‐13 atoms in the molecule eliminated the presence of undesired M+0.     

Synthesis of tritiated Cyclosporin A and FK‐506 by metal‐catalyzed hydrogen isotope exchange

Tritium‐labeled Cyclosporin A ( I ) and FK‐506 ( II ) have been prepared using a metal‐catalysed hydrogen isotope exchange procedure and high specific activity tritiated water. Specific activities of the labeled compounds were 0.15 and 0.59 TBq/mmol, respectively. Copyright © 2004 John Wiley & Sons, Ltd.     

Synthesis and characterization of [N‐methyl‐3H]loperamide

Loperamide is a piperidine butyramide mu‐opiate receptor agonist and currently employed to treat diarrhea. Because a single past report of tritiating loperamide was limited to only a very low specific activity product without technical details or extensive analysis, the synthesis of [N‐methyl‐³H]loperamide at high specific activity is now described in detail. An imine precursor was alkylated with [³H]methyl iodide to obtain a quaternary intermediate, which was then reacted with 4‐(4‐chlorophenyl)‐4‐hydroxypiperidine to afford the desired product [N‐methyl‐³H]loperamide, characterized by thin layer chromatography (TLC), HPLC, MS, UV, and proton‐decoupled tritium NMR.     

Synthesis of [3H] and [14C]genipin

[³H]Genipin was synthesized in a single step by Ir(I) catalyzed hydrogen isotope exchange. Conditions for selective exchange of the sp² CH bond ortho to the methyl ester functionality were developed through deuterium modeling studies through a catalyst screen. Optimized conditions so obtained were then utilized with tritium gas to generate [³H]genipin at a specific activity of 18.5 Ci/mmol. Racemic [¹⁴C]genipin was prepared in eight steps in overall 5.4% radiochemical yield from potassium [¹⁴C]cyanide.     

Synthesis of [3H] vardenafil,Levitra®, using a new labeling technique

Tritium was introduced into the new orally active, selective phosphodiesterase type V (PDE V) inhibitor vardenafil (Levitra^®), by reduction of a suitable amide precursor with freshly prepared lithium aluminum tritide. A specific activity of 52.7 Ci/mmol (1.95 TBq/mmol) was achieved. In order to overcome the usual technical difficulties during the preparation of complex tritides a new and easy labeling technique which has considerable potential for various tritia‐tion procedures, was developed. Copyright © 2003 John Wiley & Sons, Ltd.     

Asymmetric [3H]‐labeling using ruthenium catalyzed transfer hydrogenation

The first general method for asymmetric tritiation using ruthenium catalyzed transfer hydrogenation and its application to the radiosynthesis of an exemplary prostaglandin EP4 receptor‐selective Prostaglandin E₂ analog is described. The methodology should be of general utility for the enantioselective preparation of α‐tritiated chiral secondary alcohols. Copyright © 2010 John Wiley & Sons, Ltd.     

Meta‐substituent effects on organoiridium‐catalyzed ortho‐hydrogen isotope exchange

Reports in the literature appear to differ on the effects of some C3 substituents on the relative efficiencies of isotope exchange in the nonidentical C2‐ and C6‐positions catalyzed by organoiridium complexes. Controlled experiments were conducted using a set of model substrates in attempts to clarify these effects. The results clearly showed that, in common with most previous findings, alkyl substituents at C3 reduced the rate of isotope incorporation into C2 relative to C6, as expected on steric grounds. In contrast, all substituents possessing electron lone pairs resulted in a lessening of the inhibition of C2‐vs‐C6 labeling or promoted C2 labeling to such a degree that it became faster than that at C6. NMR measurements on equimolar mixtures of active iridium complex with selected substrates revealed that the ratios of C2‐ and C6‐iridacycles present in solution correlated with the relative rates of ortho‐deuteration in the rate studies. The results of the two studies, taken together, suggest that conventional explanations for the origin of the positive meta‐effect may not be adequate for the present system. An alternative hypothesis is advanced. Copyright © 2009 John Wiley & Sons, Ltd.     

3H]Dihydroergocryptine binding to alpha-adrenergic receptors of human platelets. A reassessment using the selective radioligands [3H]prazosin, [3H]yohimbine, and [3H]rauwolscine

Which subtype(s) of the alpha-adrenergic receptor occurs on human platelets? Studies of platelet responsiveness to adrenergic compounds and indirect radioligand binding studies addressing this question have yielded contradictory conclusions. These binding studies employed the ligand [³H]dihydroergocryptine ([³H]DHE), an alpha-adrenergic antagonist that does not select between alpha₁- and alpha₂-adrenergic receptors and that also binds to other receptor types in some tissues. To determine the subtype of the platelet alpha-adrenergic receptor, we have examined the binding to intact human platelets of [³H]prazosin (alpha₁-selective), [³H]yohimbine (alpha₂-selective), and [³H]rauwolscine (alpha₂-selective), and we have compared the binding of these selective radioligands with that of [³H]DHE. [³H]Yohimbine and [³H]rauwolscine both bound with high affinity (K_D = 2.7 and 4.6 nM, respectively) to an equal number and a single class (Hill coefficient ～1.0) of sites (～300 per platelet), but [³H]yohimbine yielded lower nonspecific binding than did [³H]rauwolscine. In paired experiments, [³H]DHE bound to 1.5 times as many (phentolamine-displaceable) sites as did [³H]yohimibine or [³H]rauwolscine. Unlabeled vohimbine and epinephrine competed for fewer [³H]DHE binding sites than did phentolamine. Thus, in addition to binding to the alpha₂-adrenergic receptors identified by [³H]yohimbine and [³H]rauwolscine, [³H]DHE seems to bind to other sites on human platelets. The nature of these sites is not clear. We found that [³H]prazosin did not identify alpha₁-adrenergic receptors on platelets, and that phenoxybenzamine only inhibited [³H]yohimbine and [³H]DHE binding at higher concentrations than usually observed for alpha₁-adrenergic receptors. We conclude that (1) all alpha-adrenergic sites on human platelets are of the alpha₂ subtype, (2) [³H]DHE may bind to additional, as yet ill-defined, sites in addition to those sites identified by [³H]yohimbine and [³H]rauwolscine, and (3) [³H]yohimbine is the preferred antagonist radioligand for studying the alpha₂-adrenergic receptors on human platelets.     

Synthesis of 2-deoxy-D-glucose [2,6,6′-3H] with high specific activity

A new tritiation method was used to obtain 2-deoxy-D-glucose [2,6,6¹-³H] starting from a mixture of protected bromo deoxy glucose derivatives. Intermediates were characterized by ¹H- and ³H-NMR.     

Synthesis of [3α‐3H] 17α‐Hydroxy pregnenolone and [3α‐3H] Pregnenolone

For the first time, [3α‐³H] 17α‐hydroxy pregnenolone (1) was synthesized through a multiple step sequence. The presence of [3β‐³H] isomer in RP‐HPLC purified product was identified by tritium NMR. The [3β‐³H] isomer was then separated from [3α‐³H] 17α‐hydroxy pregnenolone with chiralPAK AD‐H column. [3α‐³H] pregnenolone (2) was synthesized from commercial available 5‐pregnen‐3,20‐dione in one step with an improved procedure.     

Synthesis of [3H]fenobam,a radioligand for the mGlu5 receptor

Fenobam is a clinically efficacious anxiolytic that acts as metabotropic glutamate receptor 5 (mGlu5) antagonist by binding to an allosteric site. Other mGlu5 receptor antagonists such as MPEP and MTEP bind to the same allosteric site and are efficacious in preclinical models of anxiety and depression. Consequently, the allosteric‐binding site of the mGlu5 receptor is an attractive target for the discovery of novel psychiatric therapies. Radioligands of this binding site can be used for in vitro and in vivo pharmacodynamic studies. We report here a short synthesis of such a radioligand for the allosteric mGlu5 receptor‐binding site, [³H]fenobam. Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis of N-acetyl-l-aspartyl-l-glutamic acid; [3H]NAAG

[³H]NAAG, N-acetyl-l -aspartyl-l -glutamic acid, has been widely used as a substrate in glutamate carboxypeptidase II (GCPII) reactions, either to determine the inhibitory constants at 50% inhibition (IC₅₀) of novel compounds or to measure GCPII activities in different tissues harvested from various disease models. The importance of [³H]NAAG, combined with its current commercial unavailability, prompted the development of a reliable eight-step synthetic procedure towards [³H₂]NAAG starting from commercially available pyroglutamate. Pure [³H]NAAG of high molar activity (49.8 Ci/mmol) and desired stereochemistry was isolated in high radiochemical yield (67 mCi) and radiochemical purity (>99%). The identity was confirmed by mass spectrometry and co-injection with unlabeled reference.