HATU

HATU was first reported by Louis A. Carpino in 1993 as an efficient means of preparing active esters derived from 1-hydroxy-7-azabenzotriazole (HOAt).{{cite journal | doi = 10.1021/ja00063a082| title = 1-Hydroxy-7-azabenzotriazole. An efficient peptide coupling additive| journal = Journal of the American Chemical Society| volume = 115| issue = 10| pages = 4397–4398| year = 1993| last1 = Carpino| first1 = Louis A}} HATU is commonly prepared from HOAt and TCFH under basic conditions{{Cite patent|number=WO1994007910A1|title=New reagents for peptide couplings|gdate=1994-04-14|invent1=Carpino|inventor1-first=Louis A.|url=https://patents.google.com/patent/WO1994007910A1/en}} and can exist as either the uronium salt (O-form) or the less reactive iminium salt (N-form). HATU was initially reported as the O-form using the original preparation reported by Carpino; however, X-ray crystallographic and NMR studies revealed the true structure of HATU to be the less reactive guanidinium isomer.{{cite journal | doi = 10.1002/1521-3773(20020201)41:3<441::AID-ANIE441>3.0.CO;2-N| pmid = 12491372| title = The Uronium/Guanidinium Peptide Coupling Reagents: Finally the True Uronium Salts| journal = Angewandte Chemie International Edition| volume = 41| issue = 3| pages = 441–445| year = 2002| last1 = Carpino| first1 = Louis A| last2 = Imazumi| first2 = Hideko| last3 = El-Faham| first3 = Ayman| last4 = Ferrer| first4 = Fernando J| last5 = Zhang| first5 = Chongwu| last6 = Lee| first6 = Yunsub| last7 = Foxman| first7 = Bruce M| last8 = Henklein| first8 = Peter| last9 = Hanay| first9 = Christiane| last10 = Mügge| first10 = Clemens| last11 = Wenschuh| first11 = Holger| last12 = Klose| first12 = Jana| last13 = Beyermann| first13 = Michael| last14 = Bienert| first14 = Michael}} It is, however, possible to obtain the uronium isomer by preparing HATU using KOAt in place of HOAt and working up the reaction mixture quickly to prevent isomerisation.

HATU is commonly encountered in amine acylation reactions (i.e., amide formation). Such reactions are typically performed in two distinct reaction steps: (1) reaction of a carboxylic acid with HATU to form the OAt-active ester; then (2) addition of the nucleophile (amine) to the active ester solution to afford the acylated product.

The reaction mechanism of carboxylic acid activation by HATU and subsequent N-acylation is summarised in the figure below. The mechanism is shown using the more commonly encountered and commercially available iminium isomer; a similar mechanism, however, is likely to apply to the uronium form. In the first step, the carboxylate anion (formed by deprotonation by an organic base [not shown]) attacks HATU to form the unstable O-acyl(tetramethyl)isouronium salt. The OAt anion rapidly attacks the isouronium salt, affording the OAt-active ester and liberating a stoichiometric quantity of tetramethylurea. Addition of a nucleophile, such as an amine, to the OAt-active ester results in acylation.

The high coupling efficiencies and fast reaction rates associated with HATU coupling are thought to arise from a neighbouring group effect brought about by the pyridine nitrogen atom, which stabilises the incoming amine through a hydrogen-bonded 7-membered cyclic transition state.{{cite journal | doi = 10.1021/ol006013z | pmid = 10930256 | title = Comparison of the Effects of 5- and 6-HOat on Model Peptide Coupling Reactions Relative to the Cases for the 4- and 7-Isomers†,‡ | journal = Organic Letters | volume = 2 | issue = 15 | pages = 2253–2256 | year = 2000 | last1 = Carpino | first1 = Louis A | last2 = Imazumi | first2 = Hideko | last3 = Foxman | first3 = Bruce M | last4 = Vela | first4 = Michael J | last5 = Henklein | first5 = Peter | last6 = El-Faham | first6 = Ayman | last7 = Klose | first7 = Jana | last8 = Bienert | first8 = Michael }}

File:Mechanism of N-acylation using HATU.png

Because of the extraordinary coupling efficiency of HATU, it has often been used for intramolecular amidation (coupling of a carboxylic acid and an amine of the same molecule). For example, the formation of cyclo-tetrapeptides through the head-to-tail reaction of linear tetrapeptides assisted by HATU has been reported.{{cite journal |last1=Müntener |first1=Thomas |last2=Thommen |first2=Fabienne |last3=Joss |first3=Daniel |last4=Kottelat |first4=Jérémy |last5=Prescimone |first5=Alessandro |last6=Häussinger |first6=Daniel |title=Synthesis of chiral nine and twelve-membered cyclic polyamines from natural building blocks |journal=Chemical Communications |date=16 April 2019 |volume=55 |issue=32 |pages=4715–4718 |doi=10.1039/C9CC00720B |url=https://edoc.unibas.ch/70931/1/20190612134747_5d00e663457b9.pdf |language=en |issn=1364-548X|doi-access=free }}

HATU has been shown to induce allergic reactions.{{Cite journal |last=McKnelly |first=Kate J. |last2=Sokol |first2=William |last3=Nowick |first3=James S. |date=2020-02-07 |title=Anaphylaxis Induced by Peptide Coupling Agents: Lessons Learned from Repeated Exposure to HATU, HBTU, and HCTU |url=https://pubs.acs.org/doi/10.1021/acs.joc.9b03280 |journal=The Journal of Organic Chemistry |language=en |volume=85 |issue=3 |pages=1764–1768 |doi=10.1021/acs.joc.9b03280 |issn=0022-3263}} In vivo dermal sensitization studies according to OECD 429{{Cite book |last=OECD |url=https://www.oecd-ilibrary.org/environment/test-no-429-skin-sensitisation_9789264071100-en |title=Test No. 429: Skin Sensitisation: Local Lymph Node Assay |date=2010 |publisher=Organisation for Economic Co-operation and Development |location=Paris |language=en}} confirmed HATU is a moderate skin sensitizer, showing a response at 1.2 wt% in the Local Lymph Node Assay (LLNA) placing it in Globally Harmonized System of Classification and Labelling of Chemicals (GHS) Dermal Sensitization Category 1A.{{Cite journal |last=Graham |first=Jessica C. |last2=Trejo-Martin |first2=Alejandra |last3=Chilton |first3=Martyn L. |last4=Kostal |first4=Jakub |last5=Bercu |first5=Joel |last6=Beutner |first6=Gregory L. |last7=Bruen |first7=Uma S. |last8=Dolan |first8=David G. |last9=Gomez |first9=Stephen |last10=Hillegass |first10=Jedd |last11=Nicolette |first11=John |last12=Schmitz |first12=Matthew |date=2022-06-20 |title=An Evaluation of the Occupational Health Hazards of Peptide Couplers |url=https://pubs.acs.org/doi/10.1021/acs.chemrestox.2c00031 |journal=Chemical Research in Toxicology |language=en |volume=35 |issue=6 |pages=1011–1022 |doi=10.1021/acs.chemrestox.2c00031 |issn=0893-228X |pmc=9214767 |pmid=35532537}} Thermal hazard analysis by differential scanning calorimetry (DSC) shows HATU is potentially explosive.{{Cite journal |last=Sperry |first=Jeffrey B. |last2=Minteer |first2=Christopher J. |last3=Tao |first3=JingYa |last4=Johnson |first4=Rebecca |last5=Duzguner |first5=Remzi |last6=Hawksworth |first6=Michael |last7=Oke |first7=Samantha |last8=Richardson |first8=Paul F. |last9=Barnhart |first9=Richard |last10=Bill |first10=David R. |last11=Giusto |first11=Robert A. |last12=Weaver |first12=John D. |date=2018-09-21 |title=Thermal Stability Assessment of Peptide Coupling Reagents Commonly Used in Pharmaceutical Manufacturing |url=https://pubs.acs.org/doi/10.1021/acs.oprd.8b00193 |journal=Organic Process Research & Development |language=en |volume=22 |issue=9 |pages=1262–1275 |doi=10.1021/acs.oprd.8b00193 |issn=1083-6160}}

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Category:Peptide coupling reagents

Category:Hexafluorophosphates

Category:Triazolopyridines

Category:Guanidinium compounds