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Benzyl (2S,3R)-2-((tert-butoxycarbonyl)amino)-3-hydroxy-3-(4-methoxyphenyl)propanoate (CAS 1629681‑57‑7) is a non‑natural β‑hydroxy‑α‑amino acid derivative with well‑defined stereochemistry. The molecule features a Boc‑protected amino group, a benzyl ester, and a 4‑methoxyphenyl moiety — offering orthogonal protection and versatile synthetic handles.

Our internally designated N-[(S)-(2,3,4,5,6-pentafluorophenoxy)phenoxyphosphinyl]-L-alaninel-Methylethyl ester (CAS: 1334513-02-8), holds multiple identities in pharmaceutical chemistry: it is both Sofosbuvir Impurity 75 and the core Sofosbuvir Intermediate 5, as well as a key reactant. Although it does not directly face patients like the finished drug, it is an indispensable precision gear in the entire Sofosbuvir synthesis pathway, directly impacting the quality and safety of the final API.

This molecule looks nearly identical to its more common isomer, 2-Methylbenzothiazole-7-Carboxylic Acid. Both share the same molecular formula (C₉H₇NO₂S) and the same molecular weight (193.22). But they differ in one critical detail: the location of the carboxylic acid group. Here, the –COOH is attached at the 7‑position (carbon atom adjacent to the ring fusion point), whereas the more familiar isomer places it at the 6‑position. This small change in connectivity creates a distinct physicochemical profile — a predicted pKa of 2.34±0.10 (significantly lower than the 3.61 pKa of the 6‑isomer, indicating a more acidic carboxyl group) — and opens up different opportunities in drug discovery and chemical biology.

C20-OtBu-Glu(OtBu)-AEEA-AEEA-OH is the long‑chain fatty acid side chain used in the synthesis of Tirzepatide (a GIP/GLP‑1 dual agonist for type 2 diabetes and obesity) and related peptide therapeutics. Its full IUPAC name is (S)‑22‑(tert‑butoxycarbonyl)‑45, 45‑dimethyl‑10,19,24,43‑tetraoxo‑3, 6, 12, 15, 44‑pentaoxa‑9, 18,23‑triazahexatetracontanoic acid, but it is more commonly referred to by its abbreviated structure: C20‑OtBu‑Glu(OtBu)‑AEEA‑AEEA‑OH (molecular formula C₄₅H₈₃N₃O₁₃, MW 874.15). The molecule consists of four structural modules: (i) a C20 fatty diacid chain terminated with a tert‑butyl ester (OtBu) at the distal end; (ii) a L‑glutamic acid residue with its side‑chain carboxylate protected as an OtBu ester; (iii) two consecutive AEEA (2‑(2‑(2‑aminoethoxy)ethoxy)acetic acid) PEG2 spacers; and (iv) a free terminal carboxylic acid.

A single‑molecular‑weight, heterobifunctional PEG linker – this compound contains Aeea-Aeea ‑like units joined through an internal amide bond, providing a terminal primary amine (–NH₂) and a free carboxylic acid (–COOH) at opposite ends. The exact structure (C₁₂H₂₄N₂O₇) incorporates five ethylene glycol units in total, giving a hydrophilic spacer length of approximately 2.0–2.5 nm. Unlike polydisperse PEG mixtures, this discrete molecule offers a well‑defined mass (308.33 g/mol), a sharp melting point (133–135 °C), and predictable conjugation chemistry, making it a preferred building block for ADCs, PROTACs, and bioconjugation.

2-[2-[2-(Boc-amino)ethoxy]ethoxy]acetic Acid DCHA is the dicyclohexylamine (DCHA) salt of Boc-NH-PEG2-CH₂COOH — a Boc-protected, PEG2-spaced amino acid building block. The molecular formula is C₂₃H₄₄N₂O₆ (MW 444.61), and it presents as a white to off-white crystalline powder. The structure combines three functional elements: a tert-butoxycarbonyl (Boc)-protected primary amine, a diethylene glycol (PEG2) spacer, and a terminal acetic acid (–CH₂COOH), paired with one equivalent of dicyclohexylamine (DCHA). Unlike the free acid form, which is often a viscous oil or sticky semi-solid, the DCHA salt is a crystalline, free-flowing powder that is stable, easy to weigh, and convenient to handle on a laboratory scale.