Isatin-3-Oxime (CAS 607-28-3): Properties, Tautomerism and Its Role in Oxindole Routes

Isatin-3-oxime (CAS 607-28-3) is a small-volume intermediate that appears at a very specific point in synthetic routes: it is the activated, nitrogen-bearing form of isatin, and the shortest path to a 3-amino oxindole. Demand for it is project-driven – one campaign, one customer, one specification – which is why it is rarely stocked in bulk and why the useful question is not “what is your catalogue price” but “can you make this to my specification, reliably, twice”. This article covers what the material is, how it is made, where it goes, and what to put on a specification.

1. Identity: one compound, many names

Nomenclature for this material is unusually scattered, and a buyer searching for it will land on different names depending on which database they use. All of the following refer to the same compound.

Name	Where you meet it
Isatin-3-oxime	The dominant trade name and the one used in most catalogues
Isatin β-oxime / β-isatoxime	Older literature; distinguishes it from the 2-oxime regioisomer
3-(Hydroxyimino)indolin-2-one / 3-hydroxyiminoindolin-2-one	Systematic oxime naming; common in patent examples
1H-Indole-2,3-dione 3-oxime	CAS-indexed systematic name
3-Nitroso-1H-indol-2-ol	The tautomeric form under which the compound is also indexed – see section 2

Formula C₈H₆N₂O₂, molecular weight 162.15. The 3-oxime is the only commonly traded monoxime of isatin; the 2-oxime is a different compound with different reactivity, and confusing the two is a real (and expensive) sourcing error.

2. Tautomerism: the property that governs everything else

Drawn on paper, isatin-3-oxime is a ketoxime. In reality the compound exists in equilibrium with an o-nitrosophenol-type tautomer, 3-nitroso-1H-indol-2-ol, and both descriptions are used as index names. Two visible consequences:

Colour. The material is pale yellow. A pure ketoxime of this size would be expected to be colourless to off-white; the colour comes from the nitroso chromophore of the enol form.
Solvent- and pH-dependent behaviour. The position of the equilibrium shifts with solvent polarity, pH and temperature. This is why two lots that both pass a 99% HPLC assay can still behave differently in a reaction, and why “purity” and “performance” are not the same specification line.

Aspect	Oxime form	Nitroso-enol form
Structure	3-(hydroxyimino)indolin-2-one	3-nitroso-1H-indol-2-ol
Reactive site offered	Oxime oxygen / oxime nitrogen	Enol oxygen (and a nitroso group)
Typical reaction used for	O-acylation or sulfonylation, then rearrangement; reduction to the amine	O-alkylation under basic conditions; metal chelation
Practical implication	Solvent, base and temperature decide which face of the molecule your reagent meets – so process conditions are not transferable between suppliers without re-verification

3. Oxime geometry (E/Z)

Like every oxime, this one exists as E and Z geometric isomers, and the ratio is set by the oximation conditions rather than by the purification step. The (3E) configuration is the one usually encountered and the one most catalogues imply without saying so. If your downstream reaction is sensitive to oxime geometry – and rearrangement chemistry usually is – then E/Z ratio belongs on the specification as a method-defined item, not as an assumption. Ask for the ratio on the first lot and agree how it will be measured and reported.

4. How it is made

The commercial route is oximation of isatin with a hydroxylamine salt. It is a short route, and almost all of the difficulty sits in the work-up and the crystallisation rather than in the reaction itself.

Parameter	What it controls	Typical failure if it is wrong
Hydroxylamine stoichiometry	Conversion and residual isatin	Under-charge leaves isatin in the product; over-charge costs money and complicates work-up
pH of the oximation	Selectivity of oxime formation versus side reactions of the dicarbonyl	Too acidic hydrolyses the oxime back; too basic promotes by-products
Temperature	Rate versus impurity profile	Hotter is faster and dirtier; the ratio of E to Z can shift as well
Crystallisation solvent	Crystal form, colour, filterability, occluded solvent	Wrong solvent gives a slow-filtering solid that traps mother liquor and fails residual-solvent limits
Drying	Water content and colour stability	Over-drying darkens the product; under-drying fails the water specification

This is the reason the material is a custom item rather than a commodity: the customer’s specification is often written against one supplier’s process, and switching source requires re-qualification of the downstream step. That is a switching cost on the buyer’s side and a moat on the supplier’s side, and any honest conversation about it should acknowledge both.

5. Where it goes downstream

Downstream step	Product	Why the oxime is the convenient entry point
Reduction (catalytic hydrogenation, zinc/acetic acid, or iron)	3-Aminooxindole (3-aminoindolin-2-one)	One step from a stable, filterable solid to the amine; avoids handling the free amine as a crude oil
Acylation of 3-aminooxindole	3-Amido oxindole scaffolds	The gateway to oxindole-based kinase inhibitor chemotypes
O-activation then base treatment	Neber-type rearrangement products	Gives α-aminoketone equivalents, a route to ring-functionalised indoles
Condensation with aldehydes and hydrazines	Isatin-derived hydrazones and Schiff bases	Standard isatin chemistry, used extensively as building blocks and as analytical reagents

The single most important downstream application is the route to nintedanib and to other oxindole kinase programmes: an oxindole core carrying a 3-amido substituent. It is worth being clear about the scale reality here – these are kilogram to tens-of-kilogram requirements inside a multi-hundred-tonne pharmaceutical market, which is exactly the profile of a custom intermediate.

6. What belongs on the specification

Item	Why it matters for this material specifically
Appearance	Pale yellow crystal is the expected form; a darker or brownish solid points to over-drying, metal contamination or a shifted tautomer ratio
Assay (HPLC)	The headline figure. 99% min is a normal commercial grade for this intermediate
Residual isatin	Unreacted starting material is the most likely organic impurity and the easiest to control – name it explicitly rather than hoping the assay covers it
Water content	Affects both assay-basis arithmetic and colour stability in storage
Residual solvents	Oximation and recrystallisation solvents must be declared; regulated-market limits are much tighter than “none detected” implies
Heavy metals	Standard for pharmaceutical supply chains; also a useful process-quality signal
E/Z ratio (if relevant)	Not on any catalogue sheet you will see, but decisive for rearrangement chemistry

7. Buying it: five things to settle before the price is meaningful

Quantity and campaign shape – one-off kilograms or a repeat annual requirement. The second case is priced completely differently because it justifies a dedicated batch.
Specification, including the tests a catalogue does not show – residual isatin, E/Z ratio, solvent set. Building this on the first lot is far cheaper than adding it later.
Documentation package – COA and SDS as standard; method statements, third-party testing and a regulatory statement for the destination market as agreed items.
Lead time and how it is protected – for a custom intermediate the lead time is a production slot, so ask what happens if your schedule moves.
Confidentiality – if the specification gives away your route, an NDA is needed before the technical discussion, not after the quotation.

8. Packaging and supply

Supplied as a light yellow crystalline solid in foil bags or fibre drums with an inner liner, and in sealed bottles for laboratory quantities. Store sealed, cool, dry and protected from light – the solid is not aggressively hygroscopic but it is a nitroso-functional material and will darken with light and heat exposure over time. Every lot ships with a COA and SDS, and FOB or CIF terms are quoted against your destination. Because this is a project-driven intermediate, the batch size is agreed against your consumption rather than against a standard pack: shipping a tonne to a customer who needs twenty kilograms does nobody a favour.

9. The honest summary

Isatin-3-oxime is not a product where the cheapest quote is the best decision. The switching cost is in re-qualifying your downstream step, the specification is usually written against one supplier’s material, and the annual volume is small enough that the cost of a failed lot dwarfs the cost of the material. What matters is a supplier who will put residual isatin, solvent set and – where relevant – E/Z ratio on the certificate, and who will still answer the phone for a twenty-kilogram order.