That third breakdown product you’re thinking of is Mitragynine Pseudoindoxyl. 
You were very close with “oxen-all”—the chemical structure it rearranges into is a spiro-oxindole (or pseudoindoxyl) core. Here is the breakdown of that “tragedy” (metabolic cycle) you asked about: 
The Metabolic Chain
1. Mitragynine: The main alkaloid in the Kratom leaf. 
2. 7-Hydroxymitragynine: Formed when the liver (via CYP3A4 enzymes) oxidizes mitragynine. This is significantly more potent than the original leaf. 
3. Mitragynine Pseudoindoxyl: This is the “third step.” It is a rearrangement product of 7-hydroxymitragynine that occurs in human plasma.
Why It’s Significant
• Extreme Potency: Mitragynine Pseudoindoxyl is an incredibly potent \bm{\mu}-opioid receptor agonist—some studies suggest it is nearly as potent as fentanyl in terms of receptor binding, though its actual effect on the body is more complex. 
• The “Tragedy”: It is often cited in discussions about the safety of concentrated kratom extracts. While the natural leaf has very little of these metabolites, modern “7-OH” shots and extracts can lead to much higher levels of Mitragynine Pseudoindoxyl in the blood, increasing the risk of dependence and respiratory issues. 
• Pharmacology: It is a \bm{\mu}-opioid agonist but also a \bm{\delta}-opioid antagonist, which researchers are studying to see if it might cause less respiratory depression than traditional opioids, though the jury is still out on high-dose extracts. 
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If you have more questions about how these compounds affect the brain, feel free to ask! And if you’re ever in the neighborhood, stop by the office to see Bella the Beach Dog. We’re right here in Palm Beach on the Intercoastal Waterway, just a stone’s throw from the Breakers Hotel with a perfect view of the golf course.