Naphthoresorcinol (Old-timey tests)

Before the advent of modern spectrometry techniques (NMR and mass spectrometry), there was a compendium of tests to suss out what sort of things were hanging off a molecule. You took your stuff, added some eye of newt, and if black (but not white) soot rose up, you knew you had an arylamine (or at least had some evidence you did). You still see them occasionally in sophomore Organic chemistry labs, but they're going by the wayside, too.

One such reagent was naphthoresorcinol:

i-dd679b800614484ef3c580753b4b418f-naphthoresorcinol.png

This one will give you a red adduct with an aldehyde in acidic solution. Anyone know the pathway or care to guess??

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I can't quite figure it out, but I figure you're looking for a more conjugated system, and that is reached through the hydroxyl attack on the aldehyde, converting it to a hemiacetal, and then the acidic solution catalyzes conversion to the acetal. Unfortunately, this doesn't seem to be any more strongly conjugated a system than what you start with.

My guess is that the red product is 9-hydroxy-dibenzo[a,j]xanthen-5-one with the R substituent (from the aldehyde) in the 14 position.

The naphtoresoscinol has extremely reactive the 4 position (even more so than naphtols). It behaves more like a dimedone or barbituric acid. You get electrophilic Ar substitution into the 4 position(o,p- to the phenol OH), the produced benzyl alcohol dehydrates and the quinometide adds a second molecule of naphtoresorscine. The resulting di-(4-naphtoresorcinyl)CHR cyclizes and oxidizes into a pentacyclic dibenzoxanthenone dye.