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"Proto" is generally used to describe "first", "foremost" or "earliest form of (something)" indicating something primitive that transforms into something known or common. In chemical terminology, it can be used to denote the first of a series of compounds, or the one containing the minimum amount of an element. But I still cannot understand its usage. Consider below cases:

  1. Protocatechuic acid - it is the common name for 3,4-dihydroxybenzoic acid. So, why not "catechuic acid"? Is there any "catechuic acid" in the first place?
  2. (+)-proto-quercitol - It is one of the stereoisomer of quercitol. Other stereoisomers: https://pubs.acs.org/doi/10.1021/acs.jafc.8b01584. Also including (−)-gala-quercitol: https://pubs.acs.org/doi/10.1021/acsomega.9b02986. Is it a common practice to give such prefixes to stereoisomers?
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    $\begingroup$ For 2, see Fletcher Jr, Hewitt G., Laurens Anderson, and Henry A. Lardy. "The nomenclature of the cyclohexitols and their derivatives." The Journal of Organic Chemistry 16.8 (1951): 1238-1246. $\endgroup$
    – AChem
    May 1, 2023 at 6:57
  • $\begingroup$ For (1), I think the proto- sense is different. OED has a detailed paragraph on the chemical use of proto in at least two senses. I searched "proto- in chemical nomenclature organic compounds" and the first result explains it all (1909) $\endgroup$
    – AChem
    May 1, 2023 at 7:05
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    $\begingroup$ Just a guess, but I think it means "primitive form of" in this sense. The compounds catechin and epicatechin might be regarded as non-carboxylic elaborations of protocatechuic acid. $\endgroup$
    – Buck Thorn
    May 1, 2023 at 9:34
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    $\begingroup$ Catechin is a flavanoid isolated from acacia. I presume protocatechuic acid is the product of oxidative degradation of catechin. Hence, the use of the term proto. $\endgroup$
    – user55119
    May 1, 2023 at 15:54
  • $\begingroup$ According to the Merck Index, catechin is identically catechuic acid. $\endgroup$ May 2, 2023 at 15:23

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Most of the names of organic compounds from the 19th century have little or no structural meanings. Organic chemistry and structural understanding of molecules was very limited at that time. The name protocatechuic acid has a German origin from Protocatechusäure (ca. 1860s), and this also means Catechusäure (catechuic acid) also existed. However, elemental analysis and qualitative tests were well understood then with pretty good accuracy. Imagine, we are in the 1860s, Catechusäure (now catechin) has an elemental analysis of $\ce{C15H14O6}$. Protocatechusäure must be a simpler acid because the elemental composition is $\ce{C7H6O4}$. Testing for acid behaviors was easy.

I assume that the OP is from South Asia, so he might be familiar with paan (betel leaf) which is eaten with lime, kat-tha (an astringent bark extract of a certain Acacia tree), and betel nuts in weddings or other occasions. The word catechu comes from India. Today we know that Catechusäure does not have carboxylic groups, but given its astringent nature, it must be an acid for the early organic chemists.

For the second question, I already gave the reference in the comments. This time is a different era of 1950s, almost 90 years later. Structural understanding has considerably increased. This time, proto- has a specific implication*. See Fletcher Jr, Hewitt G., Laurens Anderson, and Henry A. Lardy. "The nomenclature of the cyclohexitols and their derivatives." The Journal of Organic Chemistry 16.8 (1951): 1238-1246. These authors mainly introduced some of these terms (not all). These include, allo-, cis-, epi-, gala-, muco-, neo-, proto-, scyllo-, talo-, and vibo-.

*See the table here in Wikipedia talk (?) archives under Diastereomerism: https://en.wikipedia.org/wiki/Wikipedia:Reference_desk/Archives/Science/2011_May_31

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