Question 11 of 13: Total Synthesis of Proparacaine

Question 11: Total Synthesis of Proparacaine

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Target check. Proparacaine (proxymetacaine) is 2-(diethylamino)ethyl 3-amino-4-propoxybenzoate — a benzoate ester bearing NH₂ meta to the ester and n-PrO para to the ester (ortho to the NH₂). Working forward from 4-hydroxybenzoic acid (CO₂H at C1, OH at C4, para) must therefore install NO₂ (later reduced to NH₂) at C3, and convert the C4–OH into the C4–OPr ether — exactly the two ring transformations available from steps 1→2 and 2→3.

1. 1 → 2: nitration (HNO₃) — the –OH directs, and both groups agree.
   1: 4-hydroxybenzoic acid

   2: 3-nitro-4-hydroxybenzoic acid
   –OH is a strong, activating ortho/para director; –CO₂H is a deactivating meta director. On this para-disubstituted ring the position ortho to OH (C3, or equivalently C5 by the ring's own symmetry) is simultaneously meta to CO₂H — the two groups' preferences reinforce rather than compete, giving one clean, dominant product. Mechanism: HNO₃/H⁺ generates NO₂⁺, which attacks C3; the resulting arenium (Wheland) intermediate is stabilised by direct oxygen lone-pair donation from the OH at C4 into the ring, and loss of H⁺ restores aromaticity, giving 3-nitro-4-hydroxybenzoic acid (2).
2. 2 → 3: Williamson ether synthesis (base, n-PrCl) — alkylates the phenol only.
   3: 3-nitro-4-propoxybenzoic acid
   A mild base deprotonates the acidic phenolic –OH (pK_a≈10) to the phenoxide, which performs a clean S_N2 displacement of chloride from n-PrCl. Neither the carboxylic acid nor the nitro group is affected under these conditions, giving 3-nitro-4-propoxybenzoic acid (3).
3. 3 → 4: SOCl₂ — acid to acid chloride.
   4: the acid chloride
   Standard nucleophilic acyl substitution: the carboxylic acid oxygen attacks sulfur, forming a chlorosulfite leaving group in situ; chloride then displaces it at the carbonyl carbon, expelling SO₂ and HCl (irreversible), giving acid chloride 4.
4. Ethylene oxide + Et₂NH → 5: uncatalysed S_N2 epoxide opening.
   ethylene oxide

   diethylamine

   5: 2-(diethylamino)ethanol
   Diethylamine is nucleophilic enough to open the strained epoxide directly (ring-strain release is the driving force), attacking either (equivalent) carbon by backside S_N2, giving 2-(diethylamino)ethanol (5) with no acid catalyst needed.
5. 5 + 4 → 6: ester formation (nucleophilic acyl substitution).
   6: the nitro ester
   The alcohol oxygen of 5 attacks the electrophilic carbonyl carbon of acid chloride 4, forming a tetrahedral intermediate that collapses by expelling chloride, giving ester 6, the nitro-ester.
6. 6 → proparacaine: H₂/Pd/C, chemoselective nitro reduction.
   proparacaine
   Heterogeneous catalytic hydrogenation reduces aromatic nitro groups stepwise on the catalyst surface (ArNO₂→ArNO→ArNHOH→ArNH₂+H₂O), one of the most facile transformations available under H₂/Pd — far faster than any competing reduction of the ester carbonyl or the tertiary amine (both of which are untouched under these mild conditions; reducing an ester would require a much harsher hydride such as LiAlH₄). This gives proparacaine directly.