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Ring closure via thermal decarboxylation of 2-(4-methoxyphenyl)thio)benzoic acid to 2-​methoxy-9H-​thioxanthen-​9-​one; 2-​Methoxy-9H-​thioxanthen-​9-​one

SyntheticPage 847
DOI: 10.1039/SP847
Submitted Aug 23, 2018, published Sep 05, 2018
Robert Smith (, Mariyah Sajjad (), Mark Wainwright (, Joshua Gettins ()
A contribution from Smith Group

			Reaction Scheme: <IMG src="/images/empty.gif">Ring closure via thermal decarboxylation of <SPAN id=csm1536238382938 class="csm-chemical-name csm-not-validated" title="2-(4-methoxyphenyl)thio)benzoic acid" grpid="1">2-(4-methoxyphenyl)thio)benzoic acid</SPAN> to <SPAN id=csm1536238388167 class="csm-chemical-name csm-not-validated" title=2-​methoxy-9H-​thioxanthen-​9-​one grpid="2">2-​methoxy-<EM>9H</EM>-​thioxanthen-​9-​one</SPAN><IMG src="/images/empty.gif">

Chemicals Used

2-((4-methoxyphenyl)thio)benzoic acid (prepared in house, see Synthetic Page 846)
Sulfuric acid (Fisher Scientific)
Isopropyl alcohol (Fisher Scientific)


Into a conical flask was charged 2-((4-methoxyphenyl)thio)benzoic acid (2.60 g, 10 mmol) followed by concentrated sulphuric acid (40 mL) in one full portion*. The mixture was heated to 100°C** with constant stirring for 30 minutes. The hot solution was carefully poured over ice (200 g); with vigorous stirring a yellow solid precipitated from the reaction which was isolated by vacuum filtration at the pump and washed further with water (100 mL).  The crude solid was recrystallized in the minimum amount of hot isopropyl alcohol to yield 2-​methoxy-9H-​thioxanthen-​9-​one (0.61 g, 25%) as bright yellow needles.

Author's Comments

*Upon addition of sulphuric acid an exothermic reaction occurs with effervescence.  This subsides with heat and stirring to produce a clear brown liquid.
**Temperature was established and maintained using a thermocouple which was placed into a metal heating block.  Stirring was accomplished using a standard stirring bar.


1H NMR (400 MHz, Chloroform-d) δ 8.65 (d, J = 8.6 Hz, 1H), 8.10 (d, J = 2.9 Hz, 1H), 7.66 – 7.57 (m, 2H), 7.54 – 7.46 (m, 2H), 7.28 (dd, J = 8.9, 3.1 Hz, 1H), 3.96 (s, 3H).13C NMR (101 MHz, CDCl3) δ 179.70, 158.39, 137.53, 132.03, 130.23, 129.88, 129.19, 128.58, 127.31, 126.10, 126.00, 122.78, 110.33, 55.71. LCMS – 242 [M+]

Lead Reference

Org. Synth. 1939, 19, 6 (DOI: 10.15227/orgsyn.019.0006)

Supplementary Information

1H - 40478-82-8.pdf
13C - 40478-82-8.pdf
13C vs 135DEPT - 40478-82-8.pdf
135DEPT - 40478-82-8.pdf
1H 1H COSY - 40478-82-8.pdf
MS - 40478-82-8.pdf

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Get structure file (.cdx, .sk2, .mol)

Keywords: aromatics/arenes, carboxylic acids, dehydration, ketones, thermal

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