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Synlett 2010(15): 2240-2243  
DOI: 10.1055/s-0030-1258032
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Photodecarboxylative Additions of α-Thioalkyl-Substituted Carboxylates to Alkyl Phenylglyoxylates

Su Bee Tana, Oksana Shvydkiva, Jana Fiedlera, Fadi Hatouma, Kieran Nolana, Michael Oelgemöller*b
a Dublin City University, School of Chemical Sciences and NCSR, Glasnevin, Dublin 14, Ireland
b James Cook University, School of Pharmacy and Molecular Sciences, Townsville, Queensland 4811, Australia
Fax: +61(7)47816078; e-Mail: michael.oelgemoeller@jcu.edu.au;
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Publication History

Received 25 May 2010
Publication Date:
12 August 2010 (online)

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Abstract

Irradiations of alkyl phenylglyoxylates with sulfur-containing carboxylates yielded the corresponding photodecarboxylative addition products in moderate to good yields of 26-58%. Reductive photodimerization competed with decarboxylative addition in all cases. The reaction protocol was successfully transferred to a microreactor. With potassium 2-(methylsulfanyl)propionate, photoadditions gave diastereomeric mixtures with low selectivity for the like-isomer.

Key words

photodecarboxylation - phenylglyoxolates - photochemistry - photoinduced electron transfer, microreactor

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General Procedure for Irradiation
The alkyl phenylglyoxylate (1.5 mmol) was dissolved in MeCN (50 mL). A solution of the potassium carboxylate (4.5 mmol) in H2O (50 mL) was added, and the mixture was irradiated (Rayonet Photochemical Reactor RPR-200; λ = 350 ± 30 nm) at 15-20 ˚C in a Pyrex tube (λ ³ 300 nm) while purging with a slow stream of nitrogen. The progress of the reaction was monitored by TLC analysis or by passing the departing gas stream through a sat. Ba(OH)2 solution until precipitation of BaCO3 had ceased. Most of the MeCN was evaporated, and the remaining solution was extracted with EtOAc (4 × 25 mL). The combined organic layers were washed with 5% NaHCO3 (1 × 25 mL) and brine (1 × 25 mL), dried over MgSO4, and evaporated. The products were purified by flash column chromatography (eluent: n-hexane-EtOAc = 5:1).
Selected Physical and Spectral Data for the Product Methyl-2-(1,3-dithian-2-yl)-2-hydroxy-2-phenylacetate (4e)
Yellowish solid, mp 104-106 ˚C. R f = 0.39 (SiO2, n-hexane-EtOAc = 5:1). ¹H NMR (400 MHz, acetone-d 6): δ = 1.84 (m, 2 H, CH2), 2.40-2.46 (m, 1 H, SCH2), 2.56-2.62 (m, 1 H, SCH2), 3.02-3.09 (m, 1 H, SCH2), 3.19-3.26 (m, 1 H, SCH2), 3.61 (s, 3 H, OCH3), 4.48 (s, 1 H, CH), 5.14 (s, 1 H, OH), 7.14-7.24 (br m, 3 H, Harom), 7.56 (dd, ³ J = 8.4 Hz, 4 J = 1.6 Hz, 2 H, Harom) ppm. ¹³C NMR (100 MHz, CDCl3): δ = 25.1 (t, 1 C, CH2), 28.0 (t, 1 C, SCH2), 28.3 (t, 1 C, SCH2), 50.4 (d, 1 C, CH), 53.9 (q, 1 C, OCH3), 85.1 (s, 1 C, COH), 126.0 (d, 2 C, CHarom), 128.3 (d, 1 C, CHarom), 128.4 (d, 2 C, CHarom), 139.1 (s, 1 C, Cqarom), 173.8 (s, 1 C, C=O) ppm. IR (KBr): ν = 3490, 2953, 2925, 2892, 1725, 1239, 733, 692 cm-¹. MS (EI, 70 eV): m/z (%) = 284 (<1) [M+], 119 (100) [M+ - H2O], 105 (12), 91 (4), 77 (14), 45 (5).

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The dwell-reactor is made out of Foturan glass (λ ³ 300 nm) and has a total path length of 1.15 m (20 turns) on a 118 mm × 73 mm aperture. The reactor consisted of a(bottom) serpentine reaction channel 0.5 × 2 mm (D × W), with a second(top), heat-exchanging channel through which water is passed in order to control the reactor temperature. The degassed reaction mixture (20 mL)8 was pumped through the reaction channel via a syringe pump and collected in a test tube.

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Irradiation of 2 in benzene and in the presence of 5 equiv of Me2S gave the corresponding addition product 5a in a yield of 35%. Selectivity (7 vs. 5a) was determined as 60:40. Similar results were obtained with 1,3-dithiolane.³a