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Pictet-Spengler route to Praziquantel
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25th July 2012 @ 01:14
START: 25/07/12


Hazard and Risk Assessment
HRA KAB26-19

Previous Experiments
Preparation of MNR42-1
Yb(OTf)3/PyBOX catalysed asymmetric acyl-PS formation of KAB26-17

Chemical Information
2-(3,4-dimethoxyphenyl)-N-(4-nitrobenzylidene)ethanamine (MNR42-1) - SMILES: O=[N+]([O-])C1=CC=C(/C=N/CCC2=CC(OC)=C(OC)C=C2)C=C1, InChI: InChI=1S/C17H18N2O4/c1-22-16-8-5-13(11-17(16)23-2)9-10-18-12-14-3-6-15(7-4-14)19(20)21/h3-8,11-12H,9-10H2,1-2H3/b18-12-, InChIKey: LMBMXYXQVBPKHJ-PDGQHHTCSA-N.
1-(6,7-dimethoxy-1-(4-nitrophenyl)-3,4-dihydroisoquinolin-2(1H)-yl)ethanone (KAB26-19) - SMILES: O=[N+]([O-])C1=CC=C(C2N(C(C)=O)CCC3=CC(OC)=C(OC)C=C32)C=C1, InChI: InChI=1S/C19H20N2O5/c1-12(22)20-9-8-14-10-17(25-2)18(26-3)11-16(14)19(20)13-4-6-15(7-5-13)21(23)24/h4-7,10-11,19H,8-9H2,1-3H3, InChIKey: OIFIJKJQAGAGKY-UHFFFAOYSA-N.

NOTE: All THF was dried over microwave activated 3 Å molecular sieves (2.0-2.5 mm) >72 h prior to use. All acetonitrile was dried over microwave 3 Å molecular sieves (2.0-2.5 mm) >24 h prior to use.[1] All glassware used was dried overnight at 115 °C.

To a mixture of 3 Å molecular sieves (~1.5 g) in MeCN (16 mL) was dissolved MNR42-1 (100 mg, 0.318 mmol, 1 equiv.). The mixture was cooled in an acetone ice bath (-10 °C) before the addition of acetyl chloride (0.04 mL, 0.6 mmol, 1.8 equiv.) and 2,6-lutidine (0.06 mL, 0.5 mmol, 1.6 equiv.).
A solution of Yb(OTf)3 (10 mg, 0.016 mmol, 0.05 equiv.) and 2,2′-Isopropylidenebis[(4R)-4-phenyl-2-oxazoline (21 mg, 0.063 mmol, 0.2 equiv.) in THF (~1 mL) and acetonitrile (~0.5 mL) was prepared. Once dissolved, the Yb(OTf)3 was added to the cold MNR42-1 mixture. The reaction mixture was allowed to warm to 25 °C and stirred under argon from 10:15.
After 2 hours, the reaction mixture was diluted with EtOAc (15 mL). The mixture was washed with saturated sodium bicarbonate solution (15 mL). The organic layer was separated. The aqueous layer was extracted with EtOAc (3 × 15 mL). The organic layers were combined, dried (MgSO4) and concentrated under reduced pressure to yield crude KAB26-19 (159 mg, 140%).
The crude product was purified by silica gel column chromatography (60-70% EtOAc/hexane, v/v) to yield KAB26-19 as a yellow solid (93 mg, 82%).

Summary and Conclusion

[1] D. Bradley, G. Williams and M. Lawton, J. Org. Chem. 2010, 75, 8351. DOI: 10.1021/jo101589h Paper.
[3] Rare-Earth Metal Triflates in Organic Synthesis, S. Kobayashi, M. Sugiura, H. Kitagawa and W. W.-L. Lam, Chem. Rev. 2002, 102, 2227–2302. DOI: 10.1021/cr010289i Paper.

Attached Files
HRA KAB26-19
Scheme KAB26-19
Table KAB26-19