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At room temperature, compound 1a (71.88 g, 1 eq.) and p-toluene sulfonic acid (74.4 g, 1.2 eq.) were dissolved in MeOH (640 mL). The reaction mixture was refluxed for 7 days. The solvent was removed under vacuum and the residue was recrystallised in acetone. The product was isolated by filtration, dried on P2O5 to yield compound 2a as a white solid in 76% yield. 1H NMR (CDCl3, 400 MHz) δ (ppm) 1.38 (quint, J=7.76 Hz, 2H), 1.71 (q, J=7.76 Hz, 2H), 1.99 (quint, J=6.98 Hz, 2H), 2.38 (s, 3H), 2.70 (t, J=5.17 Hz, 2H), 4.92-4.99 (m, 2H), 5.67-5.73 (m, 1H), 7.20 (brs, 2H).

1. temperatureThe reaction mixture was refluxed for 7 days
2. customThe solvent was removed under vacuum
3. customthe residue was recrystallised in acetone
4. customThe product was isolated by filtration
5. customdried on P2O5