Synthesis
The general procedure for the synthesis of tert-butyl morpholine-4-carboxylate from morpholine and di-tert-butyl dicarbonate is as follows: morpholine (1 mmol) is added to a solution of guanidine hydrochloride (15 mol%) and di-tert-butyl dicarbonate (1.2 mmol) in ethanol (1 mL) containing a magnetic stirrer, and the mixture is stirred at 35-40 °C until the reaction is complete (monitored by TLC or GC). Upon completion of the reaction, the ethanol is evaporated under vacuum and the residue is washed with water to remove the catalyst or dissolved in dichloromethane (or ethyl acetate) and filtered to separate the catalyst. If an organic solvent is used for post-treatment, evaporation of the organic solvent gives an almost pure product. If excess di-tert-butyl dicarbonate is used, the product can be washed with petroleum ether or hexane to recover the residual di-tert-butyl dicarbonate. If necessary, the product can be further purified by crystallization (using a solvent mixture of hexane and dichloromethane, or ethyl ether and petroleum ether) or by silica gel column chromatography (using ethyl acetate-hexane, 1:6, as eluent).
References
[1] Tetrahedron Letters, 2006, vol. 47, # 46, p. 8039 - 8042
[2] Synthesis, 2006, # 16, p. 2784 - 2788
[3] Tetrahedron Letters, 2008, vol. 49, # 16, p. 2527 - 2532
[4] Tetrahedron Letters, 2011, vol. 52, # 12, p. 1260 - 1264
[5] Monatshefte fur Chemie, 2011, vol. 142, # 10, p. 1035 - 1043
