Hello again
As I mentioned in my initial post, the boiling points are taken from Beilstein, and correspond to the boiling points of the proline
freebase ethyl ester. I don't see why they should be considered strange; what is it about them which makes you think this?
As Nicodem said, it is impossible to accurately answer your question without more information. It seems likely that proline ethyl ester freebase is a liquid. Are your crystals the freebase or a salt?
The melting point of plain proline is suspiciously similar to that of your product. Beilstein reports melting points ranging from 203
oC up to 220-222
oC. Recrystallising proline from ethanol will give a product with a melting point of 208-210
oC. If you are talking about the freebase having such a high melting point, then there is a possibility you have proline instead of the ester.
If you want to try a HCl catalysed esterification, you may like to take a look at the following articles which deal with this:
J. Amer. Chem. Soc., 71, 1949, 3100.
Justus Liebigs Ann. Chem.,
326?, 1903, 99.
Tetrahedron,
52 (1), 1996, 225-232. [Using HCl gas to give a quantitative yield]
An alternative is to first form the acyl chloride with SOCl
2 then react this with ethanol. The highest yielding and/or most accessible references are:
J. Org. Chem,
55 (7), 1990, 2254-2256. [97% yield]
Tetrahedron Asymmetry,
11 (6), 2000, 1367-1374.
Can. J. Chem. 80, 2002, 1662-1667. [99% yield]
Edit: Here is the experimental from
Tetrahedron,
52 (1), 1996, 225-232. The title refers to the N-protected amino ester. I suppose you'll have to be the judge of what is an 'enough amount' of HCl gas:
Preparation of Ethyl N-(2,2-Dimethylethoxycarbonyl)-(L)-prolinate (6a):
To a solution of (L)-proline (10 g, 87 mmol) in EtOH (150 mL) was introduced enough amount of hydrogen chloride gas and the resulting solution was heated at reflux for 1 h. Evaporation of the solvent gave ethyl (L)-prolinate hydrochloride (15.6 g, 100%) as a colorless oil.