Im having a bit struggle with this proedure:
It was reported in Paz, M. M.; Sardina, F. J. J. Org. Chem. 1993, 58 (25), 6990–6995.
Dimethyl N-(9-Phenylfluoren-9-yl)-3,4-Didehydro- glutamate (1). A solution of glutamate 2 (2.150 g, 5.18 mmol) in THF (11 mL) was added dropwise (10 min) to a solution of KHMDS (9.4 mL, 10.36 mmol, 1.1 M in THF) at -78 °C. The resulting pale-yellow solution was stirred for 1.5 h at -78 °C and then transferred via cannula (cooled to -78 °C) onto a solution of PhSeCl (2.182 g, 11.4 mmol, 220 mol %) in THF (9 mL) at -78 °C. THF (5 mL) was added to the flask that contained the enolate solution, cooled to -78, and cannulated to the PhSeCl solution. The reaction mixture was stirred for 2h at -78 °C, HOAc (1.5 mL) was added, and the resulting suspension was allowed to warm up to rt, diluted with EtOAc/hexane (2:1,100 mL), washed with saturated aqueous NaHCOa (2 x 50 mL), dried over MgS04, filtered, and concentrated to give 3 as a sticky yellow oil...
the procedure then continues by mcpba oxidation to yield dehydroglutamate which I followed and in the end got maybe 10% yield instead of 85%.
I did not have KHMDS so I used NaHMDS instead. But this was used in similar reactions as well, together with LDA... I did try LDA and LiHMDS as well but in all cases I still see lots of unreacted starting material and PhSeCl.
Could the choice of base really have such an impact on the reaction? I would warm it up after adding the base but since its diester, Im worried about side reactions such as ester Dieckman condensations.
I also try to isolate the selenyl derivative which I was able to get but also in small yield (around 30%) with lots of unreacted starting material and bunch of impurities.
From this it seems the formation of the enolate is not going to completion. Can you please offer any insight or help? I never did chemistry like this so Im bit struggling on where to go next and what conditions to try.