wt/vol

 

wt/vol

MW

moles

density

equivs.

yield

I

3.876 g

104.11

0.0372

 

1.0

 

II

12.292 g

274.87

0.0447

 

1.2

 

III

3.080 g

68.08

0.0452

 

1.2

 

IV

100 mL

-

-

 

 

 

V

12.495 g

342.52

0.0365

 

 

(98%)

 

Procedure:  250 mL 1-neck flask, stirbar, septum, N2 inlet

                  Dissolved 3.876 g of ester I and 12.292 g of II in 100 mL of dry DMF.  The mixture was stirred at rt and 3.080 g of imidazole was added.  After 16 h, TLC showed product spot at Rf 0.79 (3:7 EtOAc-hexanes, UV).  The reaction mixture was then concentrated by rotary evaporation.  A white solid was obtained which was triturated with ether and filtered.  The filtrate was concentrated by rotary evaporation and the product was isolated by flash chromatography on silica gel using 1:9 EtOAc-hexanes as eluant.  The product was a clear, colorless oil. 

 

1H NMR (CDCl3, 300 MHz) d 7.67 (m, ArH, 4H), 7.40 (m, ArH, 6H), 4.28 (q, J = 6.6 Hz, CH, 1H), 3.56 (s, CO2CH3, 3H), 1.37 (d, J = 6.6 Hz, CH3, 3H), 1.09 (s, SiC(CH3)3, 9H). 

 

notes

 

The protection of an alcohol using DPSCl most likely proceeds through an N-dimethyl-tert-butylsilylimidazole intermediate as shown below.  This is the mechanism first proposed by Corey and Venkateswarlu for protections involving TBSCl.