-BIS(2,2,2-TRIFLUOROETHYL)-N,N'-ETHYLENETARTRAMIDE - AN IMPROVED CHIRAL AUXILIARY FOR THE ASYMMETRIC ALLYLBORATION REACTION

-Bis(2,2,2-trifluoroethyl)-N,N'-ethylenetartramide (8), synthesized by a simple four-step sequence from ethylenediamine and benzylidenetarta ric acid, was designed in anticipation that the derived allylboronates 9-11 would display enhanced reactivity owing to the inductive effect of the N-trifluoroethyl substituents that would increase the Lewis aci dity of the boron atom of the B-allyl-1,3,2-dioxaborolanes. Reagents 9 -11 were synthesized by transesterification of 8 with the crystalline and easily purified allylboronate diethanolamine complexes 13, 19, and 25. Allylboronate 9 is ca. 100-fold more reactive than 6 and is also substantially more useful than the previously reported allyboronate 4, which suffers from very poor solubility in toluene at -78 degrees C. Most importantly, allylboronates 9-11 are significantly more enantiose lective than the parent tartrate allylboronates 1-3 and rank among the most highly enantioselective allylboron reagents yet reported. Reacti ons of 9-11 with aldehydes are performed in THF at- -78 or -55 degrees C for 5-12 h periods. The enantioselectivity realized in reactions wi th achiral aldehydes is 92-95% ee (Table 2), and excellent diastereose lectivity is achieved in double asymmetric reactions with chiral aldeh ydes 15a, 15b, and 33 (Tables 3 and 4). For example, 16 and 28 are now available with a minimum selectivity of 92% from reactions of 15a and 15b with allylboronate 9, while the crotylboration products 29, 30, a nd 31 are available with a minimum selectivity of 90% (usually greater than or equal to 95%) from reactions of 15a and 15b with crotylborona tes 10 and 11; the fourth isomer, 32a, is available with 83% selectivi ty. Chiral reagents 9-11 thus appear well suited for application to co mplex problems in organic synthesis.