Powdered KOH in DMSO: An Efficient Base for Asymmetric Cyclization via Memory of Chirality at Ambient Temperature
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- 作者：Takeo Kawabata ; Katsuhiko Moriyama ; Shimpei Kawakami ; Kazunori Tsubaki
- 刊名：Journal of the American Chemical Society
- 出版年：2008
- 出版时间：March 26, 2008
- 年：2008
- 卷：130
- 期：12
- 页码：4153 - 4157
- 全文大小：84K
- 年卷期：v.130,no.12(March 26, 2008)
- ISSN：1520-5126
文摘
Enolate chemistry has been extensively used for stereoselective C-C bond formation, in whichmetal amide bases are frequently employed in strictly anhydrous solvents at low temperatures. However,we found that asymmetric intramolecular C-C bond formation via axially chiral enolate intermediatesproceeded in up to 99% ee at 20 
C using powdered KOH in dry or wet DMSO as a base. Theenantioselectivity was even higher than that of the corresponding reactions with potassium hexamethyldisilazide in DMF at -60 C. The racemization barrier of the axially chiral enolate intermediate was estimatedto be ~15.5 kcal/mol. On the basis of the barrier, the chiral enolate intermediate was supposed to undergocyclization within ~10-3 sec at 20 C after it is generated to give the product in 
99% ee. Thus, enolatesgenerated with powdered KOH in DMSO were expected to be extremely reactive.
C using powdered KOH in dry or wet DMSO as a base. Theenantioselectivity was even higher than that of the corresponding reactions with potassium hexamethyldisilazide in DMF at -60 C. The racemization barrier of the axially chiral enolate intermediate was estimatedto be ~15.5 kcal/mol. On the basis of the barrier, the chiral enolate intermediate was supposed to undergocyclization within ~10-3 sec at 20 C after it is generated to give the product in 99% ee. Thus, enolatesgenerated with powdered KOH in DMSO were expected to be extremely reactive.