New vistas in organoelement chemistry
| Autor: | Beritte L. Christenson, Noriaki Murase, Stephen P. O'Connor, Scott E. Denmark |
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| Rok vydání: | 1996 |
| Předmět: | |
| Zdroj: | Pure and Applied Chemistry. 68:23-27 |
| ISSN: | 1365-3075 0033-4545 |
| DOI: | 10.1351/pac199668010023 |
| Popis: | AbstracL A selective and efficient protocol for the catalytic asymmetric cyclopropanation of allylic alcohols has been developed. The reaction involves preformation of the ethylzinc alkoxide of the alcohol and reaction with the FurukawdSimmons-Smith reagent. Zinc iodide has been shown to have a beneficial effect on the rate and enantioselectivity of the reaction. The structural variability in the allylic alcohol as been examined and the reaction was found to proceed smoothly with both E and Z allylic alcohols with one or two Psubstituents, but a-substitution was not well tolerated. The asymmetric cyclopropanation of alkenes has become an area of considerable interest in the past decade. While considerable advances in the catalytic cyclopropanation of simple alkenes using diazoacetate as the carbenoid source are on record,l a general method for catalytic asymmetric methylene transfer is still lacking. Early studies on cyclopropanation of asymmetrically modified substrates such as ketals? acetals? enol ethers4 and allylic alcohols5a were rewarding. More recently Charette has described a stoichiometrically modified reagent which is capable of high levels of enan tioselec tivity .5b Our interest in developing a catalytic enantioselective cyclopropanation of allylic alcohols which employed the Furukawa modification of the Simmons-Smith reaction led us to investigate the solution and solid state structure of bis(iodomethyl)zinc.6 We recently described the optimization of a protocol for the enantioselective cyclopropanation of cinnamyl alcohol (1) with this reagent. using the bis(methanesulfonamide) of (R, R)-l,2cyclohexanediamine (2) as a catalyst (Scheme 1). We first established a set of control conditions which led to a low conversion in the absence of catalyst. In the presence of 10 mo196 of the diethylzinc complex of 2. the reaction proceeded rapidly under the Same conditions but with disappointing enantiomeric excess. This was especially surprising since Kobayashi Scheme 1 |
| Databáze: | OpenAIRE |
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