Diastereoselectivity in the Cyclopropanation of 3,3-Bimetallic Allylic Alcohols. Preparation of Diastereomeric Cyclopropyl Carbinols via a Simple Oxidation-Reduction Sequence
摘要:
A highly diastereoselective cyclopropanation of allylic alcohols, containing a silyl and/or stannyl group using samarium/dihalomethane, provides a variety of bimetallic cyclopropane carbinols in good yield. A comparison with the more traditional Simmons-Smith conditions and its variants is made. The diastereoselectivity varies from 1:10 to >50:1 and depends on the substituents on the carbinol side chain (R-group) and on the substituents on the alkene portion of the allylic alcohol. Excellent diastereoselectivity (>50:1) was always obtained whenever a eis-substituent was present. Moreover, the minor diastereomer from the cyclopropanation can easily be obtained via a simple oxidation of the major cyclopropyl carbinol, followed by a selective reduction of the corresponding cyclopropyl ketone. Using LiAlH4 at 0 degrees C, facial selectivities of 15 to 20:1 are obtained for this reduction, while slightly higher selectivities up to (29:1) can be obtained using DIBAL-H. The combination of the cyclopropanation and oxidation/selective reduction sequence provides access to both diastereomeric bimetallic cyclopropanes.
AgAsF6 as safe alternative to AgClO4 for generating cationic zirconocene species: Utilities in lewis acid-promoted selective CC bond forming reactions
synthesis, AgAsF6 proved to be an efficient catalyst that serves as a safe alternative to AgClO4. Scope and limitation is discussed on this new catalyst in the processes including (1) alkyl/alkenyl transfer reaction from organozirconocene chloride to aldehyde, (2) two- and four-carbon homologation of aldehyde, (3) dual synthetic methods of 1,3-dienes from aldehydes/ketones via 1,3-bimetallic species
Inter- and Intramolecular Additions of 1-Alkenylboronic Acids or Esters to Aldehydes and Ketones Catalyzed by Rhodium(I) Complexes in Basic, Aqueous Solutions
Grignard-type addition reaction of 1-alkenylboronic acids or their esters to aldehydes or ketones were carried out in aqueous MeOH or DME in the presence of KOH ( 1 equivalent) and an RhCl(dppf) or Rh(OH)(dppf) catalyst (3 mol%). The utility of the protocol was demonstrated in the corresponding intramolecular reaction giving cyclic homoallylic alcohols.
Nucleophilic addition of alkenylzirconocene chloride [Cp2Zr(Cl)CH=CHR] to aldehyde, which is ordinarily a slow reaction, is remarkably accelerated by a catalytic amount of AgClO4. The rate acceleration effect is also valid for the alkyl congener, Cp2Zr(Cl)R.