Control of the regioselectivity of oxidative free-radical cyclizations by addition to haloalkenes
摘要:
Chlorine substituents on the alkene control the regioselectivity of the cyclization of 5-hexenyl or 6-heptenyl radicals generated by oxidation of an acetoacetate ester or 1,3-diketone with Mn(OAc)3.2H2O. 6-Exo-cyclization of the radicals obtained by oxidation of benzoylacetone derivatives 10 and 13, containing a chlorine on the terminal double bond carbon, gives alpha-chloroalkyl radicals that add to the aromatic ring to give 11 and 16, respectively. Loss of HCl leads to naphthols 8 and 17 indicating that this reaction may be useful for aureolic acid synthesis. Exo-cyclization is the exclusive process with acetoacetates 28b, 44, 55b, and 59 containing a chlorine on the terminal double bond carbon. 6-Endo-cyclization is the exclusive process with acetoacetates 18c and 51b containing a chlorine on the internal double bond carbon. Intra- and intermolecular competition experiments indicate that these effects are primarily steric. The chlorine substituent controls the regioselectivity of the cyclization by sterically hindering attack of the radical on the chlorine bearing double bond carbon thereby retarding formation of the beta-chloroalkyl radical. The chlorine substituent does not electronically accelerate attack on the other end of the double bond to give the alpha-chloroalkyl radical.
Ring Opening of Cyclic Vinylogous Acyl Triflates Using Stabilized Carbanion Nucleophiles: Claisen Condensation Linked to Carbon−Carbon Bond Cleavage
作者:David M. Jones、Marilda P. Lisboa、Shin Kamijo、Gregory B. Dudley
DOI:10.1021/jo100249g
日期:2010.5.21
Addition of stabilized carbanionic nucleophiles to cyclic vinylogous acyl triflates (VATs) triggers a ring-opening fragmentation to give acyclic β-keto ester and related products, much like those observed traditionally in the Claisen condensation. Unlike in the classical Claisen condensation, however, the VAT-Claisen reaction described herein is rendered irreversible by C−C bond cleavage, not by deprotonation
Claisen-Type Condensation of Vinylogous Acyl Triflates
作者:Shin Kamijo、Gregory B. Dudley
DOI:10.1021/ol0527781
日期:2006.1.1
The Claisen-type condensation reaction of cyclic vinylogous carboxylic acid triflates with lithium enolates and their analogues produces acyclic alkynes bearing a 1,3-diketone-type moiety. The present transformation is proposed to proceed via a 1,2-addition of the enolate to the vinylogous acyl triflate, followed by fragmentation of the aldolate intermediate.