A tellurium transposition route to allylic alcohols: overcoming some limitations of the Sharpless-Katsuki asymmetric epoxidation
摘要:
Good yields of enantiomeric allylic alcohols can be obtained in high enantiomeric excess (ee) by combining the Sharpless-Katsuki asymmetric epoxidation process (SAE) with tellurium chemistry. The advantages of the tellurium process are as follows: (1) the 50% yield limitation on the allylic alcohol in the Sharpless kinetic resolution (SKR) can be overcome; (2) allylic tertiary alcohols which are unsatisfactory substrates in the SKR can be obtained in high optical purity; (3) optically active secondary allylic alcohols with tertiary alkyl substituents (e.g. tert-butyl) at C-1 can be obtained in high ee; (4) optically active sterically congested cis secondary alcohols can be obtained in high ee; and (5) the nuisance of the slow SAE of some vinyl carbinols can be avoided. The key step in the reaction sequence is either a stereospecific 1,3-trans position of double bond and alcohol functionalities or an inversion of the alcohol configuration with concomitant deoxygenation of the epoxide function in epoxy alcohols. Trans secondary allylic alcohols can be converted to cis secondary allylic alcohols by way of erythro epoxy alcohols (glycidols); threo glycidyl derivatives are converted to trans secondary allylic alcohols. These transformations are accomplished by the action of telluride ion, generated in situ from the element, on a glycidyl sulfonate ester. Reduction of elemental Te is conveniently done with rongalite (HOCH2SO2Na) in an aqueous medium. This method is satisfactory when Te2- is required to attack a primary carbon site of a glycidyl sulfonate. In cases where Te2- is required to attack a secondary carbon site, reduction of the tellurium must be done with NaBH4 or LiEt3BH. Elemental tellurium is precipitated during the course of the reactions and can be recovered and reused.
A tellurium transposition route to allylic alcohols: overcoming some limitations of the Sharpless-Katsuki asymmetric epoxidation
摘要:
Good yields of enantiomeric allylic alcohols can be obtained in high enantiomeric excess (ee) by combining the Sharpless-Katsuki asymmetric epoxidation process (SAE) with tellurium chemistry. The advantages of the tellurium process are as follows: (1) the 50% yield limitation on the allylic alcohol in the Sharpless kinetic resolution (SKR) can be overcome; (2) allylic tertiary alcohols which are unsatisfactory substrates in the SKR can be obtained in high optical purity; (3) optically active secondary allylic alcohols with tertiary alkyl substituents (e.g. tert-butyl) at C-1 can be obtained in high ee; (4) optically active sterically congested cis secondary alcohols can be obtained in high ee; and (5) the nuisance of the slow SAE of some vinyl carbinols can be avoided. The key step in the reaction sequence is either a stereospecific 1,3-trans position of double bond and alcohol functionalities or an inversion of the alcohol configuration with concomitant deoxygenation of the epoxide function in epoxy alcohols. Trans secondary allylic alcohols can be converted to cis secondary allylic alcohols by way of erythro epoxy alcohols (glycidols); threo glycidyl derivatives are converted to trans secondary allylic alcohols. These transformations are accomplished by the action of telluride ion, generated in situ from the element, on a glycidyl sulfonate ester. Reduction of elemental Te is conveniently done with rongalite (HOCH2SO2Na) in an aqueous medium. This method is satisfactory when Te2- is required to attack a primary carbon site of a glycidyl sulfonate. In cases where Te2- is required to attack a secondary carbon site, reduction of the tellurium must be done with NaBH4 or LiEt3BH. Elemental tellurium is precipitated during the course of the reactions and can be recovered and reused.
efficient method for the synthesis of synthetically useful non-racemic allylic alcohols from 4-methylbenzenesulfonates of non-racemic 2,3-epoxy alcohols is described. Satisfactory yields are obtained by treatment of 4-methylbenzenesulfonates of non-racemic 2,3-epoxy alcohols with potassiumiodide followed by zinc powder and ammonium chloride in a one-pot manner. The method has been successfully applied to
A simple synthetic method for the preparation of scalemic allylicalcoholsfrom toluene-p-sulfonates of scalemic 2,3-epoxy alcohols is reported. Satisfactory yields are obtained by treatment of toluene-p-sulfonates of 2,3-epoxy alcohols with potassium iodide followed by triphenylphosphine and iodine in a one-pot synthesis.