Stereoselective epoxidation of asymmetrized 2-alkenyl-1,3-propanediols.
摘要:
Asymmetrized 2-alkenyl 1,3-propanediols are stereoselectively epoxidized to the same main diastereoisomer both with 3-chloroperbenzoic acid and the VO(acac)2/t-butyl hydroperoxyde system. Using the latter epoxidating reagent in combination with a protection-deprotection sequence involving the two homoallylic hydroxy groups, all the four cis epoxides were easily achieved in stereoisomeric pure form starting from a single common (Z) precursor.
Chemoenzymic preparation of asymmetrized tris(hydroxymethyl)methane (THYM*) and of asymmetrized bis(hydroxymethyl)acetaldehyde (BHYMA*) as new highly versatile chiral building blocks
摘要:
A series of asymmetrized tris(hydroxymethyl)methanes 2 and bis(hydroxymethyl)acetaldehydes 3 have been prepared in both enantiomeric forms through a chemoenzymatic methodology. The key step is the highly enantioselective PPL-catalyzed monohydrolysis of 2(E)-alkenyl-1,3-diacetoxypropanes 25-27. A careful study on the effect of unsaturations adjacent to the prochiral center in a series of 2-substituted 1,3-diacetoxypropanes has confirmed the suggested beneficial effect of a pi-system in that position but has also unveiled an unprecedented dramatic effect of double-bond configuration on enantioselectivity. A new empirical model for the interpretation of these and other results, based both on polarity and steric arguments, is proposed. This study provides a general protocol for the efficient synthesis of asymmetrized 1,3-propanediols bearing in position 2 saturated or unsaturated carbon chains.
Cis epoxides of any desired absolute stereochemistry have been obtained in a high enantio- and diastereodivergent manner via diastereospecific epoxidation of asymmetrized (Z)-2-alkenyl-1,3-propanediols, in turn obtained through a chemoenzymatic route, and a protection-deprotection ‘trick’ on hydroxy groups.