An iterative direct aldol reaction using a C3 propionate unit as an aldol donor offers expeditious access to polyketide assembly in a highly diastereo- and enantioselective manner. An all-syn polyketide array with four consecutive stereogenic centers was efficiently constructed by an aldol reaction of thiopropionamide via soft Lewisacid/hard Brønsted base cooperative catalysis. This iterative aldol
The Thiopyran Route to Polypropionates: Enantioselective Synthesis of Membrenone B from Racemic Fragments
作者:Vishal Jheengut、Dale E. Ward
DOI:10.1021/jo701546f
日期:2007.9.1
[GRAPHICS](6S,7S,8S,9R,10S)-(-)-Membrenone B was synthesized in nine steps (9.4% overall yield) beginning with two-directional aldol coupling of tetrahydro-4H-thiopyran-4-one with racemic 1,4-dioxa-8-thiaspiro[4.5]decane-6-carboxaldehyde. The first aldol reaction occurs with dynamic kinetic resolution to give a single adduct (>98% ee). The second aldol reaction is highly diastereoselective (three of eight possible adducts), and both major products are converted to membrenone B. The route also constitutes a formal synthesis of membrenone A.