Chiral alpha-hydroxy aldehydes generated in situ by the ADH reaction of vinyl sulfones undergo a borono-Mannich reaction with beta-styrenyl boronic acid and primary amines to give anti-1,2-amino alcohols in high enantiomeric purities (83-95% ee). This new method allows much more rapid access to these valuable chiral building blocks that has been used in a short formal synthesis (10 synthetic steps from 4-penten-1-ol) of (-)-swainsonine.
The total synthesis of hyacinthacines B3, B4, and B5 and purported hyacinthacine B7, 7-epi-hyacinthacine B7, and 7a-epi-hyacinthacine B3 from a common anti-1,2-amino alcohol precursor is described. These syntheses confirmed that the proposed structures and absolute configurations of hyacinthacines B3, B4, and B5 were correct and disclosed that the proposed structure of hyacinthacine B7 was incorrect. Our synthetic and spectroscopic studies suggest that the natural hyacinthacines B5 and B7 are the same compounds; however, without access to authentic samples this cannot be unequivocally proven.
Abstract:We have demonstrated that the borono-Mannich reaction is a versatile and efficient reaction for the diastereoselective preparation of chiral 1,2-amino alcohols. These Mannich products are valuable starting materials as shown in this report by the synthesis of bioactive polyhydroxylated pyrrolizidine and indolizidine alkaloids. Initial studies, directed at the more complex Stemona alkaloids and using the borono-Mannich reaction on cyclic N-acyliminium ions, are encouraging, as demonstrated by the synthesis of the pyrido[1,2-a]azepine core structure of stemocurtisinol.
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