Electrophile-Induced Ether Transfer: Stereoselective Synthesis of 2,4,6-Trisubstituted Tetrahydropyrans

Total Synthesis and Structural Reassignment of Lyngbyaloside C Highlighted by Intermolecular Ketene Esterification

Lyngbyaloside C, a classic macrolide, isolated from Lyngbya bouilloni, has shown moderate anticancer activity against several cancer cell lines. Here, we report the first total synthesis and stereochemical configuration reassignment of lyngbyaloside C. The synthesis highlights a one-pot intermolecular ketene esterification reaction to form the crucial tertiary ester and tetrahydropyran. In addition, a novel and concise synthetic pathway towards the 1,3-syn secondary, tertiary diol fragment is described using a regio- and stereospecific electrophilic ether transfer reaction.

Enantiopure aminopyrans by a Lewis acid promoted rearrangement of 1,2-oxazines: versatile building blocks for oligosaccharide and sugar amino acid mimetics

1,3-Dioxolanyl-substituted 1,2-oxazines, such as syn-1 and anti-1, rearrange under Lewis acidic conditions to provide bicyclic products 2-5. Subsequent reductive transformations afforded enantiopure 3-aminopyran derivatives such as 7 and 9 or their protected diastereomers 16 and 18, which can be regarded as carbohydrate mimetics. An alternative sequence of transformations including selective oxidation of the primary hydroxyl groups in 21 and 24 led to two protected beta-amino acid derivatives with carbohydrate-like backbone (sugar amino acids). Treatment of bicyclic ester 23 with samarium diiodide cleaved the N--O bond and furnished the unusual beta-lactam 27 in excellent yield. Alternatively, gamma-amino acid derivative 29 was efficiently prepared in a few steps. Fairly simple transformations gave azides 32 and 35 or alkyne 30 which are suitable substrates for the construction of oligosaccharide mimetics such as 34 by copper iodide catalyzed cycloadditions. With this report we demonstrate that enantiopure rearrangement products 2-5 are protected precursors of a variety of polyfunctionalized pyran derivatives with great potential for chemical biology.