Regio- and Stereoselective Synthesis of Acetallic Tetrahydropyrans as Building Blocks for Natural Products Preparation, via a Tandem [4+3]-Cycloaddition/Ozonolysis Process

• Published in: European journal of organic chemistry 2016-09, Vol.2016 (27), p.4674-4695
• Main Authors: Montaña, Ángel M., Corominas, Albert, Chesa, Juan F., García, Francisca, Font-Bardia, Mercè
• Format: Article
• Language: English
• Subjects: Cycloaddition; Oxygen heterocycles; Ozonolysis; Stereoselectivity; Synthesis design
• ISSN: 1434-193X; 1099-0690
• DOI: 10.1002/ejoc.201600590

A highly versatile synthetic pathway is presented for the preparation of polyfunctionalized acetallic tetrahydropyrans from conveniently substituted 1‐methoxy‐8‐oxabicyclo[3.2.1]oct‐6‐en‐3‐one derivatives, as intermediates in the total synthesis of natural and unnatural products with structural, functional and/or biological importance. This synthetic methodology involves two key steps: a [4 + 3] cycloaddition reaction between an oxyallyl cation and 2‐methoxyfuran as a diene, followed by oxidative and/or reductive ozonolysis of the cycloheptenone subunit. This sequence renders polyfunctionalized 2‐methoxytetrahydropyranic products capable of being easily opened under acidic conditions. The key steps, cycloaddition and subsequent ozonolysis were both fully studied under different reaction conditions and using several substrates in order to optimize yields and stereoselectivities and to study the scope of the methodology. It is noteworthy that both reactions proceed with high diastereoselectivity and, in the case of the oxidative ozonolysis, outstanding regioselectivity as well. A chemical library of 14 polyfunctionalized tetrahydrofurans, having five or seven stereocenters, has been prepared using the detailed approach. A highly versatile route to polyfunctionalized acetallic tetrahydropyrans from 1‐methoxy‐8‐oxabicyclo[3.2.1]oct‐6‐en‐3‐ones is presented. The pathway entails: i) [4 + 3] cycloaddition between an oxyallyl cation and 2‐methoxyfuran, followed by ii) oxidative and/or reductive ozonolysis of the cycloheptenone; diverse 2‐methoxytetrahydropyrans are generated with both regio‐ and stereoselectivity.