Biaryl hydroxy aldehydes as intermediates in the metal-assisted atropo-enantioselective reduction of biaryl lactones: Structures and aldehyde-lactol equilibria

• Published in: Tetrahedron 1998-09, Vol.54 (36), p.10677-10690
• Main Authors: Bringmann, Gerhard, Breuning, Matthias, Endress, Heike, Vitt, Daniel, Peters, Karl, Peters, Eva-Maria
• Format: Article
• Language: English
• ISSN: 0040-4020; 1464-5416
• DOI: 10.1016/S0040-4020(98)00618-8

The synthesis of substituted 1-(2′-hydroxyphenyl)naphthalene-2-carbaldehydes 4 and 6-alkoxy-6H-pyrans 7 and 8, analogs of the postulated metallated intermediates in the atropo-enantioselective ring cleavage of configuratively unstable biaryl lactones 2, is described. While the equilibria between the open hydroxy aldehydes 4 and the cyclic lactol structures 3 are completely shifted towards 4 for the derivatives 4c-g with substituents ortho to the biaryl axis, the lactol forms are the dominating structures ( ca. 50–100%) for the ortho-unsubstituted compounds. For the lactols 3 and their acetalic analogs 6, 7, and 8, those diastereomeric conformations are preferred (77–100%) that have the exo-oxygen function axial. Only for sterically unhindered biaryls, 3 was observed in the hydroxy aldehyde-lactol equilibrium 3 ⇌ 4 . In the corresponding acetals 7, the exo-alkoxy group occupies an axial position as in ax- 7, preferentially.