Enantioselective Synthesis of 3‐Methylisochromans and Determination of Their Absolute Configurations by Circular Dichroism

Abstract

AbstractSeven (S)‐3‐methylisochromans with different substitution patterns on their aromatic rings, and hence with different directions of their sum electric transition moments, were synthesized by ring‐closure of optically active (S)‐1‐arylpropan‐2‐ol derivatives. The (S)‐1‐arylpropan‐2‐ols were obtained by kinetic resolution and their absolute configurations were determined with the aid of a zinc porphyrin tweezer and by Mosher’s method. A systematic CD study of substituted isochroman derivatives revealed that, unlike in the cases of chiral tetralin and 2,3‐dihydrobenzo[b]furan chromophores, the presence of achiral substituents of large spectroscopic moment (e.g., OMe) on the aromatic ring does not change the helicity rule of the “unsubstituted” isochroman chromophore: (P)/(M) helicity of the isochroman heteroring resulted in positive/negative 1Lb band Cotton effects (CE) regardless of the nature(s) and position(s) of the substituent(s). (S)‐3‐Methylisochromans were oxidized at C‐1, allowing access to the corresponding dihydroisocoumarins, in which positive CE of the n→π* transitions were correlated with (P) helicity and (S) absolute configuration. On DDQ‐assisted oxidation, two trans‐1‐methoxy‐3‐methylisochroman derivatives were prepared and used to study the effect of the axial benzylic C‐1 methoxy group on the conformation of the heteroring and the 1Lb band CE. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)