Biaryl hydroxy aldehydes as intermediates in the metal-assisted atropo-enantioselective reduction of biaryl lactones: Structures and aldehyde-lactol equilibria
摘要:
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. (C) 1998 Elsevier Science Ltd. All rights reserved.
Atropisomerization Barriers of Configurationally Unstable Biaryl Compounds, Useful Substrates for Atroposelective Conversions to Axially Chiral Biaryls
摘要:
Configurationally unstable biaryl lactones of type (M)-1 reversible arrow (P)-1 and ring-opened 2-acyl-2'-hydroxy biaryl compounds of type (M)-4 reversible arrow (P)-4 are versatile precursors for the atroposelective preparation of axially chiral biaryls. The activation barriers of their atropisomerization process, which constitutes a fundamental precondition for the dynamic kinetic resolution, were determined by dynamic NMR spectroscopy for rapid processes and by HPLC-monitored racemization of enantiomerically enriched material for smaller interconversion rates. For the lactones, the free activation energies Delta G(298)(double dagger) increase with the steric demand of the substituent R ortho to the biaryl axis in the series H < OMe (t(1/2) approximate to ms) < Me (t(1/2) approximate to s) < Et < i-Pr (t(1/2) approximate to min) < t-Bu (t(1/2) approximate to d). The formally ring-opened 2-acyl-2'-hydroxy biaryls, which interconvert via the lactol isomers 5 as the cyclic (and thus configurationally less stable) intermediates, have a significantly slower atropisomerization rate as a result of the high loss in activation entropy Delta S-double dagger as a consequence of the required intermediate ring closure 4 --> 5.
Configurationally unstable biaryl lactones of type (M)-1 reversible arrow (P)-1 and ring-opened 2-acyl-2'-hydroxy biaryl compounds of type (M)-4 reversible arrow (P)-4 are versatile precursors for the atroposelective preparation of axially chiral biaryls. The activation barriers of their atropisomerization process, which constitutes a fundamental precondition for the dynamic kinetic resolution, were determined by dynamic NMR spectroscopy for rapid processes and by HPLC-monitored racemization of enantiomerically enriched material for smaller interconversion rates. For the lactones, the free activation energies Delta G(298)(double dagger) increase with the steric demand of the substituent R ortho to the biaryl axis in the series H < OMe (t(1/2) approximate to ms) < Me (t(1/2) approximate to s) < Et < i-Pr (t(1/2) approximate to min) < t-Bu (t(1/2) approximate to d). The formally ring-opened 2-acyl-2'-hydroxy biaryls, which interconvert via the lactol isomers 5 as the cyclic (and thus configurationally less stable) intermediates, have a significantly slower atropisomerization rate as a result of the high loss in activation entropy Delta S-double dagger as a consequence of the required intermediate ring closure 4 --> 5.
Biaryl hydroxy aldehydes as intermediates in the metal-assisted atropo-enantioselective reduction of biaryl lactones: Structures and aldehyde-lactol equilibria
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. (C) 1998 Elsevier Science Ltd. All rights reserved.