UNEXPECTED STABILITY OF DELTA-LACTONES WITH AXIAL SUBSTITUENTS RATHER THAN EQUATORIAL ONES - CONFORMATIONAL EVALUATION BY MOLECULAR MECHANICS AND MOLECULAR ORBITAL CALCULATIONS

摘要

A 1:1 mixture of the trisubstituted delta-lactones 5a and 5b was subjected to thermodynamically equilibrated renditions to give predominantly 5b with axial substituents rather than 5a with all equatorial ones, in contrast to the Prelog-Djerassi lactone derivatives 3a and 3b. Further surprisingly, it has been found that the disubstituted lactone 15 also adopts a half-chair conformation with axial substituents. The conformational analyses of these compounds by molecular mechanics and molecular orbital calculations have emphasized gauche effects and electrostatic interactions as a cause of the preference for axial conformers. (C) 1997, Published by Elsevier Science Ltd. [References: 45]