CONFORMATIONAL LOCKING BY INTRAMOLECULAR HYDROGEN-BONDING AND UNLOCKING BY SOLVATION USING 7-VINYLNORCARADIENES

Norcaradiene derivatives 1, penyl)-3-oxatricyclo[4.4.0.0(1,5)]deca-7,9 -dien-4- one, showed a restricted rotation in the H-1 NMR spectra. VT H-1 NMR measurements in CDCl3 revealed the existence of two rotamers a t low temperature with Delta G(not equal) of 11.5 (250 K), 12.1 (263 K ), and 13.9 kcal mol(-1)(296 K) for norcaradienes with R = H, Me, and t-Bu, where R is the para substituent of the benzyloxy group, respecti vely. This restricted rotation was stopped in CDCl3 at ambient tempera ture by introducing intramolecular hydrogen bonding via reducing of th e lactone moiety to the corresponding lactols. The observed conformati onal locking due to the intramolecular hydrogen bonding was unlocked b y solvation, resulting in the formation of three conformers in methano l-d(4) and dimethyl-d(6) sulfoxide. VT H-1 NMR measurements of the lac tol derivative in dimethyl-d(6) sulfoxide showed that two of the confo rmers interconverted with Delta 6(not equal) of 17.4 kcal mol(-1).