Camille Ganter

Nucleophilic Addition to CC Bonds, Part X

A mechanistic model is presented for the base-catalyzed intramolecular cyclization of polycyclic unsaturated alcohols of type A to ethers D (Scheme 1). The alkoxide anion B is formed first in a fast acid-base equilibrium. For the subsequent reaction to D, a carbanion-like transition state C is proposed. This mechanism is in full agreement with our results regarding the influence of substituents on the regioselectivity and the rate of cyclization. We studied the effect of alkyl substituents in allylic position (alkylated endocylic olefinic alcohols 1–3) and, especially, at the exocyclic double bond (12–15). The fastest cyclization (krel=1) is 1216, which proceeds via a primary carbanion-like transition state (E: R1=R2=H). The corresponding processes 1317 and 1417 are characterized by a less-stable secondary carbanion-like transtition state (E: R1=Me, R2=H, or vice versa) and are slower by a factor of 104. The slowest reaction (krelca. 10−6) is the cyclization 1518via a tertiary carbanion-like transition state (E: R1=R2=Me).

Nuclear magnetic resonance spectroscopy: Studies of maleic anhydride in aromatic solvents☆

The chemical shift of the protons of maleic anhydride was studied as a function of structure in aromatic solvents, of solvent composition in mixed solvents, of concentration, and of temperature. The large changes in chemical shifts which were observed are attributed to stereospecific association of maleic anhydride with solvent molecules of charge transfer type. The ^(13)C-H and H-H couplings in maleic anhydride were also determined in some of the solvents.