Conformation in the cyclotriveratrylene series

Abstract

Oxidation of cyclotriveratrylene (CTV) with chromic acid gives cyclotriveratrylen-5-one (3) accompanied by some cyclotriveratrylen-5,10,15-trione (9), which is formed in higher yield by similar oxidation of the monoketone. The ketone (3) is converted into the corresponding secondary alcohol (4) by hydride reduction and into the tertiary carbinol (6) by methyl-lithium. The latter alcohol easily loses water to form the 5-methylene derivative (7); the isopropyl analogue cannot be isolated, since it spontaneously dehydrates to the 5-isopropylidene derivative (8).

The relative stabilities of the various possible conformations of the ring system, and their rates of interconversion when more than one is observable, depend on repulsion between non-bonded atoms, ring-strain, and (when C-5 is trigonal) conjugation. The conformations have been established by ¹H n.m.r., i.r., and u.v. spectra. CTV itself exists only in the ‘crown’ conformation (A); the ¹H n.m.r. spectrum remains unchanged up to 200°. The ‘crown’ conformation with pseudo-equatorial OH is also the most stable one for the secondary alcohol (4): both the labile secondary alcohol (5), isolated by reduction of the ketone (3), and the methyl carbinol (6) exist in the ‘flexible’ form (C).

In the compounds with C-5 trigonal, conjugation, which is not possible in the ‘crown’ conformation, also plays a part, so that the ketone (3) adopts only the ‘flexible’ conformation (B) with C₂ symmetry and an enlarged C–CO–C angle. In the less highly conjugated methylene derivative the ‘crown’ conformation (A) also occurs, although the ‘flexible’ one (B) still predominates (K= 3·2). The two conformers equilibrate slowly at room temperature. The isopropylidene derivative (8), however, exists predominantly in the ‘crown’ conformation.

In the presence of dilute acid both the ‘crown’ and ‘flexible’ alcohols react with ethanol to give the ethyl ethers; the rate for the latter is at least 6 × 10⁴ times faster. That the ‘crown’⇌‘flexible’ conformational equilibrium for the intermediate carbonium ion is faster than its chemical reaction and lies, like that of the ketone (3), on the side of the flexible form is indicated by the fact that the flexible ether (12) is the primary product of etherification of both the alcohols (4 and 5).