Patai S., Rappoport Z. 2000 The chemistry of functional groups. The chemistry of dienes and polyenes. V. 2 / 0739 884

fragment in the cationoid intermediate 290 is assisted by subsequent aromatization. Analogous transformations take place during a dienone –phenol rearrangement including conjugated systems (see Section IV.A).

(97)

Me

COOEt

Me Me

[H]

Me

Vitamin A

In the course of dolastane synthesis (the dolastanes are a group of marine diterpenes) interesting rearrangements catalyzed by Lewis acids were found. Treatment of the trienone 293 with excess (1.5 eq) ethylaluminum dichloride at low temperatures ( 5 °C, 48 h) gave the tetracyclic enone 295 in 53% yield while the tricyclic dienone 296 (50%) was formed at room temperature (equation 102)156. It was assumed that both products can be derived from the common zwitterion 294 which undergoes intramolecular alkylation at low temperatures (path a) whereas an alkyl shift takes place at elevated temperatures (path b), followed by a 1,2-hydride shift (equation 102).

The rearrangement of the conjugated diene 298 to the non-conjugated one 299 was found in the course of the investigation of lauren-1-ene conversions in the presence of p-toluenesulfonic acid (equation 103)157. Tricyclic ketone 297 in cold benzene transforms slowly into diene 298, but a further conversion to diene 299 occurs in refluxing benzene.

H+

IV. REARRANGEMENTS OF NON-CONJUGATED DIENES AND POLYENES

A. Dienone–Phenol Rearrangements

In general, the dienone –phenol rearrangements can be represented by acid-catalyzed transformation of the dienones 300 to phenols 301 which proceed with migration of group R and aromatization of the ring (equation 104). However, there are numerous variants of this reaction depending upon the structure of starting cyclic dienes and the nature of substituents as well as on the reaction conditions. These various pathways of the dienone – phenol rearrangement were already shown in one of the first reviews158. It is emphasized in Miller’s very detailed survey159 that both linearly-conjugated (‘ortho’) 2,4-cyclohexadienones and cross-conjugated (‘para’) 2,5-cyclohexadienones are incapable of undergoing the thermal rearrangements. In contrast, cyclohexadienones containing allyl substituents rearrange easily at relatively low temperatures (20 – 80 °C)159. It was shown that acid-catalyzed rearrangements of cyclohexadienones can occur, including [1,2]-, [1,3]-, [1,4]-, [1,5]-, [3,3]-, [3,4]- and [3,5]-migrations of carbon – carbon bonds159.

The mechanism of dienone –phenol rearrangement was investigated very thoroughly in work of Vitullo and colleagues160 – 164. It was established that the first step is a protonation (or coordination with Lewis acid) of the carbonyl oxygen to form a cyclohexadienyl cation. The second step includes a migration of a group (aryl or alkyl) to the adjacent electron-deficient carbon atom. The subsequent elimination of proton leads to the stable phenol (equation 104). By using deuterium isotope effects it was shown161,164 that the rate-determining step is unequivocally the migration, which occurs even in the presence of very high acid concentrations. The competitive migratory aptitudes established for various groups (e.g. Me, MeO) as well as the kinetic parameters of the dienone –phenol rearrangement depending on the acid concentration162,163 confirm the reaction mechanism assumed.

COMe

(110)

An attempt to carry out the Thiele –Winter acetoxylation of triketone 320 (Ac2O, H2SO4, 20 °C) gave rise to the product of dienone –phenol rearrangement 321 in 83% yield (equation 111)171. This reaction was claimed to be a convenient entry into the tetrahydrophenanthrene-1,4-quinone 323, starting from the spiro(cyclopentanonaphthalene) triones 322 (equation 112). The rearrangements of cyclohexadiene systems can also be catalyzed by Lewis acids; for example, under metalation conditions with HgCl2 (HgO, NaHCO3, THF, 20 –45 °C) products 324 and 325 are formed in a 5 : 1 ratio (equation 113)172.

In contrast to well investigated acid-catalyzed rearrangements of the cross-conjugated cyclohexadienones mentioned above, the isomerizations of linearly-conjugated cyclohexadienones have not received much attention, except for special cases in which allyl groups migrate. Acid-catalyzed rearrangements of conjugated 2,4-cyclohexadienones can occur via [1,2]- and [1,5]-shifts depending on the nature of the migrant. Thus, dienone 326 isomerizes by a [1,2]-shift of a methyl group (equation 114) whereas the dienone 327