2.4- Cyclohexadienone ring

Very recently, Cook and Danishefsky [24] reported an interesting regioselectivity of intramolecular Diels-Alder reactions reversed by the change in the dienophihc moieties from vinyl to allenyl group (Scheme 19). For R = 2-propenyl group, C is bonded to the methyl substituted carbon Cj of the cyclohexadienone ring. For R = 2,3-butadienyl, C is bonded to Cy... 

The most reactive site of the diene part is Cj of the cyclohexadienone ring with the alkoxy gronp. This corresponds to Cj of 2-alkoxybutadiene (Scheme 15), which has the largest HOMO amplitnde. The preferable frontier orbital interactions (Scheme 22) are in agreement with the reversed regioselectivities. 

In a novel total synthesis of the tricyclic sesquiterpene (—)-longifolene, an intramolecular diazoalkane cycloaddition to a cyclohexadienone ring followed by thermal ring contraction of the resulting pyrazoline gave the tricychc vinylcyclo-propane 261 and this constitutes the key steps in this synthesis (314) (Scheme 8.63). The interesting features of this sequence are the separation of dipole and dipolarophile by five atoms and the formation of a seven-membered ring in the cycloaddition step. 

The conformation and dimensions of the substituted spirocyclic chromans (160) have been ascertained. The heterocyclic ring adopts a C-2, C-3 half-chair form, whilst the spiro-annelated cyclohexadienone ring is a C-T envelope (80TL4973, 8lAX(B)1620, 82AX(B)1001>. 

A second rearrangement can place the third ring at the head of the sequence if rearrangement proceeds until one of the terminal rings becomes the cyclohexadienone ring, enolization yields a polymeric phenol identical in every way, except for the order of succession of the rings, with that which would be produced by the direct head-to-tail coupling of aryloxy radicals (Reaction 13). The formation of a polymeric phenol... 

Thus, the formation of symmetrical (caused by perpendicular arrangement of the naphthalene and cyclohexadienone ring systems) cyclohexadienone 215 proceeds not as a [4 + 2]-cycloaddition of two carbocation species but rather as a two-step electrophilic substitution, the first stage of which is intermolecular and the second intramolecular. The reasons for the different direction of the cyclodimerisation reactions of carbocation 211 in protic media and in the presence of Lewis catalysts have been discussed elsewhere29,207. 

The above-mentioned families yield, beside the proper cyclohexadienone alkaloids (types IV and XIV, Schemes 1 and 2), their partially reduced derivatives (types V, VII, VIII, IX, and XII). The reduction involves either the ketonic group, which gives rise to two dienol isomers (VI and VII or XVIII), or it involves only one or both of the double bonds of the cyclohexadienone ring and the ketonic group does/or does not remain intact (V and VIII). Provided that the cyclohexadienone ring... 

Similar to proaporphine bases which are intermediates in the biosynthesis of the aporphine alkaloids, the promorphinanes, having a cyclohexadienone ring D, form an intermediate in the biosynthesis of the morphinane alkaloids. They also arise from benzyltetrahydroisoquino-line bases by phenolic oxidation. The first known alkaloid of this group was salutaridine (36) (first isolated from Groton salwtaris) (see Scheme 3) which in all probability is identical with the already known floripavine... 

FIGURE 31.1 Two-laser-two-color photolysis of a phenyl ester (Abs at 330 nm vs. time). The jump at 5 ps of the kinetic trace corresponds to the first laser shot (266 nm) and the jump at 15 ps to the second one (308nm), producing the cyclohexadienone ring opening. Reproduced from Jimenez et al. [31] with permission from the Royal Society of Chemistry. 

Hop "bitter" acids are isomeric mixtures of cyclohexadienone stmctures in both keto and enol forms, substituted at various positions on the ring by hydroxyl, acyl, and alkenyl groups. See Figure 2. 

An electrocyclic ring closure then leads to a cyclohexadienone complex 7, which upon migration of a proton, yields the chromium tricarbonyl-hydroquinone complex 3. 

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