2.4- Cyclohexadienone ring Cyclohexadienones

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. 

Cyclohexadienones also undergo photochemical rearrangements, but the products are different, generally involving ring opening. ... 

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. 

For all QM complexes, the ring signals in both and 13C JH NMR spectra indicate the lack of aromaticity. The carbonyl group appears in the 13C JH NMR spectrum in the range of 18418-20116ppm, which is the region observed for 2,5-cyclohexadienones and quinones.38... 

In aqueous acid cross-conjugated cyclohexadienones are principally photoconverted to one or more hydroxy ketones. In the case of a-santonin (1), isophotosantonic lactone (5) is formed in about 50% yield. A series of papers by Kropp and co-workers has aided in understanding this reaction/32-39-411 They have shown that the presence of a 4-methyl group (steroid numbering) results in the preferential formation of the 5-7 fused ring system (isophoto-... 

The initially formed cyclopropyl ketones from the photolysis of cross-conjugated cyclohexadienones are also photolabile. Usually the photochemical rearrangements of cyclopropyl ketones involve cleavage of the cyclopropyl bond which forms part of the cyclopentenone ring followed by either (a) a substituent migration or (b) rearrangement through a spiro intermediate. While the literature in this area is too voluminous to review in... 

Linearly conjugated cyclohexadienones usually photorearrange with ring fusion to a czs-diene-ketene. The reaction is reversible, so that in the absence of a nucleophile little change is observed. A good example of this type of transformation is the formation of photosantonic acid ... 

Synthesis of [1,2,3]triazolo[1,5-c]pyrimidines and [1,2,4]triazolo[1,5-c]pyrimidines A novel approach to [l,2,3]triazolo[l,5-c]pyrimidines is shown in Scheme 55. Batori and Messmer - in the course of their investigations on fused azolium salts - described a synthetic pathway to l,3-disubstituted[l,2,3]triazolo[l,5-c]-pyrimidinium salts <1994JHC1041>. The cyclization was accomplished by transformation of the hydrazone 436. This compound was subjected to an oxidative ring closure by 2,4,4,6-tetrabromo-2,5-cyclohexadienone to give the bicyclic quaternary salt 437 in acceptable yield. 

An unusual one-pot intramolecular sulfoxide alkylation-elimination reaction was found by Gibson et al. <2001SL712>. These authors found that treatment of 459 with potassium bis-trimethylsilylamide resulted in a ring closure to 460 in acceptable yield. Furthermore, Batori and Messmer found an effective method for preparation of [l,2,3]triazolo[l,5- ]pyrimidinium salts <1994JHC1041> oxidative cyclization of hydrazones 461 by 2,4,4,6-tetrabromo-2,5-cyclohexadienone gave rise to the quaternary salts 462. Under certain reaction conditions, the formation of 6-bromo-salts 462 (R6 = Br) was also experienced. As neither the starting compound nor the quaternary triazolopyridinium salt underwent bromination in this position, the authors assumed that this bromination process occurred on one of the intermediates in the course of the above-mentioned cyclization reaction. 

Systems in which one of the C-atoms in the diene unit is replaced by a heteroatom also undergo photoelectrocyclic ring closure in selected cases. For example, Adam and coworkers have recently reported the synthesis of an extended series of benzoxete derivatives (60) via photocyclization of the cyclohexadienones 59 (equation 20)148. 

Disubstituted 2,4-cyclohexadienones (112) undergo photoinduced electrocyclic ring opening to the transient ketene derivatives 113, which can be trapped by nucleophiles to prepare the corresponding carboxylic acid derivatives (114 equation 44)196 197 j le reaction has been employed successfully for the synthesis of various carboxylic acids, esters and amides. 

These results led to the proposal of the following mechanism. Decomplex-ation of the central C2 fragment allows coordination of the alkyne (intermediate 119), which then inserts to form the metallacycle 120. Deinsertion (reductive eliminate of the cobalt moiety allows ring closure to give the cyclohexadienone complex 121, which upon decomplexation yields the desired phenol. The regiochemistry of the alkyne insertion determines the ratio of 116 117 (for simplicity, only the sequence leading to 116 has been shown). 

It is seen from Table 7 that all these carbenes in solution react nonstereospeci-fically, indicating triplet character. For some carbenes such as CF3CH, PhCH, (CH3)sSiC(C02C2H5), 2,6-di-t-butyl-carbena-cyclohexadienone and 4,4-dimethyl-carbena-cyclohexadienone, the degree of non-stereospecifity is quite small. Other carbenes usually contain 7t-bonds, aromatic rings or heavy atoms as functional groups. 

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>. 

Fluorinated phenols generally undergo oxidation to cyclohexadienone and benzoquinone derivatives, and to the products of ring rearrangement and ring degradation. 

Fluonnatedphenols are usually oxidized to cyclohexadienone and benzoqui-none derivatives and to products of ring rearrangement and ring degradation Phenols with a tnfluoromethyl group at the nng are hydroxylated by potassium persulfate [57], whereas chloric acid causes oxidation to quinone, mg contraction, and incorporation of chlorine in the product [58] (equation 53)... 

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