Cyclohexadienone equivalents,

Wang used method D to fashion a key intermediate for the synthesis of rishirilide B (Fig. 4.20).21 The 2,4-bis-OBoc-3-methyl-benzyl alcohol (31) undergoes the addition of two equivalents of corresponding Grignard reagent to afford phenol 32 in 75% yield (Fig. 4.20). This material was subsequently elaborated by Mejorado in three steps (61% yield) to the corresponding 2,5-chiral cyclohexadienone 33, which was ultimately transformed into ( + )-rishirilide B (34).22... 

Rawal s group developed an intramolecular aryl Heck cyclization method to synthesize benzofurans, indoles, and benzopyrans [83], The rate of cyclization was significantly accelerated in the presence of bases, presumably because the phenolate anion formed under the reaction conditions was much more reactive as a soft nucleophile than phenol. In the presence of a catalytic amount of Herrmann s dimeric palladacyclic catalyst (101) [84], and 3 equivalents of CS2CO3 in DMA, vinyl iodide 100 was transformed into ortho and para benzofuran 102 and 103. In the mechanism proposed by Rawal, oxidative addition of phenolate 104 to Pd(0) is followed by nucleophilic attack of the ambident phenolate anion on o-palladium intermediate 105 to afford aryl-vinyl palladium species 106 after rearomatization of the presumed cyclohexadienone intermediate. Reductive elimination of palladium followed by isomerization of the exocyclic double bond furnishes 102. 

The analogous 2,6-cycloheptadien-i-ones display zwitterion-derived reactivity as weii52,53 pqj. example, the parent compound 113, upon irradiation in either acetic acid or t-butanol, gave diastereomeric solvent adducts 114 and 115 (Scheme 28) . On the other hand, tetramethoxy derivatives 116 furnished rearranged products 117, which are equivalent to the cyclopropyl ketone 1,4-shift products seen with cyclohexadienones . 

Vinyl sulfoxides are useful Diels-Alder dienophiles. Thermolysis of the adducts gives rise to alkenes via sulfoxide elimination and so over the two steps the vinyl sulfoxides are the synthetic equivalents of alkynes, e.g. phenyl vinyl sulfoxide reacts with anthracene and 1,4-diphenylbuta-1,3-diene with concomitant sulfoxide elimination under the Diels-Alder conditions to give adducts (70) and (71 Scheme 15). 3-Phenylsulfinyl-a,3-unsaturated-carbonyl compounds have also been used as Diels-Alder dienophiles in chemistry developed for a synthesis of disodium prephenate and epiprephenate. With the electron rich diene (72), vinyl sulfoxides (73) and (75) gave rise to products (74 equation 33) and (76 equation 34) after hydrolysis. For the prephenate synthesis the phenylsulfinylmethylenelactone (77) was used, giving cyclohexadienone (78 Scheme 16) after sulfoxide elimination and hydrolysis. ... 

Then the computations on the 2,5-cyclohexadienones were used to obtain 3, 3-bond orders of the five electronic states listed in Scheme 1.5. As noted above, such bond orders between nonconjugated centers are equivalent to a perturbation computation and provide information on tendencies of the two centers to bond. There are positive bond orders for three of these states. However, it is known that the Type A rearrangement occurs selectively from a triplet, and only the n-x triplet has a positive bond order, thus providing theoretical support for the reaction mechanism. 

Oxidative cyclization gives radical 32 oxidation of 32 gives 33, probably as a mixture of double bond positional isomers. The unsaturated cyclic -keto ester 33 is more acidic than 31 and is rapidly oxidized further by two equivalents of Mn(III) to give a cyclohexadienone that tautomerizes to phenol 34. The overall reaction consumes four equivalents of Mn(OAc)3. 

Deprotonation of phenol (P20.2) gives an anion which is equivalent to the enolate anion of cyclohexadienone (P20.3). This can be alkylated on carbon, rather than oxygen, if the reaction conditions are correct. The product ketone (P20.4) can be re-enolised to give o-cresol (P20.5) and the whole process repeated. After the addition of three methyl groups, the product ketone (P20.1) cannot enolise and so the alkylation process ceases (Figure S20). 

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