Cyclohexadienone synthesis

ZINCKE - SUHL Cyclohexadienone synthesis Synthesis of cyclohexadienones from phenols by Fnedel Crafts alkylafion... 

Ruonne-Contairung Cyclohexadienones Synthesis and Properties Kobnna, L S, Shteingarts, V D J Fluorine Ckem 41, 111-162 122 n... 

ZINKE - SUHL Cyclohexadienone synthesis 435 ZINCKE - ZIEGLER Synthesis of caibcarene 436 ZININ Benzidine (semxline) rearrangement 437... 

Cyclohexadienone synthesis. Danishefsky et al. have developed a cyclo-hexadienone synthesis based on Diels-Alder reactions of this diene (1) with... 

ZINCKE-SUHL Cyclohexadienone Synthesis Synthesis of cyclohexadienones form phenols by Friedel-Crafts alkylation. 

The procedure described is essentially that of Belleau and Weinberg and represents the only known way of obtaining the title compound. One other quinone acetal, 1,4,9,12-t6traoxadispiro[4.2.4.2]tetradeea-6,13-diene, has been synthesized by a conventional method (reaction of 1,4-cyclohexanedione with ethylene glycol followed by bromination and dehydrobromination ) as well as by an electrochemical method (anodic oxidation of 2,2-(l,4-phenylenedioxy)diethanol ). Quinone acetals have been used as intermediates in the synthesis of 4,4-dimethoxy-2,5-cyclohexadienone,. syw-bishomoquinone, - and compounds related to natural products. ... 

A plausible pathway is that the aromatisation of the cyclohexadienone 92 by a proton shift is accelerated in the presence of Ac20 under formation of acetate 93. The simultaneously generated acetic acid then cleaves the acetate to form the free phenol 94 (Scheme 44). This effect was observed for the first time during studies towards the total synthesis of the lipid-alternating and anti-atherosclerotic furochromone khellin 99 [64].The furanyl carbene chromium complex 96 was supposed to react with alkoxyalkyne 95 in a benzannulation reaction to give the densely substituted benzofuran derivative 97 (Scheme 45). Upon warming the reaction mixture in tetrahydrofuran to 65 °C the reaction was completed in 4 h, but only a dimerisation product could be isolated. This... 

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... 

In contrast to 2-cyclohexenones where no a-cleavage is observed this reaction occurs efficiently for 2,4-cyclohexadienones 216,217). Among different synthetic applications the synthesis of dimethylcrocetine (a heptaene dicarb-oxylic acid) may serve as an illustration (2.11) 218),... 

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. 

Common error alert Many cume points have been lost over the years when graduate students used cyclohexadienone or cyclopentadienone as a starting material in a synthesis problem ]... 

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. 

Nucleophilic additions to (cyclohexadienone)Fe(CO)3 complexes (218) occur in a dia-stereospecific fashion (Scheme 56)197. For example, the Reformatsky reaction of ketone (218a) affords a simple diasteromeric alcohol product19715. The reduction of (1-carbo-methoxycyclohexa-l,3-dien-5-one)Fe(CO)3 (218b) to give 219 has been utilized in the enantioselective synthesis of methyl shikimate. In a similar fashion, cycloadditions of (2-methoxy-5-methylenecyclohexa-l,3-diene)Fe(CO)3 (220) occur in a diastereospecific fashion198. 

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 voltammetric response of curcumin and carthamin must, in principle, be dominated by the oxidation of the phenol and/or methoxyphenol groups (see Scheme 2.2). The electrochemistry of methoxyphenols has claimed considerable attention because of their applications in organic synthesis [159-163]. As studied by Quideau et al., in aprotic media, 2-methoxyphenols are oxidized in two successive steps into cyclohexadienone derivatives [163], whereas a-(2)- and a-(4-methoxyphenoxy) alkanoic acids undergo electrochemically induced spirolac-tonization to develop synthetically useful orthoquinone bis- and monoketals. In the presence of methanol, the electrochemical pathway involves an initial one-electron loss, followed by proton loss, to form a monoketal radical. This undergoes a subsequent electron and proton loss coupled with the addition of alcohol to form an orthoquinone monoketal. The formal electrode potential for the second electron transfer... 

Some recent synthesis of ketencs involve interesting chemistry. A hutadienylkeiene is obtainable at -11)0 to - 150 C hv photolysis (mercury lamp) of the appropriate cyclohexadienone ... 

Another synthesis of a bridged hydrocarbon takes advantage of high electron release from the para-position of phenolate anions, which may be used to transform the phenol moiety into a substituted cross-conjugated cyclohexadienone system (S. Masamune, 1961, 1964). 

The hypervalent iodine reagents PIFA and PIDA have also been used in the synthesis of naturally occurring structures, primarily the amaryllidaceae alkaloids and related species. Work by White s group showed the feasibility of this method for the synthesis of 6a-epipretazettine and (-)-codeine [45, 46]. In the early 1990s, Rama Krishna and co-workers demonstrated that PIDA can promote the oxidative phenolic coupling of diaryl substrates 38a-e to deliver cyclohexadienones 39a-e, respectively, in consistent 30 % yields for all of the substrates examined (Scheme 10) [47]. 

The photochemistry of a,p nsaturated ketones has attracted much attention and is still a field cld of current interest. 1/Numer-ous examples of such photochemical transformations are well-documented for cyclic enones and dienones, including both cycloaddition ructions and rearrangements. For example, cyclopentenones / and/readily rearrange to cyclopropyl ketenes upon irra/idiation. Recently, the related cyclohexadienone/butadienyl ketene rearrangement has been shown to be a highly useful tool in the synthesis of natural products and macrocydic lactone /... 

Liu Q, Rovis T (2006) Asymmetric synthesis of hydrobenzofuranones via de-symmetrization of cyclohexadienones using the intramolecular Stetter reaction. J Am Chem Soc 128 2552-2553... 

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