2-Cyclohexenones 1,4-dihydro

The structure of the products is determined by the site of protonation of the radical anion intermediate formed after the first electron transfer step. In general, ERG substituents favor protonation at the ortho position, whereas EWGs favor protonation at the para position.215 Addition of a second electron gives a pentadienyl anion, which is protonated at the center carbon. As a result, 2,5-dihydro products are formed with alkyl or alkoxy substituents and 1,4-products are formed from EWG substituents. The preference for protonation of the central carbon of the pentadienyl anion is believed to be the result of the greater 1,2 and 4,5 bond order and a higher concentration of negative charge at C(3).216 The reduction of methoxybenzenes is of importance in the synthesis of cyclohexenones via hydrolysis of the intermediate enol ethers. 

DIENOPH1LES Acrolein. Ethynylp-tolyl sulfonc. 3-Mcthylsulfony 1-2,5-dihydro-furane. 3-Nitro-2-cyclohexenone. Phenyl vinyl sulfone. 

Sequential addition of hydrogen sulfide to an a,/3 -unsaturated ketone, followed by condensation with an a -dicarbonyl compound, leads directly to thiophenes. Thus addition of hydrogen sulfide to a benzene solution of cyclohexenone, followed by an a -dicarbonyl compound and reflux, gave the respective 6,7-dihydro-5//-benzo[6]thiophene-4-ones (274). Compounds were obtained by this method where R1 and R2 = H, R1 = Ph or Me and R2 = H, and where R1 = R2 = Me (71lJS(A)(i)62). This is closely related to a patented process previously described (cf. Section 3.15.3.4). 

Alkeuyl-3-methyl-2-eyclohexene-t-ones. The kinetic enolate of 3-methyl-3-cyclohexene-l-one is alkylated only by reactive halides such as allyl bromide. An alternate route to compounds of type 2 starts with the 2-diethy1aminoethyl ether of 2,5-dihydro-m-cresol (1), obtained by Birch reduction. (The corresponding methyl ether is not readily metalated.) n-Butyllithium-HMPT in THF can metalate 1. The resulting carbanion is alkylated by a variety of alkyl bromides to give, after acid hydrolysis, cyclohexenones 2 in yields of 80-90%. ... 

The Birch reduction of aromatic hydrocarbons and ethers to the 2,5-dihydro derivatives proceeds most satisfactorily when the substitution pattern allows the addition of hydrogen to two unsubstituted positions in a para relationship. If this requirement is satisfied, better yields are obtained from more highly substituted aromatic rings than from (say) anisole itself, which affords a substantial amount (20%) of 1-methoxycyclohexene (c/. Scheme 1). Extra substitution presumably hinders protonation at the terminus of the dienyl anion (which would lead to a conjugated diene and overreduction). The utilization of anisole moieties as precursors to cyclohexenones has been of very limited value with many 1,2,3-substitution patterns and more densely substituted derivatives. Compounds (23) to (26), for example, have only been reduced by employing massive excesses (200-600 equiv.) of lithium metal,2 while the aromatic ring in (28) is completely resistant to reduction. ... 

The vinylogous aza-enone, A -methoxycarbonyl-5,6-dihydro-4-pyridone 31, has several advantages. It reacts with electron-rich as well as electron-poor alkenes to render cycloadducts in high yield and with extremely high regioselectivity [56], It should be noted that photoaddition of the analogous cyclohexenone derivatives show moderate or no regioselectivity toward the electron-rich or electron-poor alkenes 32a and 32c. 

Intramolecular Additions to Cyclohexenones and Related Systems. 3-Styrylcyclohex-2-enone undergoes photochemical cyclization to afford 2,3-dihydro-[177]-phenanthrene-4-one. Work carried out on the photostability of curcumin has also examined the photolability of the chalcone (38). This undergoes cyclization to (39) on irradiation. ... 

A Birch reduction of 40, followed by acylation of the amino group in the resulting dihydro derivatives 41 with cyanoacetic acid and subsequent hydrolysis of the enol ether moiety gave cyclohexenones 42. Treatment of 42 with a substoichiometric amount of NaOEt caused the isomerization of the carbon—carbon double bond to give an a,P-enone and the closure of the piperidine B ring by an intramolecular Michael addition, leading to the ds-fused perhydroisoquinoUne derivatives 43 as mixtures of C-9 epi-mers. A stereoselective allylation from the most accessible face of 43... 

Irradiation of 2,2-dimethyl-2,3-dihydro-4H-thiin-4-one (57) — a 4-thiacyclohex-2-enone — in neat furan afforded three cycloadducts 58, 59, and 60, in a 5 4 1 ratio, whose formation is quenched by appropriate triplet quenchers. Irradiation of 57 in methanol afforded a 3 2 mixture of adducts 61 and 62 and in CD30D,a similar mixture of D4-6I and D4-62. Finally, irradiation of 57 in furan MeOH (1 1) gives only cycloadducts 58, 59, and 60 to the exclusion of adducts 61 and 62 (Scheme 19). Compound 57 was chosen for these studies because the longer C-S bonds (as compared to C-C bonds) should make such dihydrothiinones ideal model compounds for the investigation of photochemically generated, highly twisted monocyclic cyclohexenones. ... 

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