1.4- Dimethyl-l,3-cyclohexadiene

A. 1,2-Dimethyl-l,4-cyclohexadiene. Caution This step should he conducted in a hood to avoid exposure to ammonia fumes. A 5-1. three-necked flask, cooled in a dry ice-isopropyl alcohol bath, is fitted with an efficient stirrer and a dry ice condenser. The flask is charged with approximately 2.5 1. of liquid ammonia, the stirrer is started, and 450 g. of anhydrous diethyl ether, 400 g. (10 moles) of absolute ethanol, and 318.5 g. (3.0 moles) of u-xylene (Note 1) are added slowly in that order (Note 2). I hen 207 g. (9.0 g. atoms) of sodium is added in small jtieees over a 5-hoiir... 

Scheme 45 summarizes a novel method to prepare dimethyl l,4-cyclohexadiene-l,2-dicarboxylate. The method starts from 2 equiv of acetylene and 1 equiv of dimethyl acetylendicarboxylate, and uses the 0s(02CCH3)(P Pr3)2 unit as a template. This unit is introduced in the form of the dihydride-acetate complex 170 [66]. 

Electrocyclic reactions of 1,3,5-trienes lead to 1,3-cyclohexadienes. These ring closures also exhibit a high degree of stereospecificity. The ring closure is normally the favored reaction in this case, because the cyclic compound, which has six a bonds and two IT bonds, is thermodynamically more stable than the triene, which has five a and three ir bonds. The stereospecificity is illustrated with octatrienes 3 and 4. ,Z, -2,4,6-Octatriene (3) cyclizes only to cw-5,6-dimethyl-l,3-cyclohexadiene, whereas the , Z,Z-2,4,6-octa-triene (4) leads exclusively to the trans cyclohexadiene isomer. A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode... 

Bei der Umsetzung von 6-Acetoxy-3,6-dimethyl-3-nitro-l,4-cyclohexadien mit katalyti-schen Mengen Schwefelsaure erhalt man in Deutero-trichlor-methan neben 40% 1,4-Di-methyl-2-nitro-benzol noch 40% l-Methyl-4-nitromethyl-benzol, 15% Essigsaure-(4-me-thyl-benzylester) und 5% 4-Methyl-benzaldehyd4. 

The ergosterol system mentioned earlier falls in this category. Before the system is discussed, it is worthwhile to point out that, in general, substitution of even a simple group such as an alkyl seems to change the details of the photochemical primary processes in a startling manner among the dienes and trienes. Thus, in solution, 1,3,5-hexatriene cannot be converted to 1,3-cyclohexadiene photochemically, but 2,4,6-octatriene does cyclize photochemically to (rans-5,6-dimethyl-l,3 Cyclohexadiene under the same conditions. ... 

Fhe most striking feature of electrocyclic reactions is their stereochemistry. For example, (2E,4Z,6 )-2,4,6-octatriene yields only ds-5,6-dimethyl-1,3-cyclo-hexadiene when heated, and (2 ,4Z,6Z)-2,4,6-octatriene yields only trans-5,6-dimethyl-l,3-cyclohexadiene. Remarkably, however, the stereochemical results change completely when the reactions are carried out under what are called photochemical, rather than thermal, conditions. Irradiation, or photolysis,... 

Uliginosin A. Cj,HmOs. 3 -[(2,4.Dthydroxy-5.isobutyr-yt-3,3-dimethyl-6-oxo-l,4-cyclohexadien-l-yl)methyI)-2 -4, 6 -trihydroxy-2-methyl-S -(3-methyl-2-butenyl)propio-phenone. Pale yellow crystals, mp ]60.5-161.5s from acetonitrile-chloroform (4 1), uv max (cyclohexane) 229, 293 nm [Pg.1550]

Antkracyclinones. The use of latent quinone reagents such as 2-lithio-3,3,6,6-tetramethoxy-l,4-cyclohexadiene has been extended to a synthesis of an anthra-cyclinone. The starting material 1 was converted into the quinone bisketal 2 by anodic oxidation. The corresponding lithio compound was then condensed with dimethyl 3-methoxyphthalate (3). The reaction fortunately was stereoselective and resulted in 4 in satisfactory yield. The conversion of 4 to the anthracyclinone 5 was conducted in three steps without isolation of intermediates reductive hydrolysis to the hydroquinone, saponification, and finally cyclization with hydrogen fluoride. The overall yield of 5 from 3-bromo-2,5-dimethoxybenzalde-hyde, the precursor of 1, was 8%. ... 

A soln. of 1,2-dimethyl-1,4-cyclohexadiene in 3.6 10 water/acetone added to a yellow slurry of bis(acetonitrile)dichloropalladium(II), KHCO3, and CUCI2 in acetone at 20°, and worked up after 24 h di-p-chlorobis[(l,2,3-r )-5-hydroxy-l,2-dimethyl-2-cyclohexen-l-yl]dipalladium (Y 77%), in methanol containing Na-propionate pressurized in a Fischer-Porter tube with 3.8 atm. CO for 4.5 h methyl trans-S hydroxy-... 

However, there are cases where the 1,2-addition product is more stable and would be the product of thermodynamic control. For example, addition of bromine to l,4-dimethyl-l,3-cyclohexadiene under conditions of thermodynamic control gives 3,4-dibromo-l,4-dimethylcyclohexene because its trisubstituted double bond is more stable than the disubstituted double bond of the 1,4-addition product. 

Can these observations be generalized Let us look at the stereochemistry of the cis-1,3.5-hexatriene-cyclohexadiene interconversion. Surprisingly, the six-membered ring is formed thermally by the disrotatory mode, as can be shown by using derivatives. For example, heated trans,cis,trans-2,4,6-oclatriene gives cii-5,6-dimethyl-l,3-cyclohexadiene, and cw,cw,trfl 5 -2,4,6-octatriene converts into irfl i-5,6-dimethyl-l,3-cyclohexadiene, both disrotatory closures. 

H.7 Here we need a conrotatory ring opening of tran5-5,6-dimethyl-l,3-cyclohexadiene (to produce lrans,cis,tmns-2,4,6-octatriene) then we need a disrotatory cyclization to produce cw-5,6-dimethyl-1,3-cyclohexadiene. 

METHOXY-2,4-CYCLOHEXADlEN-l-YL] -5,5-DIMETHYL-l, 3-CYCLO-HEXANEDIONE]lRON,57, 16 TRICARBONYL [(1,2,3,4,5-n)-2-METH-OXY-2.4-CYCLOHEXADIENE-1-YL ] -IRON(l+)HEXAFLUOROPHOS-PHATE(l-), 57, 107... 

Isoprene or 2,3-dimethyl-l,3-butadiene or 1,3-cyclohexadiene (with Et2NH), 2,3-dimethyl-1,3-butadiene (with n-BuNH2 or piperidine) and 1,3-hexadiene or 2,4-hexa-diene (with PhNH2) similarly give 1 1 telomers in fair to good yields [186]. 

Upon treatment with nickeltetracarbonyl, dimethyl 3-vinyl-l,2-dichlorocy-clobutane-l,2-dicarboxylate 169 is rearranged, to dimethyl 1,4-cyclohexadiene-1,2-dicarboxylate 171 with concomitant loss of the chlorine atoms [87], Reduction to dimethyl 3-vinylcyclobutene-l,2-dicarboxylate 170 is involved in the initial step. (Scheme 64)... 

The products of electrochemical oxidation of conjugated dienes are considerably affected by the reaction conditions such as the material of the electrode, the supporting electrolyte and the solvent. The oxidation of butadiene with a graphite or carbon-cloth anode in 0.5 M methanolic solution of NaClCU mainly yields dimerized products along with small amounts of monomeric and trimeric compounds (equation 5)1. The use of platinum or glassy carbon mainly gives monomeric products. Other dienes such as isoprene, 1,3-cyclohexadiene, 2,4-hexadiene, 1,3-pentadiene and 2,3-dimethyl-l,3-butadiene yield complex mixtures of isomers of monomeric, dimeric and trimeric compounds, in which the dimeric products are the main products. 

Scheme 6.71 Generation of l-oxa-3,4-cyclohexadiene (333) from 6,6-dichloro-3-oxabicyclo[3.1. OJhexane (332) by n-butyllithium and interception of333 by n-butyllithium, styrene, 1,3-butadiene, isopreneand 2,3-dimethyl-l,3-butadiene.

Scheme 6.76 Generation of l-oxa-2,3-cyclohexadiene (351) from 5-bromo-3,4-dihydro-2H-pyran (376) and trapping products of351 obtained from furan, 2,3-dimethyl-1,3-butadiene, 1,1-diphenyl-ethylene, ( )-l-phenylpropene, ( )-2-butene, (Z)-2-butene and tert-butyl alcohol , according to Schlosserand co-workers.

D-Erythrose undergoes self-aldolization in alkali solution, to form d- / co-L- /3 C6 TO-3-octulopyranose by combination of the 1,2-enediol and aldehyde forms. In weak alkali at 105°, syrupy D-erythrose yields d- /ycero-tetrulose, jS-D-a/tro-L-g/ycero-l-octulofuranose, and a-Ti-gluco-i -g/ycero-3-octulopyranose. At 300° in alkali, the major products from syrupy D-erythrose were 1-5% of butanedione (biacetyl) with smaller proportions of pyrocatechol, 33, 2,5-dimethyl-2,5-cyclohexadiene-l,4-dione (2,5-dimethylbenzoquinone), and 2,5-dimethyl-1,4-benzenediol (2,5-dimethylhydroquinone). It was assumed that D-erythrose is reduced to erythritol by a Cannizzaro type of reaction, followed by dehydration of erythritol to form biacetyl. However, very low proportions (<1%) of biacetyl are formed from erythritol compared with D-erythrose itself. Apparently, some other mechanism predominates in the formation of biacetyl. 

Similar results were achieved when benzene was reduced with alkali metals in anhydrous methylamine at temperatures of 26-100°. Best yields of cyclohexene (up to 77.4%) were obtained with lithium at 85° [396]. Ethylamine [397] and especially ethylenediamine are even better solvents [398]. Benzene was reduced to cyclohexene and a small amount of cyclohexane [397, 398] ethylbenzene treated with lithium in ethylamine at —78° gave 75% of 1-ethyl-cyclohexene whereas at 17° a mixture of 45% of 1-ethylcyclohexene and 55% of ethylcyclohexane was obtained [397], Xylenes m- and p-) yielded non-conjugated 2,5-dihydro derivatives, l,3-dimethyl-3,6-cyclohexadiene and 1,4-dimethyl-1,4-cyclohexadiene, respectively, on reduction with sodium in liquid ammonia in the presence of ethanol (in poor yields) [399]. Reduction of diphenyl with sodium or calcium in liquid ammonia at —70° afforded mainly 1-phenylcyclohexene [400] whereas with sodium in ammonia at 120-125° mainly phenylcyclohexane [393] was formed. 

Experimental evidence of the involvement of a biradical intermediate in the decomposition of 3,3-dimethyl-l,2-dioxetane (10) has been obtained by radical trapping with 1,4-cyclohexadiene (CHD). Decomposition of 10 in neat CHD was shown to result in the formation of the expected 1,4-dioxy biradical trapping product, 2-methyl-1,2-propanediol (11) ° . However, more recently, it has been shown that the previously observed trapping product 11 was formed by induced decomposition of the dioxetane, initiated by the attack of the C—C double bond of the diene on the strained 0—0 bond of the cyclic peroxide (Scheme 9)"°. 

Iron, tricarbonyl[2-[(2,3,4,5-i))-4-methoxy-2,4-cyclohexadien-l-yl]-5,5-dimethyl-1,3-cyclohexane one] -]... 

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