Elimination reactions diene formation

O-isopropylidene derivative (57) must exist in pyridine solution in a conformation which favors anhydro-ring formation rather than elimination. Considerable degradation occurred when the 5-iodo derivative (63) was treated with silver fluoride in pyridine (36). The products, which were isolated in small yield, were identified as thymine and l-[2-(5-methylfuryl)]-thymine (65). This same compound (65) was formed in high yield when the 5 -mesylate 64 was treated with potassium tert-hx Xy -ate in dimethyl sulfoxide (16). The formation of 65 from 63 or 64 clearly involves the rearrangement of an intermediate 2, 4 -diene. In a different approach to the problem of introducing terminal unsaturation into pento-furanoid nucleosides, Robins and co-workers (32,37) have employed mild base catalyzed E2 elimination reactions. Thus, treatment of the 5 -tosylate (59) with potassium tert-butylate in tert-butyl alcohol afforded a high yield of the 4 -ene (60) (37). This reaction may proceed via the 2,5 ... 

The formation of heterobutadiene 9 can be explained by an insertion-elimination reaction Insertion of sulfur into the silicon-hydrogen bond yields in the first step (2-Me2NCH2C6H4)(CH=CH2)Si(SH)2 as intermediate. This species eliminates H2S, producing the 1-thia-2-sila-1,3-diene 9, which is one of the rare examples possessing a silicon-sulfiir double-bond unit... 

Other elimination reactions reported include the formation of the l(10),5-diene from a 5,6-dibromo-19-nor-steroid, and also from a 6j -methoxyoestr-5(10)-... 

TT-Allyl and related complexes can be prepared by reactions such as 23.81-23.85 the last two reactions illustrate formation of allyl ligands by deprotonation of coordinated propene, and protonation of coordinated buta-1,3-diene respectively. Reactions 23.82 and 23.83 are examples of pathways that go via a-bonded intermediates (e.g. 23.48) which eliminate CO. 

As our computational results presented above demonstrate, it is highly unlikely that heterocycles would be good dienes for Diels-Alder reactions if formation of one or two C-N bonds were involved in the course of the reaction. This automatically eliminates some tautomeric forms of five-membered heterocycles with heteroatoms in 1 and 2 positions as well as five-membered heterocycles with heteroatoms in 1,2,3 and 1,2,5 positions. A major reason for the low reactivity of the heterocycles is because of their high aromaticity. It is obvious that diminishing or eliminating the aromaticity in these heterocycles would make them better dienophiles for Diels-Alder reactions. 

Many authors have concluded that the conjugated diene method might be used as an index of stability of lipids in place of, or in addition to PV. It is faster than iodometric PV determination, much simpler, does not depend on chemical reactions or color development, and requires a smaller sample size. However, the presence of compounds absorbing in the region of the conjugated diene formation may interfere with such determinations. It has been proposed that interference in complex systems can be minimized or eliminated by derivative spectroscopy, using photodiode detection of spectra and computer analysis of the data. ... 

Several 1,3-diene syntheses involving elimination reactions that are catalyzed by Pd(Ph3P)4 have been reported. The first involves the Et3N mediated elimination of HOAc from allylic acetates in refluxing THF. A complementary procedure involves the Pd(Ph3P)4 catalyzed decarboxylative elimination of /3-acetoxy-carboxylic acids (eq 46). The substrates are easily prepared by the condensation of enals and carboxylate enolates irrespective of the diastereomeric mixture, ( )-alkenes are formed in a highly stereocontrolled manner. The geometry of the double bond present in the enal precursor remains unaffected in the elimination and the reaction is applicable to the formation of 1,3-cyclohexadienes. 

Different products are obtained depending on substituents. Reaction of 526, which has no substituent at C-6, afforded the diene 528 (48 %) and the formate 529 (34 %). In this reaction, formaldehyde is generated and inserts to the intermediate 527 to form 530. The formate 529 was obtained from 530 by j6-H elimination to generate 531 and reductive elimination. The diene 528 was obtained in 87 % yield in the presence of t-octylamine as a scavenger of formaldehyde. Also the 1,3-dioxane 532 is formed in the presence of formaldehyde. Thus the reaction of the carbonate 533 with formaldehyde afforded the 1,3-dioxane 535 via 534 in high yield [197]. 

The elimination to a diene is a competing pathway in all these reactions. If ttiethylamine is employed as the amine, and/or the 1,3-diene has an electron-withdrawing group in the 1-position, then diene formation predominates. For example, ( , )-2,4-pentadienoic acid reacted with aryl bromides in the presence of triethylamine and the palladium catalyst to give ( ,E)-5-aryl-2,4-pentadienoic acid in good yield. The propensity for ehmination to a diene was later developed into a... 

When performing the 1,4-elimination reaction on 0-(lV-Boc-2-pyrrolidinyl) derivatives 159, areversal of the C1=C2 stereoselectivity of the diene was observed, leading to the exclusive formation of the (1E,3E) diastereomer (Scheme 69). In this case, the reaction required the use of an amide as base and the yields were highest when lithium 2,2,6,6-tetramethylpiperidide (LiTMP) in THF was employed [151]. A significant drop of the yield is observed when an alkyl substituent is present on position C2 of the substrate. 

In the reaction of aryl and alkenyl halides with 1,3-pentadiene (248), amine and alcohol capture the 7r-allylpalladium intermediate to form 249. In the reactions of o-iodoaniline (250) and o-iodobenzyl alcohol (253) with 1,3-dienes, the amine and benzyl alcohol capture the Tr-allylpalladium intermediates 251 and 254 to give 252 and 255[173-175]. The reaction of o-iodoaniline (250) with 1,4-pen tadiene (256) affords the cyclized product 260 via arylpalladiuni formation, addition to the diene 256 to form 257. palladium migration (elimination of Pd—H and readdition to give 258) to form the Tr-allylpalladium 259, and intramolecular displacement of Tr-allylpalladium with the amine to form 260[176], o-Iodophenol reacts similarly. 

When allylic compounds are treated with Pd(0) catalyst in the absence of any nucleophile, 1,4-elimination is a sole reaction path, as shown by 492, and conjugated dienes are formed as a mixture of E and Z isomers[329]. From terminal allylic compounds, terminal conjugated dienes are formed. The reaction has been applied to the syntheses of a pheromone, 12-acetoxy-1,3-dode-cadiene (493)[330], ambergris fragrance[331], and aklavinone[332]. Selective elimination of the acetate of the cyanohydrin 494 derived from 2-nonenal is a key reaction for the formation of the 1,3-diene unit in pellitorine (495)[333], Facile aromatization occurs by bis-elimination of the l,4-diacetoxy-2-cyclohex-ene 496[334],... 

Carboxylic acids react with butadiene as alkali metal carboxylates. A mixture of isomeric 1- and 3-acetoxyoctadienes (39 and 40) is formed by the reaction of acetic acid[13]. The reaction is very slow in acetic acid alone. It is accelerated by forming acetate by the addition of a base[40]. Addition of an equal amount of triethylamine achieved complete conversion at 80 C after 2 h. AcONa or AcOK also can be used as a base. Trimethylolpropane phosphite (TMPP) completely eliminates the formation of 1,3,7-octatriene, and the acetoxyocta-dienes 39 and 40 are obtained in 81% and 9% yields by using N.N.N M -tetramethyl-l,3-diaminobutane at 50 in a 2 h reaction. These two isomers undergo Pd-catalyzed allylic rearrangement with each other. 

Cyclopropanes can also be obtained by the reaction of vinyltrialkylborates with aldehydes followed by treatment with phosphoms pentachloride and base (300), and by the rearrangement of 5-substituted alkynyltrialkylborates (308). It is also possible to utilize this approach for the synthesis of five- and six-membered rings (3). Trans-1,4-elimination ia cycHc systems leads to the formation of stereodefined acycHc 1,5-dienes or medium-ring dienes, depending on the starting compound (309). 

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