Six-membered ring alkene

The adduct is a six-membered ring alkene. The diene can have the two conjugated bonds contained within a ring system as with cyclopentadiene or cyclohexadiene, or the molecule can be an acyclic diene that must be in the cis conformation about the single bond before reaction can occur. 

Photochemical cycloaddition has been reported for the dione (248). Irradiation of this compound in the presence of cyclopentadiene affords the adduct (249). Similar adducts. (250) and (251) respectively, are formed when indene and cyclopentene are used as the alkenes. In contrast to the above when six membered ring alkenes such as cyclohexa-1,3-diene and dihydropyran are used cycloaddition occurs at the ring double bond to yield the adducts (252) and (253) respectively. Several examples of the thermal and/or photochemical addition of diphenyl ketene and p-tolyl diphenylketimine to heterocyclic 1,2-diones has been reported. One example of the reaction is that of the... 

By far the most common cycloalkene generated using Martin s sulfurane is cyclohexene. Isolated six-membered ring alkenes and cyclohexenes in fused polycyclic systems are all generated with regularity using the title reagent. 

Sc(OTf)3 was proved to be an effective catalyst for Diels-Alder reactions of a new type of 1-hydrazinodiene with many dienophiles (Scheme 12.16) [32]. The obtained Diels-Alder adducts could be converted to six-member ring alkenes via suprafacial 1,5-sigmatropic rearrangements of hydrogens after the loss of dinitrogens. This process enabled a 1,3-transfer of stereochemistry to a new position on the six-member ring system. 

The silyl enol ethers 209 and 212 are considered to be sources of carbanions. and their transmetallation with Pd(OAc)2 forms the Pd enolate 210. or o.w-tt-allylpalladium, which undergoes the intramolecular alkene insertion and. 1-elimination to give 3-methylcyclopentenone (211) and a bicyclic system 213[199], Five- and six-membered rings can be prepared by this reaction[200]. Use of benzoquinone makes the reaction catalytic. The reaction has been used for syntheses of skeletons of natural products, such as the phyllocladine intermediate 214[201], capnellene[202], the stemodin intermediate 215[203] and hir-sutene [204]. 

The enzyme catalyzed reactions that lead to geraniol and farnesol (as their pyrophosphate esters) are mechanistically related to the acid catalyzed dimerization of alkenes discussed m Section 6 21 The reaction of an allylic pyrophosphate or a carbo cation with a source of rr electrons is a recurring theme m terpene biosynthesis and is invoked to explain the origin of more complicated structural types Consider for exam pie the formation of cyclic monoterpenes Neryl pyrophosphate formed by an enzyme catalyzed isomerization of the E double bond m geranyl pyrophosphate has the proper geometry to form a six membered ring via intramolecular attack of the double bond on the allylic pyrophosphate unit... 

An NOE between the the alkene proton at 3h = 6.22 and the methyl protons at Sh = 1.17 establishes the relative eonfiguration (exo) of the respeetive methyl group. The exo attaehment of the six-membered ring in the stereostrueture I follows, in partieular, from the NOE between the methyl protons at 5h = 1.26 and the bridgehead proton at Sh = 3.22 as well as the absenee of effeets between the alkenyl proton pair with Sh = 5.44/6.22 and the bridgehead proton pair with Sh = 2.85/3.22. 

The effect of introducing -hybridized atoms into open-chain molecules was discussed earlier, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are noted when sp centers are incorporated into six-membered rings. Whereas the fiee-energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecyclohexane and to 4.9 kcal/mol in cyclohexanone. ... 

Fluorinated cyclobutanes and cyclobutenes are relatively easy to prepare because of the propensity of many gem-difluoroolefins to thermally cyclodimerize and cycloadd to alkenes and alkynes. Even with dienes, fluoroolefins commonly prefer to form cyclobutane rather than six-membered-ring Diels-Alder adducts. Tetrafluoroethylene, chlorotrifluoroethylene, and l,l-dichloro-2,2-difluoroethyl-ene are especially reactive in this context. Most evidence favors a stepwise diradical or, less often, a dipolar mechanism for [2+2] cycloadditions of fluoroalkenes [S5, (5], although arguments for a symmetry-allowed, concerted [2j-t-2J process persist [87], The scope, characteristic features, and mechanistic studies of fluoroolefin... 

The Diels-Alder reaction,is a cycloaddition reaction of a conjugated diene with a double or triple bond (the dienophile) it is one of the most important reactions in organic chemistry. For instance an electron-rich diene 1 reacts with an electron-poor dienophile 2 (e.g. an alkene bearing an electron-withdrawing substituent Z) to yield the unsaturated six-membered ring product 3. An illustrative example is the reaction of butadiene 1 with maleic anhydride 4 ... 

Alkenes, alkynes, and arenes (aromatic compounds) all contain carbon-carbon multiple bonds. Alkenes have a double bond, alkynes have a triple bond, and cneues have alternating double and single bonds in a six-membered ring of carbon atoms. Because of their structural similarities, these compounds also have chemical similarities. 

A method for the stereospecific synthesis of thiolane oxides involves the pyrolysis of derivatives of 5-t-butylsulfinylpentene (310), and is based on the thermal decomposition of dialkyl sulfoxides to alkenes and alkanesulfenic acids299 (equation 113). This reversible reaction proceeds by a concerted syn-intramolecular mechanism246,300 and thus facilitates the desired stereospecific synthesis301. The stereoelectronic requirements preclude the formation of the other possible isomer or the six-membered ring thiane oxide (equation 114). Bicyclic thiolane oxides can be prepared similarly from a cyclic alkene301. 

Although the intramolecular cyclopropanation of simple alkenes easily occurs in those cases where a five- or six-membered ring is formed in addition to the three-membered ring [13], the intermolecular version of this process was described by Barluenga et al. in 1997 [ 14c]. Thus, this reaction has shown a high... 

Alkyl halides and tosylates react with Na2pe(CO)4 in the presence of ethylene to give alkyl ethyl ketones. The reaction was not successful for higher alkenes, except that where the double bond and the tosylate group are in the same molecule, five- and six-membered rings can be closed. ... 

In the Diels-Alder reaction a double bond adds 1,4 to a conjugated diene (a 2 + 4 cycloaddition), so the product is always a six-membered ring. The double-bond compound is called a dienophile. The reaction is easy and rapid and of very broad scope and reactivity of dienes and dienophiles can be predicted based on analysis of the HOMOs and LUMOs of these species. Ethylene and simple alkenes make poor dienophiles, although the reaction has been carried out with these compounds. 

As is outlined for ene reactions of singlet oxygen in Scheme 15, the prototypical ene reaction starts with the electron delocalization from the HOMO of propene to the LUMO of X=Y. The delocalization from the HOMO, a combined n and orbital with larger amplitude on n, leads to a bond formation between the C=C and X=Y bonds. Concurrent elongation of the bond enables a six-membered ring transition stracture, where partial electron density is back-donated from the LUMO of X=Y having accepted the density, to an unoccupied orbital of propene localized on the bond. As a result, the partial electron density is promoted (pseudoex-cited) from the HOMO (it) to an unoccupied orbital (ct n ) of alkenes. This is a reaction in the pseudoexcitation band. 

The alkylation of olefinic G-H bonds proceeds when conjugated enones are employed in the ruthenium-catalyzed reaction with alkenes, as shown in Equation (16).1 7 Among the acylcyclohexenes, 1-pivaloyl-l-cyclohexene exhibits a high reactivity and the presence of an oxygen atom at the allylic position in the six-membered ring increases the reactivity of the enones. Some terminal olefins, for example, triethoxyvinylsilane, allyltrimethylsilane, methyl methacrylate, and vinylcyclohexane, are applicable for the alkylation of the olefinic C-H bonds. Acyclic enones also undergo this alkylation. 

The formation of a six-membered ring is also feasible but is more limited, and the reaction is found to be more sensitive to the reaction conditions (Scheme 51). The difficulty for forming cyclohexanes is ascribed to the poorer ability of 1,7-enynes to function as bidentate ligands. This problem can be partially circumvented by introducing an alkene moiety (206 vs. 207) or a substituent that can coordinate to the metal, such as a free carboxylic acid, although in this case, the actual mechanism involves hydropalladation as the first step (see Section 10.07.4.1.3.(i).). 

Several factors and observations support the route proposed in Scheme 10 (1) Due to steric factors, the styrenyl alkene is expected to react preferentially (versus the neighboring disubstituted cyclic olefin see below for further discussion). (2) Involvement of tetracyclic intermediates such as 43 provides a plausible rationale for the reluctance of six-membered ring ethers [46 in Eq. 4] to participate in the catalytic rearrangement and for the lack of reactivity of cyclopen-tenyl substrates [48 in Eq. 5] because of the attendant angle strain, the generation of the tetracyclic intermediate is not favored. (3) Reactions under ethylene atmosphere inhibit dimer formation, since 44 is intercepted with H2CCH2, rather than 41 [19]. 

---