Cyclopentenyl ring

Hydrazine hydrate can often reduce vulnerable substituents on the pyrazolcme ring. One such example is the alkene group of the cyclopentenyl ring of pyrazol-3-one 16. It was found that heating 4-(l-methoxycarbonyl-2-oxopropyl)-2-cyclo-penten-l-ol 15 with hydrazine hydrate in ethanol afforded a mixture of pyrazol-3-one derivatives 16 and 17 in a 5 2 ratio (97JHC233) (Scheme 4). 

Perhaps the first successful variation of the PPFA framework was the development of the JosiPhos family of ligands (33) [125, 131, 141, 142], Here, the two phosphorus groups are attached to the same cyclopentenyl ring rather than one to each of the rings. The C2-symmetry model is now a distant memory for these ligand families. 

In this category, fatty acids with a terminal cyclopentenyl ring are best known (Sebedio and Grandgirard, 1989). There are three main types hydnocarpic (ll-cyclopent-2-enyl-undecanoic) acid (structure IVa, Fig. 5.1), chaulmoogric (13-cyclopent-2-enyl-tridecanoic) acid (structure IVb) and gorlic (13-cyclopent-2-enyl-tridec-6-enoic) acid (structure IVc) and its positional isomers (13-cyclopent-2-enyl-tridec-4-enoate and 13-cyclopent-2-enyl-tridec-9-enoate). They usually occur in varying proportions as major components (up to 90% of total fatty acids) in seed oils along with smaller amounts of saturated, oleic and linoleic acids. 

There are also examples of electrocyclic processes involving anionic species. Since the pentadienyl anion is a six-7c-electron system, thermal cyclization to a cyclopentenyl anion should be disrotatory. Examples of this electrocyclic reaction are rare. NMR studies of pentadienyl anions indicate that they are stable and do not tend to cyclize. Cyclooctadienyllithium provides an example where cyclization of a pentadienyl anion fragment does occur, with the first-order rate constant being 8.7 x 10 min . The stereochemistry of the ring closure is consistent with the expected disrotatory nature of the reaction. 

Methyl polyfluoro-1-cyclobutenyl ethers react with sulfur trioxide to form fluoride and polyfluorocyclobutenones, but coproducts are their fluorosulfonyloxy derivatives resulting from sulfur trioxide insertion [5] (equation 5) The nonsul-fonyloxylated polyfluorocyclopentenones are favored when starting with methyl polyfluoro-1-cyclopentenyl ethers (equation 6). Ring insertion of sulfur tnoxide occurs during the reaction of 1-methoxytnfluorocyclopropene (equation 7)... 

Upon treatment of a divinyl ketone 1 with a protic acid or a Lewis acid, an electrocyclic ring closure can take place to yield a cyclopentenone 3. This reaction is called the Nazarov cyclization Protonation at the carbonyl oxygen of the divinyl ketone 1 leads to formation of a hydroxypentadienyl cation 2, which can undergo a thermally allowed, conrotatory electrocyclic ring closure reaction to give a cyclopentenyl cation 4. Through subsequent loss of a proton a mixture of isomeric cyclopentenones 5 and 6 is obtained ... 

It occurred to us that bromination of 3-cyclopentenyl hydroperoxide would yield an adduct in which there is always a frans-3-bromine, and that ring closure would afford a [2.2.1]-peroxide with a substituent at the 5-position. This additional... 

We developed a convenient synthesis of 3-cyclopentenyl hydroperoxide via hydro-boration and autoxidation of cyclopentadiene, and bromination proceeded smoothly to afford 32 40). Ring closure with silver trifluoroacetate (Eq. 26) afforded a 5-bromo-2,3-dioxabicyclo[2.2.1]heptane 34 (6%) and a 5-trifluoroacetoxy-2,3-dioxabicyclo-[2.2.1]heptane 35 (14%), and it was shown independently that 34 is rapidly converted into 35 by reaction with Ag02CCF3. To avoid the trifluoroacetate bromide substitution that accompanies and competes with the dioxabicyclization, 32 was treated with silver oxide and this slowly yielded an isomeric 5-bromo-peroxide 33 (42 %) (Eq. 26). 

The corresponding cyclohexenyl system 56 (Scheme 16) remains relatively unreactive, however, even when the reaction is performed under an ethylene atmosphere after 24 h (10mol% lb, 1 atm ethylene, CH2Cl2), only 10-20% of chromene 47 is obtained. This persistent lack of reactivity is presumably because (1) the relatively strain-free six-membered ring is less prone (relative to cyclopentenyl and cycloheptenyl structures) to react with LnRu=CH2 [21], and (2) in case ring rupture does occur with the proper regiocontrol to afford 57... 

Cyclohexenyl and cyclopentenyl iodonium tetrafluoroborates were also photolyzed in methanol. Ring-strained five-membered cyclic vinyl cation could be generated photochemically as well as the six-membered cyclic vinyl cation.25... 

More recently Miron and Lee (1962) analysed the hydrocarbons removed from strong acid catalysts in some detail, and suggested unsaturated cyclic structures. These structures contain from one to five five-membered rings with various methyl and alkenyl substituents and a minimum of two double bonds per molecule. However, during their drowning procedures, as the acid is diluted, considerable polymerization occurs. This conclusion is based on work by Hodge (1963), who showed that cyclopentenyl cations are rapidly destroyed by alkylation at 10 m concentrations in 35% H2SO4. 

Cyclopentadienyl iodide (5-iodo-l,3-cyclopentadiene) reacts approximately 10 times as rapidly as cyclopentenyl iodide (3-iodocyclopentene) with tetrabutylammonium bromide under the same conditions (Breslow, R. Canary, J. W., J. Am. Chem. Soc., 1991, 113, 3950). Under solvolysis conditions (SnI), the reactivity order is reversed. Provide a rationalization based on the frontier orbitals of the system. What would you predict for the analogous 3- and 7-membered ring systems ... 

Acylation with cyclopentenyl- or cyclohexenylacetyl chloride gave three-membered ring systems.113 114 An interesting example is the reaction of ethylene with acid chlorides complexed with excess A1C13 to give the 17 p.y-unsaturated ketones 115... 

Fig. 4.2 illustrates the first few members of the series of equilibria of conjugated ions. In cations, they are the equilibria between the allyl 4.11 and the cyclopropyl cation 4.12, the pentadienyl 4.13 and the cyclopentenyl cation 4.14, and the heptatrienyl 4.15 and cycloheptadienyl cation 4.16, In anions, they are between the allyl 4.17 and the cyclopropyl anion 4.18, the pentadienyl 4.19 and the cyclopentenyl anion 4.20, and the heptatrienyl 4.21 and cycloheptadienyl anion 4.22. There are heteroatom-containing analogues, with nitrogen and oxygen lone pairs rather than a carbanion centre, and the systems can again have substituents and fused rings. 

A ring-opening has also been seen, when the relief of strain in a cyclopropane makes it thermodynamically favourable—the cyclopentenyl anion 4.100 opens to the pentadienyl anion 4.101. This reaction had no option but to be disrotatory with the two hydrogen atoms moving outwards 4,98, since a trans double bond is impossible in the 6-membered ring. 

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