Cyclopentenyl double bond

The cyclopentenyl double bond is the most reactive nucleophile, since the double bonds of the phenyl ring are stabilised by aromaticity. HCI is the only electrophile present. 

When in addition to a cyclopentenyl double bond there is a further double bond in the aliphatic chain, as with methyl gorlate (13-cyclopent-2-enyltridec-6-enoate), the base ion is still at m/z = 67, and there is another abundant ion at m/z = 80, formally equivalent to a dihydrofulvene ion there are no ions diagnostic for the position of the double bond in the chain. 

In the case of cyclopentenyl carbamate in which a directive group is present at the homoallyl position, the cationic rhodium [Rh(diphos-4)]+ or iridium [Ir(PCy3)(py)(nbd)]+ catalyst cannot interact with the carbamate carbonyl, and thus approaches the double bond from the less-hindered side. This affords a cis-product preferentially, whereas with the chiral rhodium-duphos catalyst, directivity of the carbamate unit is observed (Table 21.7, entry 7). The presence of a hydroxyl group at the allyl position induced hydroxy-directive hydrogenation, and higher diastereoselectivity was obtained (entry 8) [44]. 

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. 

Bicyclic y-lactams.1 N-Allyltrichloroacetamides in which the double bond is associated with a cyclopentenyl or -hexenyl group undergo cyclization in the presence of CuCl in CH3CN or of Cl2Ru[P(C6H5)3]3 in QHg or C6H4(CH3)2. 

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. 

In the simplest picture of optimum resonance-derived stabilization, the two carbons of the double bond, the amine nitrogen and all five atoms affixed to these three enamine atoms lie in a common plane. Much the same demand for planarity exists for the exocyclic isopropylide-necycloalkanes. For these hydrocarbons, one finds the rearrangement of 1-isopropylcyclohexene to isopropylidenecyclohexane is exothermic by 3.1 0.6 kJ mol-1 but that of 1-isopropylcyclo-pentene to isopropylidenecyclopentane is exothermic by 4.3 0.3 kJ mol -1 [D. B. Bigley and R. W. May, J. Chem. Soc. (B), 1761 (1970)]. If one ignores error bars, the 3.1 - (-4.3) s 7 kJ mol -1 net destabilization for the exocyclic double bond in the cyclopentene vs cyclohexene hydrocarbon case is essentially identical to the 51 — 45 6 kJ mol -1 for the cyclopentenyl and cyclohexenyl enamines. 

The ketone IR absorption of 3-methyl-2-cyclohexenone occurs near 1690 cm-1 because the double bond is next to the ketone group. The ketone IR absorption of 3-cyclopentenyl methyl ketone occurs near 1715 cm-1, the usual position for ketone absorption. 

The polysulfanes formed on reaction of DCPD with liquid sulfur have been studied by extraction of sulfur cement and analysis by LC, H-NMR, MS, and other techniques.The initial products are trisulfane and pentasulfane derived from DCPD by addition of S3 or S5 units to the norbomenyl double bond. These monomers are believed to further react with elemental sulfur to form low-molecular mass polymers (CS2 soluble), and on further heating form an insoluble material. The cyclopentenyl unsaturation of DCPD is much less reactive and is still present in the CS2-soluble products. endo-T>CP D reacts more slowly with liquid sulfur at 140 °C than eco-DCPD, while the cyclic trisulfanes of endo- and gxo-DCPD react at almost the same rate with liquid sulfur at 140°C. The stmctures of DCPD-S3, DCPD-S5, and the hkely stmcture of the low-molecular mass polymer, are shown in Figure 8. 

Beyond the disrotatory or conrotatory stereochemical imperative which must accompany all Nazarov cyclizations there exists a secondary stereochemical feature. This feature arises because of the duality of allowed electrocyclization pathways. When the divinyl ketone is chiral the two pathways lead to dia-stereomers. The nature of the relationship between the newly created centers and preexisting centers depends upon the location of the cyclopentenone double bond. The placement of this double bond is established after the electrocyclization by proton loss from the cyclopentenyl cation (equation 5). Loss of H, H or in this instance generates three tautomeric products. The lack of control in this event is a drawback of the classical cyclization. Normally, the double bond occupies the most substituted position corresponding to a Saytzeff process. The issue of stereoselection with chiral divinyl ketones is iUustrated in Scheme 7. The sense of rotation is defined by clockwise (R) or counterclockwise (5) viewing down the C—O bond. Thus, depending on the placement of the double bond, the newly created center may be proximal or distal to the preexisting center. If = H the double bond must reside in a less substituted environment to establish stereoselectivity. 

The other two double bonds in 7-23 have rearranged. The allyl double bond and the double bond in the new cyclopentenyl group are situated suitably for a [3,3] sigmatropic shift. 

The pericyclic process comes next and it is a Nazarov reaction (p. 962), a conrotator electrocyclic closure of a pentadienyl cation to give a cyclopentenyl cation. There is r stereochemistry and the only regiochemistry is the position of the double bond at the end of th -. reaction. Here it prefers the more substituted side of the ring. 

Unsaturated acyl chlorides react with substituted alkynes to give mixtures of linear and cyclopentenyl products, the latter being formed in greater yield at higher temperatures (Scheme 22). ° This offers a convenient access to a variety of S-chlorocyclopentenones bearing substituents at the 4- and S-positions, with control over the location of the double bond. Application of this method includes a short synthesis of the antibiotic methylenomycin B (17 Scheme 23). ° ... 

Steroidal systems can be achieved by initiating cyclization with a cyclopentenyl cation. Asymmetrically substituted initiators are attacked at the least alkylated terminus. With a central E double bond an anti-trans-product is formed via a chair-chairlike transition state53. Substituents in position 6. 7, 11 or 12 lead to varying mixtures of diastereomers depending on the position (a or fi) of these substituents in the cyclized system. 

The structure of the 2-norbomyl cation 5 is indirectly proved by the solvolytic cyclization of 2-(3-cyclopentenyl)ethanol. Thus the ar tolysis rate of nosylate 36 exceeds that of a saturated analogue by a factor of 95 This fact has been interpreted in favour of direct participation of a double bond in the substitution of an arylsulphonate anion. As shown by Sargent and Bartlett the substitution of... 

On the other hand, when the 2-norbornyl cation is formed from its A -cyclopentenyl-ethyl precursor ( it-route ), the more flexible structure of the precursor makes it possible for the participating group (double bond) to move closer to the reaction centre which results in a decreasing a-effect for the exo isomer. 

Although the 3-cyclopentenyl derivatives formally resemble the 7-anti-norbornenyl ones, their solvolysis has until recently been considered to proceed without any double-bond participation due to the practically planar conformation of the cyclo-pentenyl ring and an increased angle strain Lambert, however, has shown that 3-cyclopentenyl tosylate 234 is formolyzed under practically complete (>99%)... 

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