Cyclopentene ring

The C—C double bond in the cyclopentene ring can be cleaved by the osmium tetroxide-periodate procedure or by photooxygenation. The methoxalyl group on C-17 can, as a typical a-dicarbonyl system, be split off with strong base and is replaced by a proton. Since this elimination occurs with retention of the most stable configuration of the cyclization equi-hbrium, the substituents at C-17 and C-18 are located trans to one another. The critical introduction of both hydrogens was thus achieved regio- and stereoselectively. 

Aldol reaction of campholenic aldehyde with propionic aldehyde yields the intermediate conjugated aldehyde, which can be selectively reduced to the saturated alcohol with a sandalwood odor. If the double bond in the cyclopentene ring is also reduced, the resulting product does not have a sandalwood odor (161). Reaction of campholenic aldehyde with -butyraldehyde followed by reduction of the aldehyde group gives the aHyUc alcohol known commercially by one manufacturer as Bacdanol [28219-61 -6] (82). 

Scheme 5 details the synthesis of / -cormorsterone (14) from 17. Oxidative scission of both carbon-carbon double bonds in 17 with ozone, followed by two straightforward operations, furnishes intermediate 38. The stability of the oxime in these systems is noteworthy, and is attributed to its hindered nature. At this juncture, it is instructive to note that substituted cyclopentene rings, like the... 

Scheme 7 Cyclopentene ring closure in the presence of urethane and sulfonamido groups in total synthesis of agelastatin (38) [30]...

The use of the NHC-Ni catalytic system has also been used to promote the cycloisomerisation of vinyl cyclopropanes 125 to afford the cyclopentene rings 126 in excellent yields (Scheme 5.33) [38]. The reaction required only 1 mol% of [NiCCOD) ] and 2 mol% of IPr carbene. 

Bode and co-workers have demonstrated the application of such substrates in a number of reactions, including cyclopentene ring formation and y-lactamisation [15]. For example, y-lactamisation to give 56 with saccharin-derived sulfonylimines 54 was achieved from a-hydroxyenone 53 with moderate to excellent stereocontrol (Scheme 12.10). 

While SN4 and STD exhibit essentially D2d symmetry, SN3 has Cs symmetry with a planar diene ring and an envelope-shaped cyclopentene ring. The maximum torsion in the folded ring of SN3 is 20.2°. The spiro-connection of two five-membered ring systems leads to some strain at the spiroatom (101.4° to 101.8° at A1 compared to 109.5° for tetrahedral... 

By analogy with the intramolecular insertion of phenylthiocarbenes, the reaction of (oo-oxido)diazoalkanes 93 resulted in the formation of cycloalkenes 94.38 However, the reaction was proven to proceed not via a carbene route b but a nitrene route a as shown in Scheme 26. The nitrene route is supported by the formation of heterocyclic products 98 and 99.39 This insertion reaction was used in the cydization step to the cyclopentene ring formation of isocarbacycline 97.40... 

Increased ring strain in the C-4 ( = 2) and C-5 (n — 3) products may be a reason for lower yields (and longer irradiation times). The effect of strain on cyclizations is well-documented outside of cycloaromatization chemistry, as well. For example, annealing of a cyclopentene ring leads to a decrease in the cyclization rate and an inversion of the 5-exo/6-endo selectivity for all three patterns shown in Fig. 22. 

Thus, the structure of viridenomycin was established except for the absolute configuration. This antibiotic is partially related to hitachimycin, which is a 19-membered lactam antibiotic possessing a phenyl group and a cyclopentene ring, but devoid of the tetraene systems and ester linkage. 

As a last example we mention polydicyclopentadiene (DCPD). Dicyclopentadiene is the Diels-Alder adduct of cyclopentadiene, an abundant product from the cracker in the refinery. It contains a strained norbomene ring and a less strained cyclopentene ring. ROMP leads to opening of the norbomene ring, but some reaction of the cyclopentene ring also takes place, which leads to cross-linking (Figure 16.26). 

Another illustration of the utility of this chemistry is provided by its use as the key step in a synthesis of prostaglandin PGF2Q [24]. In this case, the radical formed from bromoacetal (35) undergoes an intramolecular S-exo-trig cyclization onto the olefin of the cyclopentene ring, thereby generating a new radical capable of undergoing another reaction. When it was intercepted intermolecularly by ynone (36), enone (37) was produced in a 55% yield (Scheme 10). 

Marino and his coworkers [20a], on the other hand, studied the fluoride ion-induced desilylation of ethyl 2-silyloxycyclopanecarboxylates 24 and the resulting "homoenolate" anion 25 was allowed to react with different electrophiles, such as Michael acceptors, to give dissonant cyclopentene rings (26) via a [3 -i- 2] annulation strategy (Scheme 5.16). 

Since it is known that the cyclopentene ring of norbomene can be easily opened by methylidene carbene complex Ih, bicyclic compound 66 has been synthesized from norbomene derivative 65 having an alkene part in a sidechain in the presence... 

Treatment of compound 222, containing a 1,2,4-trioxane ring fused to a cyclopentene ring, with O2 leads to formation of a hydroperoxide (223) with ene displacement, as shown in equation 76. The structure of 223 was determined by single-crystal XRD analysis. A contact of the hydroperoxy group with the endocyclic ether O atom of a neighboring molecule (287.4 pm) points to weak H-bonding. ... 

This means that, formally, the cyclopentene ring in 65b is cleaved and two carbon-carbon bonds are formed between the double and triple bonds, respectively, to produce 66b (Figure 6). Ring size of the cycloalkene formed in this reaction corresponds to the carbon number of the original cycloalkene plus two carbons. 

Cycloalkylation (7, 148).2 Cycloalkylation of 2 with (Z)-l was used as one step in a total synthesis of (+ )-sesbanine (6), a constituent of Seshania drummondii seeds with antileukemic activity. The hydroxyl group was introduced into the cyclopentene ring of 4 by iodolactonization followed by reduction to give 5. Final steps included aminolysis of the lactone ring, intramolecular addition of the amide anion to the CN group, and hydrolysis to give 6. 

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