Cyclopentenes oxidative cleavage

The carbonyls Fe(CO)5 and [CpFe(CO)2]+ (2) form stable cationic complexes with alkenes, which are used for both protection and activation of alkenes [1]. [CpFe(CO)2]+ (2 abbreviated as Fp+) is prepared by the reaction of cyclopentadienyl anion (1) with Fe(CO)5, followed by oxidative cleavage with bromine, and used for the protection of alkenes. The electron density of the double bond is decreased by the coordination of [CpFe(CO)2]+ and hence this bond is activated to nucleophilic attacks. Introduction of nucleophiles, such as the carbon nucleophile of malonate, to cyclopentene becomes possible via the formation of the complex 3, and the stable tftmv-er-alkyliron complex 4 of cyclopentane is prepared. The vinyl ether complex 6 is obtained easily from the a-bromoacetal 5, and reacts with an enolate of ketone 7 as an... 

Flowers et al. have dealt with the thermal gas-phase reactions of methyl-oxirane, other methyl-substituted oxiranes, and ethyloxirane. The kinetics of the processes have been compared. Pyrolysis of these compounds is a first-order, homogeneous, nonradical process the reaction rate is not affected by radical scavengers. A biradical mechanism holds. The thermochemical behavior of cyclopentene oxide and cyclohexene oxide is similar. The primary products are the corresponding carbonyl compounds and unsaturated alcohols. Two mechanistic possibilities have been discussed they are obtained from a common biradical intermediate or the alcohol is formed directly from the oxirane in a concerted manner. Thermolysis of spirooxiranes leads to ketone derivatives via biradicals with homolytic bond cleavage (Eqs. 376, 377). ... 

Oxidative cleavage of bicyclic alcohols. Oxidation of either exo- or endo-norbornanol (1) with 2 equiv. of ceric ammonium nitrate in 50% aqueous acetonitrile at 50° gives three products of oxidative cleavage, 3- and 4-cyclopentene-acetaldehydes (2) and (3) and 3-nitratocyclopentaneacetaldehyde (4).3 Oxidation... 

Another prominent route to develop 1,5-difunctionlized molecular skeletons is the oxidative cleavage of cyclopentene derivatives (Scheme 2.57). In Corey s terminology of retrosynthesis this is the reconnect operation (reconnection of the two functionalities). Other reactions that faU under this heading include the Baeyer-Villiger oxidation and the Beckmann rearrangement of cyclopentanones. 

Taber (2002) plan for (-)-codeine oxidative cleavage of cyclopentene ring by ozonolysis followed by recyclization to form cyclohexone ring (steps 16 and 17). 

The 4 -azido-carbocyclic nucleoside 176, related to the furanoside 84 (Section 6), has been prepared as a racemate by the chemistry outlined in Scheme 17 the cyclopentene 175 was prepared from 3-methoxycycIohexene by oxidative cleavage and subsequent aldol condensation.206... 

Taber (University of Delaware) described an approach that focuses on the same bond construction used by Mulzer to set the quaternary center, but uses the opposite sense of polarity in construction of this bond. The key transformation was projected to be 100 to 101 via a combination of aldol-dehydration and intramolecular alkylation reactions (not in any particular order). Compound 100 is highly fimctionalized. Perhaps in an attempt to reduce the level of functionality in precursors, it was to be generated in a manner we first saw in Chapter 2, by the oxidative cleavage of a cyclopentene. 

Oxirane on thermolysis or photolysis suffers C—O homolysis to give a plethora of products (Scheme 2). Substituted oxiranes behave similarly on thermolysis although some C—C cleavage is observed (Scheme 3). Cyclopentene and cyclohexene oxides undergo only C—O cleavage (Scheme 4). 

Other derivatives were prepared by modifications of the original Ellman route. Thus, a second cyclopentene precursor 18 containing the C5 6 cis double bond was carried through to give the resin-bound intermediate 19 via a Stille coupling and an oxidation. Subsequent cuprate addition, reduction, and cleavage give 20 and 21 (the F and E series) with the C56 double bond intact. This type of approach was used to synthesize 11 more compounds in this series. 

Cleavage of epoxides. As with simpler haloboranes, these reagents cleave epoxides of cycloalkenes to give, after nonoxidative workup, halohydrins in 65-90% yield. When carried out at -78 to -100°, the cleavage can show high en-antioselectivity. Thus the halodiisopinocampheylboranes derived from (+ )-pinene react with the oxide of cyclohexene or of cyclopentene to furnish (1R,2R) halo-... 

One of the most popular ways of using the double bond cleavage sequence envisages its utilization for the preparation of l, -dicarbonyl compounds via oxidation of the respective cycloalkene derivatives. Thus oxidation of cyclohexene represents the easiest way to prepare the 1,6-dialdehyde 460. Intramolecular aldol condensation of460 proceeds with ease to give the respective cyclopentene derivatives 461 or 462 (Scheme 2.149). 

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