Cyclopentene stereospecific

For rigid alkenes, triplet sensitisation brings about photocycloaddition via the 3(Jt,7t ) state. These reactions are neither concerted nor stereospecific. Cyclopentene produces a tricyclic dimer ... 

The last synthesis to evolve which is due to Ito and his coworkers is interesting in that it relies on a stereospecific skeletal rearrangement of a bicyclo[2.2.2]octane system which in turn was prepared by Diels-Alder methodology (Scheme XLVIII) Heating of a toluene solution of cyclopentene 1,2-dicarboxylic anhydride and 4-methylcyclohexa-l,4-dienyl methyl ether in the presence of a catalytic quantity of p-toluenesulfonic acid afforded 589. Demethylation was followed by reduction and cyclization to sulfide 590. Desulfurization set the stage for peracid oxidation and arrival at 591. Chromatography of this intermediate on alumina induced isomerization to keto alcohol 592. Jones oxidation afforded diketone 593 which had earlier been transformed into gymnomitrol. 

When cyclopentene reacts with Br2, the product is a racemic mixture of 1,2-dihromocyclopentane. Addition of Bt2 to cycloalkenes gives a cyclic hromonium ion intermediate instead of the planar carhocation. The reaction is stereospecific, and gives only anti addition of dihalides. 

Further evidence that acetoxypalladation is stereospecific is provided by the fact that cyclic esters such as 1-cyclopenten-l-yl ester do not exchange. As shown by Equation 16, stereospecific addition—elimination would not permit exchange. 

The addition of bromine to cyclopentene is a stereospecific anti addition. 

The reaction of the epoxides of cyclopentene and cyclohexene with sodium azide give the trans-azido alcohols, the hydrogenation of which affords the trans isomers of 2-aminocyclopentanol and 2-aminocyclohaexanol (eq. 9.94) stereospecifically.246 By using these reactions, Swift and Swern prepared stereochemically pure cis- and trans-1,2-diaminocyclohexanes by the reaction sequences described in Scheme 9.23.247... 

The synthesis69 started with c s-cyclopentene-3,5-diol7°) 26 which had been obtained in a stereospecific reaction by addition of singlet oxygen to cyclopentadiene and reduction of the intermediate cycloperoxide. 

Asymmetric syntheses of agelastatin A rapidly followed Weinreb s racemic synthesis, with Feldman and coworkers publishing the first stereospecific approach to the natural enantiomer [68,69]. This route employed a chiral alkynyliodonium triflate-alkylidenecarbene-cyclopentene transformation to form the key intermediate 69 from which (—)-6S could be efficiently assembled. Hale et al. initially reported a chiral formal synthesis of (—[-agelastatin A through the enantiospedfic preparation of an advanced intermediate from Weinreb s racemic route [70] and later described a total... 

Alkoxy-2-vinylcyclopropanes rearrange stereospecifically to 2-cyclopentenols under very mild conditions. Thus, the lithium salt of 2-vinyl-1-cyclopropanol (204) undergoes the vinylcyclopropane-cyclopentene rearrangement at room temperature (equation 137). ... 

By controlling reaction conditions and by proper choice of the peroxy acid, it is often possible to favor the Baeyer-Villiger reaction over epoxidation. An illustrative example of the usefulness of the Baeyer-Villiger reaction is the stereospecific, and regio- and chemoselective conversion of the unsaturated bicyclic ketone shown below to a cyclopentene containing three consecutive stereogenic centers. 

With respect to the arylcarbenes, however, the carbene addition is not stereospecific the aryl-cyclopropanes are generally formed as mixtures of diastereomers. In almost all cases reported, the sj -isomer predominates, and this is the case even when the corresponding uw/z-isomer is far less sterically crowded. Thus, when 1-naphthylcarbene reacted with cyclopentene, the corresponding cyclopropane 6-(l-naphthyl)bicyclo[3.1.0]hexane (4) was obtained in 62% yield with endojexo ratio of 84 16. ... 

The exclusively stereospecific and sterically sensitive ring expansion of isomeric vinylcyclopro-panes 21 to cyclopentenes 22 was detected in the desilylation of 21 with freeze-dried potassium fluoride. ... 

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