Carbocations cyclopentenyl

If both cr-bonds form at the same time, the reaction is pericyclic, but carbocations are capable of reacting with alkenes, and so the first bond may form to give the cyclopentenyl cation 2.51 in one step, with the second bond formed in a separate step 2.51 (arrows). The reaction remains a cycloaddition whether both bonds are formed at the same time or not, but it is pericyclic only if they are both formed in the same step, as is probable in this case. Fig. 2.6 shows a small selection of other reactions that may similarly be pericyclic. >... 

Coordination to the ketone makes it more of a carbocation, and hence the conjugated system more of a pentadienyl cation 4.90. The cyclization takes place to give the cyclopentenyl cation 4.91, which loses the silyl group and picks up a proton to give the ketone 4.92. The relative stereochemistry at C-l and C-5 has the two hydrogen atoms trans, proving that the cyclization has... 

A stereochemical study of the synthesis of unsaturated 1,4-aminoalcohols via the reaction of unsaturated 1,4-alkoxyalcohols with chorosulfonyl isocyanate revealed a competition between an retentive mechanism and an SnI racemization mechanism, with the latter having a greater proportion with systems where the carbocation intermediate is more stable.254 An interrupted Nazarov reaction was observed, in which a nonconjugated alkene held near the dienone nucleus undergoes intramolecular trapping of the Nazarov cyclopentenyl cation intermediate.255 Cholesterol couples to 6-chloropurine under the conditions of the Mitsunobu reaction the stereochemistry and structural diversity of the products indicate that a homoallylic carbocation derived from cholesterol is the key intermediate.256 l-Siloxy-l,5-diynes undergo a Brpnsted acid-promoted 5-endo-dig cyclization with a ketenium ion and a vinyl cation proposed as intermediates.257... 

Considering the manifold reaction pathways available for carbocations it is not surprising that unexpected products have been isolated. Capture of the intermediate cyclopentenyl cation by nucleophiles was first shown by Shoppee (Scheme 6). This pathway can become dominant, resulting in a transposition of the enone moiety by tautomerization and dehydration. 

Originally, we rationalized the formation of 15 with the intermediacy of a P-silyl-substituted carbocation 16 which is in an equilibrium with the silylium ion 17 (see Scheme 2). The reaction course strongly depends on the nature of the substituents R at silicon and on the strength of the acid. Thus, treating the silanorbomenes 14 with R = methyl or methoxy groups with HCl/Et20 yielded the corresponding cyclopentenyl compounds 15, exclusively. 

The oligomerization of propene on zeolite H-Y has been studied [33,37] by variable-temperature MAS NMR. Alkoxy species formed between protonated alkenes and oxygens of the zeolitic framework were found to be important long-lived intermediates in these reactions. Simple secondary or tertiary carbocations are either absent in the zeolite at low temperatures, or are so transient as to be undetectable by NMR even at temperatures as low as 163 K. There was, however, evidence for long-lived alkyl-substituted cyclopentenyl carbocations, which are formed as free ions in the zeolite at room temperature. At 503 K the oligomers crack to form branched butanes, pentanes and other alkanes. The final product was highly aromatic coke. The structure, dynamics and reactivity of an alkoxy intermediate formed from acetylene on zeolite catalysts have been investigated by Lazo et. al. [32]. 

A similar type of chiraUty transfer was exploited by Tius in the total synthesis of roseophilin (69) (Scheme 3.15) [18]. A key intermediate in the synthetic pathway consisted of a substituted cyclopentanone, constructed via a Nazarov-type cyclization. The asymmetric cyclopentannulation reaction was initiated by generating the cyclopentenyl intermediate in situ using the lithioallene 64 and the amide 65. The chiral substituent on the delocahzed carbocation 66 induced... 

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