Terminal ring double bond

Let us consider a carotenoid system having a fi end group, i.e. a molecule with the possibility of conjugation between the terminal double bond of the polyenic chain and the ring double bond, for example zeaxanthin, 44. 

Product of epoxidation of the ring double bond 57%, product of epoxidation of the terminal double bond 4%, 3% diol formation. P 7% of enone. 

The 5-keto acid 11 is supposed to be formed by hydroboration and oxidation of the terminal CC double bond in 12, feasible by allylation of the enone 13. Enone 13, once again, originates from an intramolecular Knoevenagel alkenylation of the a,5-diketone 14 prepared by addition of ethylmagnesium halide to the enol lactone 15 of the 5-keto acid 16. Preceding this keto acid, the allyl compound 17 is formed by nucleophilic opening of the cyclopropane ring with hydride and mefliylation in a position of the carbonyl in the cyclopropyl ketone 18. 

Bfickvall and co-workers recently reported a solvent effect which promoted the Alder-Ene reaction to occur at a relatively mild temperature for the unactivated, all-carbon system. " At 120 °C in DMF, the cyclization of the terminal allenic double bond with simple alkenes occurred intramolecularly. While a by-product was formed, no activator was utilized. Other solvents were screened as well as ionic liquids and were found inferior to DMF in facilitating the reaction. One limitation observed was that only 5-membered rings were formed and the ene portion had to be an allene which contained two terminal methyl groups. Such requirements for reaction suggested something more than merely a solvent effect. 

The initiation step is similar to that described for beta-pinene as illustrated in Figure 6. Propagation through the terminal methylene group would be predicted. However, the determination of olefin content by NMR, ozonolysis and perbenzoic acid oxidation indicates that approximately one-half of the mer units have unsaturation. The endocyclic or ring double bond is thus involved in the polymerization and is consumed in some manner. To substantiate this theory, the structurally similar model compound 8,9-p-menthene was polymerized under identical conditions. Only dimer was obtained. The double bond in the ring thus permits successful polymerization of dipentene. 

Transition metal catalyzed intramolecular [4+2] diene-allene cycloadditions provide 6,6- and 6,5-fused ring systems in good yields under mild conditions. For example, from the tethered allene 404 in the presence of Ni(COD)2 as the catalyst a 97 % yield of the cycloadduct 405 is obtained resulting from addition to the terminal allene double bond. From the same substrate, using [Rh(COD)Cl]2 as the catalyst, the hydrindane derivative 406 is obtained in 90 % yield. 

In general, different mechanistic pathways can be taken into account for these domino reactions (Scheme 11.19). The first would involve initial attack at the terminal exocycUc double bond, which would then undergo metallocycloaddition onto the endocyclic olefin. Alternatively, ring opening may occur first leading to two possible intermediates A and D. As observed in early studies [35], favored formation of A or D, and therefore B or E, depends on the ring size of the final product as well as on the presence of ethylene in the reaction mixture. Under these conditions, it is possible to convert A into C and shift the equilibrium toward... 

The opening of the oxirane ring is accompanied by inversion except when the oxirane ring is in the terminal position of an aliphatic chain the ultimate result is equivalent to trans addition to the double bond. Thus cyciohexene yields trans-1 2- f/ohexanediol ... 

The benzene derivative 409 is synthesized by the Pd-catalyzed reaction of the haloenyne 407 with alkynes. The intramolecular insertion of the internal alkyne, followed by the intermolecular coupling of the terminal alkyne using Pd(OAc)2, Ph3P, and Cul, affords the dienyne system 408, which cyclizes to the aromatic ring 409[281]. A similar cyclization of 410 with the terminal alkyne 411 to form benzene derivatives 412 and 413 without using Cul is explained by the successive intermolecular and intramolecuar insertions of the two triple bonds and the double bond[282]. The angularly bisannulated benzene derivative 415 is formed in one step by a totally intramolecular version of polycycli-zation of bromoenediyne 414[283,284],... 

When this stereoelectronic requirement is combined with a calculation of the steric and angle strain imposed on the transition state, as determined by MM-type calculations, preferences for the exo versus endo modes of cyclization are predicted to be as summarized in Table 12.3. The observed results show the expected qualitative trend. The observed preferences for ring formation are 5 > 6, 6 > 7, and 8 > 7, in agreement with the calculated preferences. The relationship only holds for terminal double bonds. An additional alkyl substituent at either end of the double bond reduces the relative reactivity as a result of a steric effect. 

Chemoselectivity plays an important role in the benzannulation reaction as five-membered rings such as indene or furan derivatives are potential side products. The branching point is again the rf-vinylcarbene complex D intermediate which maybe formed either as a (Z)- or an ( )-metallatriene the (E)-configuration is required for the cyclisation with the terminal double bond. (Z)-Metallatriene D, however, leads to the formation of furan derivatives H (Scheme 8). Studies on the formation of (E)- and (Z)-isomers discussing stereoelectronic effects have been undertaken by Wulff [17]. 

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