Alkenes into allylic esters

In 1992, Tomoda and Iwaoka reported the first catalytic chemical conversion of alkenes into allylic ethers and esters [310]. The reactions were performed anploy-ing 10 mol% of diselenides 455 or 456 (Fig. 7.27) and copper (11) nitrate trihydrate in methanol or acetic acid in the presence of excess of sodium persulfate as an oxidizing agent. The best results were achieved when using the diselenide 456 in the presence of molecular sieves 3 A in acetic acid (Scheme 7.69). This suggests that... 

Yang12 has effected an intramolecular asymmetric carbonyl-ene reaction between an alkene and an a-keto ester. Reaction optimization studies were performed by changing the Lewis acid, solvent, and chiral ligand. Ligand-accelerated catalysis was observed for Sc(OTf)3, Cu(OTf)2, and Zn(OTf)2 (Equation (6)). The resulting optically active m-l-hydroxyl-2-allyl esters provide an entry into multiple natural products. 

Electron withdrawing groups on the alkene considerably retard the epoxidation. For example, acrylic esters and acrylonitrile are not reactive, while allyl chloride is about one tenth as reactive as propylene towards TBHP/MoVI.240 The epoxidation of alkenes is completely stereoselective eri-alkenes are exclusively transformed into ds-epoxides and (runs-alkenes into trans-epoxides. Oxygen addition to the double bond preferentially occurs from the less shielded face of the substrate, e.g. in the selective epoxidation of terpenes by r-pentyl hydroperoxide (t-amyl hydroperoxide, TAHP) (equations 67 and 68).225... 

An intramolecular allyl silane/N-sulfonyl iminium ion cyclization has also been used as a pivotal step in an approach to the tricyclic core of the unique marine alkaloid sarain A [46]. The starting material was aziridine ester 129 (Scheme 25) which was elaborated to amide 130. An important step in the synthetic strategy was thermolysis of 130 to an azomethine ylide, which underwent stereospecific intramolecular 1,3-dipolar cycloaddition with the Z-alkene to produce bicyclic lactam 131 [47]. This compound was then elaborated into allyl silane 132. It was then possible to replace the lactam N-benzyl functionality with a tosyl moiety, leading to 133, and subsequent reduction of the carbonyl group afforded the desired cyclization precursor a-hydroxy sulfonamide 134. Exposure of 134 to ferric chloride promoted cyclization to a single stereoisomeric tricyclic amino alkene 136 having the requisite sarain A nucleus. It is believed that the intermediate N-sulfonyl iminium ion cyclizes via the conformation shown in 135. 

A variant on this approach is to incorporate the enantiotopic alkenes into a five-membered ring such as a cyclopentadiene. 3-Hydride elimination is then obviated, as a if-allyl complex 5.130 is formed after insertion, which may be intercepted by an added nucleophile, such as acetate (Scheme 5.39). Other nucleophiles, including carbon nucleophiles can also be used to intercept the tt -allyl intermediate (see Section 9.2.9). This chemistry was used in a synthesis of capnellene 5.137 (Scheme 5.40). The T]p-allyl intermediate 5.130 was intercepted with a functionalized malonate nucleophile 5.132. The malonate was used to construct the third five-membered ring of the natural product while one ester group was removed by Krapcho... 

Removal of the 0-substituted Fp group can be achieved by conversion into the cationic alkene-Fp complex using Ph3CPF6 and subsequent treatment with iodide, bromide or acetonitrile. Oxidative cleavage with ceric ammonium nitrate in methanol provides the methyl esters via carbon monoxide insertion followed by demetallation. The [3 + 2]-cydoaddition has been successfully applied to the synthesis of hydroazulenes (Scheme 1.11) [34]. This remarkable reaction takes advantage of the specific nucleophilic and electrophilic properties of V-allyl-, cationic t 5-dienyl-, cationic ri2-alkene- and ti4-diene-iron complexes, respectively. 

This strategy has been applied to the synthesis of the Cis Cecropia juvenile hormone (JH) (163 Scheme 13) using the ketal (160) rather than an alkoxy butadiene. Surprisingly, an a,p-unsaturated ester is obtained as an -3 1 mixture from which the undesired ( )-isomer can be separated by fractional distillation. The iterated allylic alcohol (163) is transformed into its isomeric allylic cohol which is subsequently reduced to an alkene and epoxidized to realize the target (163). The scheme also lends itself to... 

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