Silyl dienol ethers, preparation

The endocyclic silyl dienol ether is also obtained as the major product when the conjugated cyclic enone is successively treated with the Fe complex prepared according to Scheme 2, then with one equivalent of methylmagnesium bromide (Table 2). This is interesting since these products are difficult to obtain otherwise. The mechanism of this unusual reaction remains obscure. 

Recently, Filrstner and coworkers have prepared a super-ate complex of iron(II) as shown in Scheme 5. The structure was fully characterized by X-ray crystallography. They have shown that methylmagnesium bromide reacts with pulegone in the presence of this complex to give the corresponding endocyclic silyl dienol ether. Consequently, they have proposed that a similar ate-complex is probably involved when the reaction is performed under the Kharasch conditions. 

The precursors (1) to the silyl dienol ethers are prepared as a mixture of (E)- and (Z)-isomers by Wittig reactions with R3SiOCH2CHO. 

Five- and six-membered rings can be prepared by this reaction [217], and the reaction has been plied to syntheses of natural products. Now the cycloalkeny-lation can be carried out with a catalytic amount of Pd(OAc)2 in DMSO under O2. The bicyclo[3.2.1]octenone 532, a partial skeleton of gibberellin, was obtained in 81 % yield by the cycloalkenylation of the silyl dienol ether 531 in DMSO using... 

Another preparative method for the enone 554 is the reaction of the enol acetate 553 with allyl methyl carbonate using a bimetallic catalyst of Pd and Tin methoxide[354,358]. The enone formation is competitive with the allylation reaction (see Section 2.4.1). MeCN as a solvent and a low Pd to ligand ratio favor enone formation. Two regioisomeric steroidal dienones, 558 and 559, are prepared regioselectively from the respective dienol acetates 556 and 557 formed from the steroidal a, /3-unsaturated ketone 555. Enone formation from both silyl enol ethers and enol acetates proceeds via 7r-allylpalladium enolates as common intermediates. 

Oxidation with mepba — -hydroxyketones, using the reaction of isophorone dienol ether (detailed preparation given), and Et3N.HF cleavage of the intermediate silyl ether. 

Pd(OAc)2 to give the cyclohexenone 256. In clavulone synthesis, only the silyl enol ether in 257 reacts with Pd(OAc)2 to give the enone 258 [153]. The dehydrogenation can be carried out with a catalytic amount of Pd(OAc)2 using benzoquinone as the reoxidant. Cyclopentenone (260) is prepared from cyclopentanone (259) by using a supported Pd catalyst under 02 atmosphere [154], The enone 261 is converted to the dienone 263 via the dienol silyl ether 262 [155],... 

The hexasubstituted benzenoid compound (145) is synthesized in one step by the condensation of 2 mol of the dienol silyl ether (146) with 1 mol of methyl orthoacetate and TiCU. The reaction is believed to proceed through the intermediate (147). Compound (147) can be considered as a potential 1,3,5-trielectrophile. The more reactive sites in (147) are at C-5 and at C-3 respectively. Condensation of (147) with a second mole of (146) gives therefore regioselectively the aromatic compound (145). By this method, the plant growth substance sclerin (148) can easily be prepared. 

Grieco and coworkers19 employed the trimethyliodosilane/hexamethyldisilazane (HMDS) combination, first introduced by Miller and McKean20, to prepare dienol silyl ether 2 (equation 13). This enol silyl ether was converted via the Rubottom reaction to the sensitive a-hydroxy enone A ring of Quassimarin, 3. 

Enones can be reacted with tm-butyldimethylsilyl triflate and triphenylphosphine to produce the regiospecifically generated enol er -butyldimethylsilyl ether of /toriphenyl-phosphonium ketones57 (equation 50). In this reaction the silyl triflate is not only present in order to form the enol silyl ether, but to catalyze the addition of the phosphine to the enone. The resultant products can be further used to prepare dienol silyl ethers and / -substituted enones. 

Oiu- finalized synthetic path toward 1 is Ulustrated in Scheme 12. The macrocyclization precursor 70 would be available from three readily prepared fragments, i.e., the aldehyde 42, the ester 71, and the dienol silyl ether 65, by... 

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