Ethers, allyl methyl reduction

Although simple ketones and esters can not be allylated by Pd catalysts, they are allylated with allyl carbonates via their enol ethers of Si and Sn. In the allylation of the silyl enol ether 202 with allyl carbonate 200, transmetallation of 202 with the n-allylpalladium methoxide 201, generated from allyl methyl carbonate (200), takes place to generate the Pd enolates 203 and 204. Depending on the reaction conditions, allyl ketone 205 is formed by the reductive elimination of 203 [100]. When the ratio of Pd Ph3P is small, the a,/i-unsaturated ketone 206 is obtained by -elimination [101]. For example, the silyl enol ether 208 of aldehyde 207 is allylated with allyl carbonate (200) to give a-allylaldehyde 210 via 209. The a-allyl carboxylate 213 is obtained by allylation of ester 211 with allyl carbonate (200), after conversion of ester 211 to the ketene silyl acetal 212 [102], As the silyl group is trapped in these... 

The enhanced reductive power of Lewis acid/LAH mixtures observed with benzyl substituents is reflected in allyl compounds. Thus allyl methyl ethers are readily demethoxylated with LAH/TiCU (equation 49), with complete rearrangement of the double bond and some (Z)-selectivity. Allyl alcohols are reduced to alkenes with the same reagent or with LAH/AICI3 (equation 50). With both these reagents double bond migration was observed to varying degrees as well as diene formation. Primary allyl alcohols have been reduced to alkenes with titanocene and LAH (equation 51). ... 

The relatively efficient cyclisation of monocyclofamesic acid with boron trifluoride etherate to methyl bicyclofarnesate (46) has led to a successful synthesis of drimenin (48) via acid-catalysed lactonisation of the allylic alcohol (47), a product of singlet oxygen addition to (46). It has also been shown that hydride reduction of drimenin to the allylic diol (49) followed by a two-step oxidation procedure yields cinnamolide (39). A Ci antifungal mould metabolite has... 

The silylated methyl ester was then a-methylated with lithium diisopropylamide and methyl iodide in tetrahydrofuran. Reduction of methyl 10-( erl-butyldimethylsilyloxy)-2-methyldecanoate with DIBAL in ether at -78°C afforded the corresponding aldehyde. The 10- tert-butyldimethylsilyloxy)-2-methyldecanal was subsequently coupled in a Wittig reaction with 1-hexyltriphenylphosphonium bromide and n-butyllithium affording (Z)- and ( )-1 -(teri-butyldimethylsilyloxy)-9-methyl-10-hexadecene in a 9 1 ratio, respectively. Deprotection with tetrabutylammonium fluroride (TBAF) in tetrahydrofuran and final oxidation with pyridinium dichromate (PDC) in dimethylformamide resulted in a 9 1 mixture of (Z)- and ( )-9-methyl-10-hexadecenoic acid as shown in Fig. (7). As was also the case with acid 6, the stereochemistry at C-9 in 7 is not known. The key step in the synthesis of the allylic methyl group was a-methylation of a methyl ester, followed by reduction to the corresponding aldehyde, which was used in the subsequent Wittig reaction. 

Reductive demethoxylation. Alkenes can be prepared by the reaction of allyl methyl ethers with this reducing agent. The reaction is accompanied by migration of the double bond. ... 

Avermectins and milbemycins are produced by fermentation as mixtures containing either a C-5 hydroxy or methoxy group, of which the first are generally more potent antiparasitic agents. A procedure for the conversion of the allylic methyl ether to the desired alcohol involving oxidation by Hg(OAc)2, hydrolysis of the intermediate vinylether to the 5-ketone [37] and stereospecific reduction to the C-5-R-(p-) alcohol has been developed [38] (Fig. 6). 

In the allylation of the silyl enol ethers with allyl carbonates, the transmetallation of the silyl enol ethers with the 7r-allylpalladium methoxides, generated from allyl methyl carbonates, gives the 7r-allylpalladium enolates. Depending on the reaction conditions, allyl ketones are formed by reductive elimination (Scheme When a ratio... 

Reactions.—Gc/icrn/. Allyl methyl ethers have been reductively transformed into predominantly (Z)-alkenes with carbon-carbon double bond rearrangement (81 82) using TiCU-LiAlH. ... 

Acyloin-type reactions of esters provide the simplest route to 1-siloxy-l-alkoxycyclopropane [21,22] Eq. (6). The reaction of commercial 3-halopropionate with sodium (or lithium) in refluxing ether in the presence of Me3SiCl can easily be carried out on a one mole scale [21]. Cyclization of optically pure methyl 3-bromo-2-methylpropionate [23], available in both R and S form, gives a cyclopropane, which is enantiomerically pure at C-2, yet is a 1 1 diastereo-meric mixture with respect to its relative configuration at C-l Eq. (7). Reductive silylation of allyl 3-iodopropionate with zinc/copper couple provides a milder alternative to the alkali metal reduction [24] Eq. (8). 

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