5- silyloxy-1-alkene 4-alken

Silyloxy)alkenes were first reported by Mukaiyama as the requisite latent enolate equivalent to react with aldehydes in the presence of Lewis acid activators. This process is now referred to as the Mukaiyama aldol reaction (Scheme 3-12). In the presence of Lewis acid, anti-aldol condensation products can be obtained in most cases via the reaction of aldehydes and silyl ketene acetals generated from propionates under kinetic control. 

Reductive silylation of a-diketones or quinones.1 a-Diketones are conveniently converted into l,2-bis(silyloxy)alkenes by reaction with ClSi(CH3)3 and zinc. Yields are increased by use of THF rather than ether as solvent and by ultrasonic irradiation. 

Modem acyloin condensations are usually executed in the presence of Me3SiCl, and a bis(silyloxy)alkene is obtained as the immediate product. The bis(silyl-oxy)alkene may then be hydrolyzed to the acyloin upon workup. The yield of the acyloin condensation is greatly improved under these conditions, especially for intramolecular cyclizations. The MesSiCl may improve the yield by reacting with the ketyl to give a neutral radical, which can undergo radical-radical combination more easily with another ketyl radical due to a lack of electrostatic repulsion. 

A Japanese group described the first U-4CR-related C-nucleophilic reaction [130].The 4-isocyano-/-silyloxy-alkenes react with Eschenmoser s salt into their bicyclic pyrroline derivatives. 

Acyloin condensations of esters conducted with TMS-Cl and sodium in toluene give 1,2-bis(trimethyl-silyloxy)alkenes. In general, the silylacyloin reaction provides higher yields than the conventional acyloin reaction and is particularly useful for the preparation of cyclic enol silyl ethers. " The synthesis of compound (47) serves as an illustration. 

In a formal synthesis of fasicularin, the critical spirocyclic ketone intermediate 183 was obtained by use of the rearrangement reaction of the silyloxy epoxide 182, derived from the unsaturated alcohol 180. Alkene 180 was epoxidized with DMDO to produce epoxy alcohol 181 as a single diastereoisomer, which was transformed into the trimethyl silyl ether derivative 182. Treatment of 182 with HCU resulted in smooth ring-expansion to produce spiro compound 183, which was subsequently elaborated to the desired natural product (Scheme 8.46) [88]. 

Alkyl and silyl nitronates are, in principle, /V-alkoxy and /V-silyloxynitrones, and they can react with alkenes in 1,3-dipolar cycloadditions to form /V-alkoxy- or /V-silyloxyisoxaz.olidine (see Scheme 8.25). The alkoxy and silyloxy groups can be eliminated from the adduct on heating or by acid treatment to form 2-isoxazolines. It should be noticed that isoxazolines are also obtained by the reaction of nitrile oxides with alkenes thus, nitronates can be considered as synthetic equivalents of nitrile oxides. Since the pioneering work by Torssell et al. on the development of silyl nitronates, this type of reaction has become a useful synthetic tool. Recent development for generation of cyclic nitronates by hetero Diels-Alder reactions of nitroalkenes is discussed in Section 8.3. 

Scheme 29 1,3-Dipolar cycloaddition of l-diazo-2-silyloxy-l-alkene and thermolysis of the cycloadduct...

As an example of the selective reactivity of borazirconocene alkenes, their hydrolysis was examined [1]. The carbon—zirconium bond is more reactive than the carbon—boron bond towards various electrophiles, and so hydrolysis can be expected to occur with preferential cleavage of the former bond. Since hydrolysis of alkenylzirconocenes is known to proceed with retention of configuration [4,127—129], a direct utility of 45 is the preparation of (Z)-1-alkenylboronates 57 (Scheme 7.17) [12]. Though the gem-dimetalloalkenes can be isolated, in the present case it is not necessary. The desired (Z)-l-alkenylboronates can be obtained in a one-pot procedure by hydrozirconation followed by hydrolysis with excess H20. The reaction sequence is operationally simple and is compatible with various functional groups such as halides, acetals, silanes, and silyloxy protecting groups [12]. 

Yttrocene complexes catalyze the cascade cyclization/hydrosilylation of trienes to form saturated silylated bicyclic compounds.For example, reaction of the 4-silyloxy-4-vinyl-l,6-hexadiene 69 and phenylsilane catalyzed by Gp 2YMe(THF) at room temperature for 1 h followed by oxidation of crude 70a gave [3.3.0]bicyclic diol 70b in 73% yield over two steps as a single diastereomer (Scheme 18). Selective conversion of 69 to 70a presumably requires initial 1,2-hydrometallation of one of the less-hindered G=G bonds to form alkylyttrium alkene complex II (Scheme 18). Selective S-exo carbometallation of II in preference to -exo carbometallation would form cyclopentyl-methylyttrium complex III (Scheme 18). Gyclization of III via a chairlike transition state would form the strained /r< /75 -fused alkylyttrium complex IIIl, which could undergo silylation to form 70a. 

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