Ethers allenyl silyl

Acylsilanes of a variety of substitution patterns have been employed (9) in routes to allenyl silyl enol ethers. 

Allenyl Silyl enol ethers, 86 Allyl alcohol trimethylsilyl ether, 84 Allyl carbonates, 114-15 9 Allyl-ay 2 octalone, 34-5 2-Allyl-2 methylcyclohexanone, 106 (Allyldimethylsilyl)methyl chloride, 58, 59 (AUyldimethylsilyl)methylmagnesium chloride, 59... 

The combination of a silyl-migration from carbon to oxygen and a prototropic isomerization leads to allenyl silyl ethers [246]. 

Whereas deprotonation and halogen-lithium exchange represent the most common methods to access allenic and propargylic lithium intermediates, several less direct routes to more functionalized analogues have also been reported. Additions of various lithium acetylides to acylsilanes followed by Mel or EtI afforded alkylated allenyl silyl ethers (Table 9.4) [10]. The adducts were analyzed after hydrolysis to the related enones. 

Allenyl silyl ethers 40 have also been prepared by the reaction of 2-lithiofurans 38 with acylsilanes 39 via the Brook isomerization (equation 15) . 

Brook rearrangement. a-Trimethylsilylpropargyl alcohols undergo Brook rearrangement to afford allenyl silyl ethers, which can be used to condense with aldehydes. Silyl group transfer from the ether four bonds apart is preferred after the Sn/Li exchange from silyl ethers of l-tributylstannyl-l,3-alkanediols. °... 

Scheme 13 shows additional examples of constructions of four-carbon chains functionalized appropriately for condensation to pyrroles. Af,Af-bis(trimethylsilyl)propargyl amines can be converted to ketones or esters by lithiation and acylation. These compounds then undergo cuprate addition. The resulting allenyl silyl ethers cyclize to pyrroles on treatment with acid <93T4603). 

Hydrostaimation of arylallenes such as 124 proceeded regio- and stereoselec-tively to provide allylstannane 125 [39]. Reaction of allenyl silyl ethers such as 126 also proceeded regioselectively to produce allylstannanes. A mixture of the stereoisomers 127 and 128 was obtained, but in this case the Z isomer 128 was obtained predominantly. As shown above, allylstannanes are obtained by metallation at the terminal carbon [40]. However, Lautens et al. reported that the vinylstaimane 130 was obtained from alkylallene 129 when ligandless heterogeneous Pd(OH)2 on carbon was used as a catalyst. It was confirmed that the allylstannane 131 was obtained when Pd(PPh3)4 was used. The catalysts changed the selectivity [41]. 

The CuBr2/amine-promoted 2 + 2-cycloaddition reaction of 1,4-bisallenes (17) produced bicyclo[4.2.0]octadiene derivatives (18) in a one-pot procedure (Scheme 7) Phosphoramidite ligands (19) activate the Au(I)-catalysed 2 + 2-cycloaddition reaction of A-allenylsulfonamides with styrenes, at -70 C, to form vinyl cyclobutane derivatives in high yields and enantioselectivity. A new Pt(II) catalyst with a hollow-shaped tri- ethynylphosphine activates the intermolecular 2 + 2-cycloaddition reaction of allenyl silyl ether with vinyl ethers to yield methylenecyclobutanes in good yield. 

Lewis acid-catalyzed ene reactions proceed between allenyl sulfides, e.g. 330, and aldehydes 329 to afford cis-trans mixtures of 1,3-butadienes 331 (Scheme 8.90) [168, 175b], Similar ene reactions observed with imines such as 332 provide the corresponding allylamines [168,177]. It was also found that the ene reaction of 1-silylated allenyl sulfide 333 with various aldehydes (or acetals) furnishes a,/l-unsaturatcd acyl compounds such as 334 and 335 under BF3-etherate catalysis [175b]. 

The alkyl allenyl ethers whose chemistry has been discussed are readily prepared and are useful for the synthesis of diverse cyclopentenones, as racemates or in enan-tiomerically enriched form. It is worth noting that silyl allenyl ethers allow entry into a distinct mechanistic manifold [16]. Triisopropylsilyloxyallene 47 can be deprotonat-... 

The most versatile synthesis of w./i-unsaUi rated acyl silanes involves the use of allene methodology, developed by a number of groups14,22. Deprotonation and silylation of allenyl ethers followed by hydrolysis gives rise directly to ,/3-unsaturated acyl silanes via their enol ethers, 1-alkoxy-l-trimethylsilylallenes (Scheme 43). Indeed, the first example of an a,/l-unsaturated acyl silane was prepared by such a route223, as was the first example of an allenic acyl silane (from a l-trimethylsilyl-l-trimethylsilyloxy-l,2,3-alkatriene)22b. 

Even if the SMS reaction typically involves allylsilanes, carbonyls and alcohols (or silyl ethers), some transformations can be considered as belonging to the same family. For example, in 2001, Yokozawa et al. described [43] a three-component reaction between aldehydes 6, alkoxysilanes 38 and propargylsilane 88 (instead of allylsilane). Tritylperchlorate was used as the catalyst and a-allenyl ethers 89 were... 

Propargylic silyl ethers 588 undergo intramolecular Friedel-Crafts alkylation to afford 4-allenyl isochromans 589 in excellent yield (Equation 243) <2001JOC4635>. 

Allenamides are functionalized at the a-position by virtue of the regioselective lithiation. Thus, on introduction of an alkynyl chain, valuable precursors of the Pauson-Khand reaction can be prepared. Lithiation of the silyl ethers of propargyl or allenyl alcohols engenders O —>C transsilylation. ... 

An intramolecular Friedel-Crafts alkylation of propargyl silyl ethers 22 gives 4-allenylisochromans presumably via an allenyl cation (Scheme 22) <01JOC4635>. 

Versatile preparation of conjugated acylsilanes involves hydrolysis of a-silylated allenyl ethers that can be derived from lithiated allenyl ethers.The method is applicable to the synthesis of not only a variety of substituted a,(3-unsaturated acylsilanes but also acetylenic acylsilanes (Scheme 6.6T... 

Trifluoromethyl)allyltrimethylsilane results frwith aldehydes and ketones gives trifluoromethyl-ated alcohols and silyl ethers. Synthesis of allyl and allenyl-silanes is stereospecific from hydroalumination or hydroboration of... 

Deprotection of the silyl ether followed by a Swem oxidation of the alcohol 46 provided the aldehyde. Subsequent condensation with aminoaziridine B in MeOH afforded allenyl aziridinyUmine 40, the cycloaddition precursor (Scheme 12). 

From the chiral epoxide 56, the TMM cycloaddition precursor 55 for construction of the tetraquinane stmcture was prepared in a straightforward manner. Oxidation of epoxy alcohol 56 by the Swem protocol followed by treatment with Bestmann-Ohira reagent (E) produced alkyne 70. Iron-catalyzed Sisr2 -type reaction of 70 afforded an allene moiety as a diastereo-meric mixture in a 1 1 ratio and this mixture was used in the next step without the separation of isomers. Allenyl alcohol 71 was protected as a TBDPS ether to give 0-silylated allene 72. Deprotection of the acetal of 72 was delicate as the allenyl moiety was not stable under acidic condition. Fortunately, treatment of 72 with p-toluenesulfonic acid monohydrate in presence of formaldehyde afforded aldehyde 73, and subsequent treatment with p-toluene sulfonyUiyrazide furnished the precursor 55 for the tandem cycloaddition reaction (Scheme 21). 

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