Allylsilanes, synthesis

The Julia alkene synthesis has been applied to allylsilane synthesis, including the synthesis of 2-cyclopentylidene- and 2-cyclobutylideneethyl(trimethyl)silanes30. 

Retro-Brook rearrangement is relatively common and enjoys widespread application in many facets of organic synthesis. For example, vinylsilane synthesis from aUyloxysUane (eqnation 103)"°, allylsilane synthesis (eqnation 104)"°, and lithium eno-late formation from silyl enol ether (equation 105)" were reported. 

The [1,4]-rearrangement of the allyloxysilane system is a potentially useful method for allylsilane synthesis (equation 106). However, in general, a [1,2]-rearrangement may occur predominantly in this system (see Section n.F.l). To this end, the author s group recently found that the rearrangement of 171 in the presence of excess amount of HMPA provides the [l,4]-retro-Brook rearrangement product 172 in excellent yield (equation 107). ... 

K. -I. Serita, S. Hiraoka, T. Yokozawa, Tetrahedron Lett. 2000, 41, 7075-7078. Lewis acid-catalyzed three-component condensation reactions of aldehydes, A-silylcarbamates, and allylsilane synthesis of A-homoallylcarbamates. 

Scheme 3-54. Chiral allylsilane synthesis from chiral allylic alcohols through the Claisen-Ireland rearrangement.

Similai selectivity is observed in the synthesis of allylsilanes where X = CF3SO3 and Y = Si(CH3) 3 (304). Alkenyl- and alkynylborates containing a leaving group in the y-position rearrange to aUyhc and aUenic boranes, respectively (305). 

Allylsilanes are available by treatment of allyl acetates and allyl carbonates with silyl cuprates17-18, with antarafacial stereochemistry being observed for displacement of tertiary allyl acetates19. This reaction provides a useful asymmetric synthesis of allylsilanes using esters and carbamates derived from optically active secondary alcohols antarafacial stereochemistry is observed for the esters, and suprafacial stereochemistry for the carbamates20,21. 

An iinportanl advance in this synthesis of allylsilanes involved the use of optically active palladium ferrocenyl complexes as catalysts to provide optically active allylsilanes with good enantiomeric excesses being obtained for (/. (-allylsilanes and less good enantiomeric excesses for (Z)-allylsilanes26,27. 

Claisen and Carroll rearrangements of hydroxyalkenylsilanes provide an asymmetric synthesis of allylsilanes from optically active secondary alcohols39,40. 

In the synthesis of spirocyclic systems via an intramolecular Sakurai reaction, allylsilanes with an allyl moiety attached to the 3-position of the 2-cyclohexenone are required as starting materials. 

Allylsilane additions were used in a formal synthesis of roflamycoin [51] (Eq. 24). A one-pot, three-component sequential coupling of bis-allylsilane 138 with 4-acetoxy-l,3-dioxanes 137 and 139 provided the C11-C22 polyol chain (140) in moderate yield. 

Recently, a new multicomponent condensation strategy for the stereocontrolled synthesis of polysubstituted tetrahydropyran derivatives was re-published by the Marko group, employing an ene reaction combined with an intramolecular Sakurai cyclization (IMSC) (Scheme 1.14) [14]. The initial step is an Et2AlCl-promoted ene reaction between allylsilane 1-50 and an aldehyde to afford the (Z)-homoallylic alcohol 1-51, with good control of the geometry of the double bond. Subsequent Lewis acid-media ted condensation of 1-51 with another equivalent of an aldehyde provided the polysubstituted exo-methylene tetrahydropyran 1-53 stereoselectively and... 

Scheme 2.79. Synthesis of benzazepanes via a Beckmann rearrangement/allylsilane-cyclization sequence.

In reactions of chiral aldehydes, TiIV compounds work well as both activators and chelation control agents, a- or A-oxygcnated chiral aldehydes react with allylsilanes to afford chiral homoallylic alcohols with high selectivity (Scheme 22).85 These chiral alcohols are useful synthetic units for the synthesis of highly functionalized chiral compounds. Cyclic chiral 0,0- and A/O-acetals react with allylsilanes in the same way.86,87 Allenylsilanes have also been reported as allylation agents. 

An intramolecular Heck cyclization of substrate 178 was the central feature in Overman s synthesis of pyridinomorphinans [139]. Octahydroisoquinoline 178, derived from an allylsilane-iminium ion cyclization as a single stereoisomer, was cyclized under forcing conditions to afford enantiomerically pure 179, an intermediate for a (-)-morphine (180) analog. 

Silyl enol ethers are powerful intermediates in organic synthesis. Reactions of silyl enol ethers with various electrophiles provide effective methods for the synthesis of various carbonyl compounds. In this section we will briefly touch on the electrochemical reactions of silyl enol ethers and related compounds. The electrochemical behaviour of silyl enol ethers is expected to be closely related to that of allylsilanes and benzylsilanes because silyl enol ethers also have a silyl group ft to the re-system. 

A diastereoselective synthesis of 3,5-disubstituted indolizidines based on an intermol. addition of an allylsilane on an acyliminium ion derived from (d )-pyroglutamic acid, is described <2006TA1253>. 

This reaction has been extensively used for the synthesis of polyfunctionalized piperidines with a wide range of nucleophiles selected and representative examples are collected in Table 15. From these results, hydrides, Grignard reagents, aluminium derivatives, allylsilane, as well as aromatics can be used as nucleophiles to give the corresponding C-8a functionalized compounds in good yields and, in most cases, excellent selectivities. 

Cycloheptane annelation.1 A new route to cycloheptanes is based on a Lewis acid-catalyzed, intramolecular addition of an allylsilane group to a 3-vinylcycloal-kenone (equation I). This annelation has been applied to a synthesis of the ses-... 

Piperidines. Grieco et al. have described a general synthesis of piperidines by reaction of the acid salt of a primary amine with an allylsilane and 2 equiv. of formaldehyde in water. The reaction involves reaction of iminium ion (a), derived from the amine and formaldehyde, with the allylsilane to form a homoallylamine (b), which can form a second iminium ion (c), which cyclizes with capture of water to the piperidine. 

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