Ethers silyl vinyl

An interesting pericyclic-anionic-pericyclic domino reaction showing a high stereoselectivity is the cycloaddition-aldol-retro-ene process depicted in scheme 20.1581 The procedure presumably starts with a [4+2]-cycloaddition of diene 98 and S02 in presence of a Lewis acid. After opening of the formed adduct reaction with (Z)-silyl vinyl ether 99 leads to a mixture of alk-2-enesulfinic acids 101. It follows a retro-ene reaction which affords a 7 3 mixture of the products 102 and 103. The reaction described by Vogel et al is a nice example for the efficient generation of polypropionate chains with the stereoselective formation of three stereogenic centers and one (0-double bond in a three-component domino reaction in its strict definition. 

A variation of GTP, referred to as aldol GTP, involves polymerization of a silyl vinyl ether initiated by an aldehyde [Sogah and Webster, 1986 Webster, 1987]. Both initiation and propagation involve nucleophilic addition of the vinyl ether to the aldehyde carbonyl group with transfer of the trialkyl silyl group from vinyl ether to the carbonyl oxygen (Eq. 5-82). The reaction has similar characteristics as GTP. The product is a silated poly(vinyl alcohol) (PVA), which can be hydrolyzed by acid to PVA (Eq. 5-83). 

In certain reactions, introduction of a bulkier silyl group in substrates may decrease the stereoselectivity. For example, photolysis of a mixture of benzaldehyde and silyl vinyl ethers 31 affords a mixture of diastereomeric [2 + 2] cycloadducts 32 and 33... 

The Lewis acid-promoted tandem inter[4 + 2]/intra[3 + 2]-cycloaddition of the (fumaroyloxy)nitroalkene (124) with the chiral /i-silyl vinyl ether (125) is the key step in the total synthesis of (+)-crotanecine (126), the necine base of a number of pyrrolizidine alkaloids (Scheme 46).237 The tandem inter[4 + 2]/intra[3 + 2]-cycload-ditions of nitroalkenes (127) with dipolarophiles attached to the /<-carbon of a vinyl ether (128) provides a method of asymmetric synthesis of highly functionalized aminocyclopentanes (129) (Scheme 47).238 trans-2-( 1 -Mcthyl-phcnylcthyl jcyclohcx-anol has been developed as a new auxiliary in tandem 4 + 2/3 + 2-cycloadditions of nitroalkenes.239 The scope and limitations of the bridged mode tandem inter-[4 + 2]/intra[3 + 2]-cycloadditions involving simple penta-1,4-dienes are described in detail.240 A tandem intermolecular/intramolecular Diels-Alder cycloaddition was successfully used to synthesize a B/C cw-fused taxane nucleus (130) in 50% overall... 

In a process related to GTP, aldehydes initiate the polymerization of silyl vinyl ethers and silyl diene ethers. Here the silyl group is present in the monomer and transfers to the aldehyde ended chains regenerating aldehyde ends [17] (Scheme 8). A Lewis acid catalyst is required. terf-Butyldimethylsilyl works best as a transfer group for vinyl ether while trimethylsilyl is suitable for diene ethers [18]. Even though aldol GTP provides a route to polyvinyl alcohol segments in the subsequent block polymer synthesis, the projected cost of the monomers discouraged further research aimed at commercialization. 

Aldol group transfer polymerization of ferf-butyldimethylsilyl vinyl ether [62] was initiated by pendant aldehyde functions incorporated along a poly(methyl methacrylate) (PMMA) backbone [63]. This backbone was a random copolymer prepared by group transfer polymerization of methyl methacrylate (MMA) and acetal protected 5-methacryloxy valeraldehyde. After deprotection of the aldehyde initiating group, polymerization proceeded by activation with zinc halide in THF at room temperature. The reaction led to a graft copolymer with PMMA backbone and poly(silyl vinyl ether) or, upon hydrolysis of the ferf-butyldimethylsilyl groups, poly(vinyl alcohol) branches. 

Poly(Silyl vinyl ether) or poly(vinyl alcohol) Styrene 33 [76-78]... 

Cycloaddition. Reaction of the 1,3-dipolar species derived from methyl 2-phenylthiocyclopropyl ketone with silyl vinyl ethers furnishes functionalized cyclopentanes. A related reaction is the trapping of a fragmented cyclobutane. ... 

Palladium/phosphine-sulfonate neutral complexes, reported by Drent, Pugh, and coworkers, could incorporate several polar comonomers such as methyl acrylate, vinyl ethers, methyl vinyl ketones, and silyl vinyl ethers. Other common comonomers such as vinyl acetate, acrylonitrile, and vinyl chloride showed very low comonomer incorporation, between 1% and 2% [48]. 

Cycloadditions involving terminal C-silylated vinyl ethers proceeded stereospecifically except reactions with olefins having a bulky ferf-butyldimethyl-... 

Binaphthol Lewis Acids. In search of Lewis acids which might provide better stereocontrol in these cycloadditions, we turned our attention to l,l -bi-2-naphthol as a potentially useful ligand for the titanium promoter complex. For synthetic reasons, we continued to use benzylic vinyl ethers and silyl vinyl ethers so that these protecting groups could be cleaved to provide the free alcohol later. 

A related technique called aldol GTP makes use of the reaction of an aldehyde with a silyl vinyl ether. The cocatalysts used are again Lewis acids, and if used with t-butyl dimethyl silyl vinyl ether, the combination will polymerize aldehydes ... 

Grubbs and coworkers [127,128] reported two independent transformation reactions for block copolymer synthesis. The first report [128, 129] involved changing the mechanism from hving metathesis polymerization of cycloalkene to a group transfer polymerization of silyl vinyl ether. In a second report, block copolymers of NB and ethylene were prepared by transforming a metathesis polymerization to a... 

Organometallic, Thermal, and Photochemical Methods. The Simmons-Smith cyclopropanation procedure has now been successfully applied to silyl vinyl ethers and... 

Transition metal-catalyzed hydrosilylation of Q ,/3-unsaturated ketones and aldehydes with PhMe2SiH proceeds in a 1,4-fashion to give silyl vinyl ethers, which are hydrolyzed to give ketones... 

Silyl Vinyl Ethers. Enolizable ketones (16) are readily transformed to silyl vinyl ethers (17) by the action of disilane and a catalytic amount of sodium in HMPA (eq 8). The regioselec-tivity of the silyl vinyl ether formation depends on the structure of the ketones. 

More recently the workers at Du Pont have developed aldol group transfer polymerization, in which a silyl vinyl ether is polymerized using an aldehyde as the initiator to give a living silylated poly(vinyl alcohol), e.g. 

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