Allylsilane products cycloaddition

The class of 3-silyl-substituted reagents provides, upon addition with aldehydes, allylic silanes that offer many options for further derivatization. Oxidative processes are described in previous sections (see the sections on Preparation of 1,2-Diols and 1,4-Diols). If the appropriate silicon substituents are chosen, formal [3+2] cycloadditions with aldehydes can be promoted under Lewis acid catalysis. For example, the mismatched addition of the Z-3-propyl-3-benzhydryldimethyl allylsilane 183 to an a-benzyloxy aldehyde proceeds with low diastereofacial selectivity in favor of product 184 however, after protection of the secondary alcohol, an efficient [3+2] annulation provides the polysubsubstituted furan 185 in good yield and acceptable stereoselectivity (Scheme 24). ° The latter is brought forward to a tricyclic unit found in the antitumor natural product angelmicin B. 

In the cycloaddition of triisopropylallylsilane to a, /J-unsaturated lactams 197, cyclobutane adducts 198 have been found to be the kinetic products whereas the formation of cyclopentanes 199 is thermodynamically controlled285. Reactions of allenylmethylsilanes with activated unsaturated esters and nitriles (equation 163)286 and allylsilanes with unsaturated esters287 are other examples of using [2+2] cycloaddition to construct cyclobutane derivatives. 

Photocycloaddition proceeds between allylsilane and A-methylphthalimide to yield a mixture of [2 + 2] and [4 + 2] adducts along with the allylated product291. Intramolecular cycloadditions of the vinylsilanes with the cyclopentenone moieties in 200 furnish good yields of cyclic products stereoselectively (equation 164)292. In the presence of 1,4-dicyanonaphthalene, diallylsilane 201 undergoes an intramolecular photocycloaddition reaction in an aromatic solvent to give a four-membered ring product (equation 165)293. 

It is noteworthy that the ene product 227 is obtained exclusively when allylsilane 226 is treated with butynone in the presence of Znl2 catalyst and molecular sieve (ms) 4A(equation 185)331. Methyl vinyl ketone behaves similarly. High enantioselectivity has been observed in product 228 when triphenylallylsilane is coupled with methyl glyoxylate in the presence of (R)-(BINOL)TiCl2 catalyst (equation 186)332. In addition to [2 + 2] cycloaddition, TCNE undergoes a regiospecific ene reaction with y-alkyl substituted allylsilanes to yield the substituted olefins 229 (equation 187)333. 

Some silyl groups are known to serve as efficient latent hydroxy groups.160,160a Allyltriisopropylsilane achieves an efficient [3 + 2]-cycloaddition to a-enones however, it is difficult to convert the silyl group of products into a hydroxy group. Therefore, to enhance synthetic utility of silylated cycloadducts, new allylsilanes bearing a bulky, easily oxidizable silyl group have been developed (Scheme 14). 

In presence of dienes (Scheme 1), heterodienes, and heterotrienes (Scheme 2) rapid cycloadditions take place which prevent other reaction modes of the silylenes (insertions, polymerization, or HX-elimination-polymerization). Although a concerted [l+4]-cycloaddition is symmetry-allowed a stepwise mechanism (Scheme 3) via a three-membered intermediate is prefered or at least partly be competing to account for the formation of double-bond isomers. Ususally the formal [4+l]-cycloadducts (allylsilane-type) are the main products while the isomers with vinylsilane-units are side-products (< 30 %). Exceptions are... 

Silylated 1,3-butadienes, e.g. (161), are potentially attractive reaction partners in [4 + 2] cycloadditions since the adducts are prone to various function izations of the allylsilane. For example, addition product (162) gave allylic cohol (163) on hydrolysis/oxidation (Scheme 39). However, addition of diene (161) to methyl acrylate is inefficient and largely devoid of regio- and stereo-chemical control. 

Butyn-2-one undergoes tandem [3+2] cycloadditions of allyltriisopropylsilane to give a bicyclo[3.3.0]octane as a mixture of three possible diastereomers in a good yield (Scheme 10.169) [468]. The use of a reduced amount of the allylsilane forms a silylated cyclopentene as a minor product. This indicates that the 1 1 cycloadduct is the precursor of the bicyclic product. In contrast, the ZnL or Me2AlCl-promoted reaction of 3-butyn-2-one with an isocyclic allyltrimethylsilane affords a [2+2] adduct wifhout [3+2] adducts [448a, 469] (vide infra). 

The Lewis acid-promoted reactions of acrylates and propiolates with allylsilanes usually afford [2-1-2] adducts as described in the next section [470-473]. The corresponding [3-1-2] adducts are obtained as minor products although there are a few exceptions. The ratio of the two kinds of cycloadduct depends on the reaction temperature [470] - the proportion of [3-1-2] adducts increases with increasing temperature. The product ratio from cycloaddition to alkylidenemalonates and their derivatives is markedly temperature-dependent (Scheme 10.170) [474, 475]. Cyclobutanes are major products at low temperature, and [3-1-2] cycloaddition proceeds predominantly at higher temperature. In addition, the [2-1-2] cycloadducts are smoothly isomerized to the [3-1-2] adducts in the presence of a Lewis acid. This behavior clearly shows that [3-1-2] cycloaddition is thermodynamically favored. 

Nitrosium tetrafiuoroborate (NOBF.,) also undergoes [3+2] cycloaddition to a-substituted allylsilanes without use of a promoter (Scheme 10.176) [490]. The A -isoxazoline products would be formed by deprotonation and the subsequent proto-desilylation of fhe initial [3+2] adducts, Abisoxazolines. A -Isoxazolines serve as precursors of /i-hydroxy ketones and y-amino alcohols. 

The cycloaddition using allylsilanes as 1,2-dipole equivalents is apphcable to fhe construction of six-membered rings. j5-Oxyaldehydes as well as u-oxyaldehydes undergo Lewis acid-promoted cycloaddition with allylsilanes to provide substituted THP derivatives in moderate yields (Scheme 10.190) [509]. When N-t-butoxycarbo-nyl-O,N-acetals are used as the substrates, silylated oxazinones are obtained as major products [510]. In this reaction the t-butoxycarbonyl group adds to a /7-silyl-carbenium ion intermediate for ring-closure. 

N-Acyliminium ion pools react with various carbon nucleophiles as summarized in Scheme 5.16. For example, allylsilanes, silyl enol ethers, Grignard reagents, and 1,3-dicarbonyl compounds serve as good nucleophiles. Aromatic and heteroaromatic compounds also react as nucleophiles with N-acyliminium ion pools to give Friedel-Crafts-type alkylation products.N-Acyliminium ions are known to serve as electron-deficient 4n components and undergo [4 -F 2] cycloaddition with alkenes and alkynes. Usually these reactions take place very quickly, and therefore N-acyliminium ion pools serve as effective reagents for flash chemistry. 

Knoevenagel products are highly reactive compounds because of their low energy LUMO. They can act as dienophiles in the normal Diels-Alder reaction, as heterodienes in the hetero Diels-Alder reaction with inverse electron demand,- as dipolarophiles in 1,3-dipolar cycloadditions, as enophiles in the ene reaction and as acceptors for the addition of allylsilanes. Sigmatropic rearrangements and photochemical reactions have been described. 

Although it was stated that allyltrialkylsilanes were unsuccessful in the Lewis acid mediated addition to acryloylFp complexes, a stereoselective procedure has been developed for [3 + 2] cycloadditions of allylsilanes to enones10. The reaction occurs in the presence of titanium(IV) chloride (TiCl4) as Lewis acid and the single cycloaddition product formed is the m-fused eyclopentane with the trimethylsilyl group in the endo orientation. The reaction appears to follow a similar pathway to the previous process although, as yet, yields and experimental procedures have not been documented. 

DeShong (Scheme 20). Thus, reaction of nitrones with vinyl silanes followed by reduction provides Peterson - type intermediates which can then be eliminated to either Z- or E-products. Homoallylic amines were also prepared using allylsilane in the initial cycloaddition. Substitution reactions of allylic nitro compounds have received considerable attention and some examples are outlined in Scheme 21. In each case, examination of the regiochemistry of the reaction was of paramount concern, the results using palladium being superior to the SnCl mediated process. Allylic sulphides constitute yet another group of... 

Chiral oxetanes can be synthesized by the BF3-OEt2 catalyzed [2 + 2] cycloaddition reactions of 2,3-O-isopropylidenealdehyde-D-aldose derivatives with allylsilanes, vinyl ethers, or vinyl sulfides. The regiospecificity and stereoselectivity is greater than in the photochemical reaction fra s-2-alkoxy- and trans-2-phenylthiooxetanes are the resulting products. 

West and coworkers [28] reported the efficient formation of bicyclic bridgehead ketones 110 via the electrocyclization/formal intermolecular [3+2]cycloaddition of dienones 108 with allyltrimethylsUane 109 (Scheme 3.24). LA-mediated electrocyclization of 108 provides oxyallyl intermediate 111, which reacts with allylsilane 109 to give intermediate 112. At this point, ring closure provides the bridgehead ketone 110. In some cases, a minor product 113 was observed, the result of the elimination of the TMS group from the same silicon-stabilized carbocation 112. 

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