Allylsilanes cycloadditions

The ring opening of an aziridine can also occur in an intramolecular fashion, as observed in the formal [3+2] azitidine-allylsilane cycloaddition reaction of 142. These substrates were used for the preparation of both 5-5 and 6-5 fused ring systems <99T8025>. 

When the related aziridine 195 was used containing one less carbon in the tether, an unusual cyclization occurred which corresponds to a formal [3+2] aziridine allylsilane cycloaddition to give the bicyclic pyrrolidine 196. This substrate was used for the preparation of other fused ring systems (Scheme 53) <1999T8025>. 

Other reactions not described here are formal [3 -i- 2] cycloadditions of a,p-unsaturated acyl-fluorides with allylsilanes [116], or the desymmetrization of meso epoxides [117]. For many of the reactions shown above, the planar chiral Fe-sandwich complexes are the first catalysts allowing for broad substrate scope in combination with high enantioselectivities and yields. Clearly, these milestones in asymmetric Lewis-base catalysis are stimulating the still ongoing design of improved catalysts. 

Knolker and coworkers also used a domino [3+2] cycloaddition for the clever formation of a bridged tetracyclic compound 4-172, starting from a cyclopentanone 4-168 and containing two exocydic double bonds in the a-positions (Scheme 4.36) [57]. The reaction of 4-168 with an excess of allylsilane 4-169 in the presence of the Lewis acid TiCLj led to the spiro compound 4-170 in a syn fashion. It follows a Wag-ner-Meerwein rearrangement to give a tertiary carbocation 4-171, which acts as an electrophile in an electrophilic aromatic substitution process. The final step is the... 

Another interesting example of a photochemi-cally induced domino process is the combination of the photocyclization of aryl vinyl sulfides with an intramolecular addition as described by Dittami et al. [901 as intermediate a thiocarbonyl ylide can be assumed. The domino-Norrish I-Knoevenagel-allyl-silane cyclization developed by us allows the efficient stereoselective formation of 1,2-trans-subsituted five- and six-membered carbocycles.1911 A photochemical cycloaddition of enamino-aldehydes and enamino-ketones with the intermediate formation of an iminium salt followed by addition to allylsilanes gives access to novel bicyclic heterocy-des. New examples of photochemically induced... 

Allylsilanes also serve as a two-carbon component for the analogous formal [4+2]-cydoaddition to provide tetrahydropyridines [188]. The reaction of 270b with 82 provided 271b, indicating that the enol ether moiety rather than the allylsilane moiety determines the orientation of the cycloaddition reaction. 

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 a related strategy, the dihydropiperidine skeleton of the macrocyclic alkaloid cannabisativine (208) (Fig. 1.6), isolated from the leaves and roots of the common cannabis plant, was prepared with regio- and stereoselectivity using an intramolecular allylsilane-nitrone cycloaddition reaction as a key step (260). 

Lewis acid treatment of 1,2,4-trioxolanes gives metallated carbonyl oxides which may be trapped by cycloaddition to allylsilanes to give 1,2-dioxolanes 78 <99TL6553>. 

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. 

Lewis acids play a dominant role on the chemoselectivity of the cycloaddition of allylsilanes and 3-butyn-2-ones. AICI3 and EtAlCl2 promote [2+2] cycloaddition predominantly, whereas TiCLt-mediated reaction gives significant amount of the [3+2] adduct288,289. The reaction of 2,2-bis(trifluoromethyl)ethylene-l,l-dicarbonitrile with allylsilane without any Lewis acid catalysts yields the cyclobutane derivative via a nonconcerted fashion290. 

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. 

Another approach leading to pyrrolidine 210 via [3 + 2] cycloaddition is the reaction of A-protected a-amino aldehydes with allylsilanes in the presence of a catalytic amount of BF3 OEt2 (equation 172). No 1,2-silyl group migration occurs in these annulation processes313. 

As for oxygen heterocycles, several reports involving the diastereoselective synthesis of highly substituted tetrahydrofuran derivatives have appeared. SnCLj-mediated [3 + 2] cycloaddition of allylsilane with optically active a-ketoesters affords 211 with excellent diastereoselectivity (equation 173)314-317. 

Formation of A2-isoxazolines 214 can be achieved by the cycloaddition of allylsilanes to NOBF4 in excellent diastereoselectivity (equation 175)318. 

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. 

The synthesis of polysubstituted tetrahydrofurans was also achieved in a stereoselective manner by a formal [3+2] cycloaddition of an allylsilane with a-triethylsiloxy aldehydes. An example showing the mechanism is illustrated <02JA3608>. In another approach, allylsilane was also allowed to react with a-keto esters in a [3+2] annulation reaction, providing highly substituted tetrahydrofurans in good yields as single diastereomers <02OL2945>. 

An interesting method to synthesize dihydrobenzo[6]furans was developed by employing a [3+2] dipolar cycloaddition between allylsilanes and o-benzoquinones <02TL5349>. 

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). 

The reaction of homochiral allylsilanes with a-enones and a-enals is very valuable for the asymmetric synthesis of multisubstituted cyclopentanes (Equation (40)).161,162 The [3 + 2]-cycloaddition of allylsilanes is applicable to />-quinones163 and />-quinoneimines.164 3-Butyn-2-one undergoes a double cycloaddition with an excess amount of allyltriisopropylsilane to give a bicyclo[3.3.0]octane in good yield (Equation (41)).165... 

The Lewis acid-promoted reaction of aldehydes with a-substituted allylsilanes forms 3-silyltetrahydrofurans in good to high yields.169-172 The use of homochiral allylsilanes is very valuable for highly diastereo- and enantio-selective syntheses of tri- and tetrasubstituted tetrahydrofurans (Equation (43)). Catalytic asymmetric [3-1-21-cycloaddition of a-substituted allenylsilanes to aldehydes can be achieved by a chiral scandium complex.173... 

Certain ketones are also usable for [3 + 2]-cycloaddition with allylsilanes.174,174a-174c,17S Interestingly, the reaction of unsubstituted allylsilanes at — 78 °C gives 2-silylmethyloxetanes ([2 + 2]-cycloadducts) mainly, although only [3 + 2]-cycloadducts are obtained at 0°C (Equation (44)).176... 

Imines and iminium salts also undergo [3 + 2]-cycloaddition of allylsilanes to afford 3-silylpyrrolidines with high diastereoselectivity.121,177,178... 

Spontaneous reaction of iV-chlorosulfonyl isocyanate with a-substituted allylsilanes is valuable for the synthesis of 4-silyl-2-pyrrolidinones.179-181 The [3 + 2]-cycloaddition of crotylsilanes proceeds stereospecifically as does the... 

Benzyl cations generated from benzyl alcohols or quinone methides by the action of S11CI4 undergo [3-(-31-cycloaddition of allylsilanes leading to tetrahydronaphthalenes.1 With secondary and tertiary benzyl cations, a competing [3 + 2]-pathway forms dihydro(177)indenes. 

The first example of Lewis acid-promoted [2 + 2]-cycloaddition of allylsilanes was introduced by Snider et al. in 1979.185 In the last decade, similar cycloadditions to various electron-deficient unsaturated bonds have been developed for efficient syntheses of cyclobutanes, cyclobutenes, oxetanes, and azetidines as described below. 

The [2 + 2]-cycloaddition of allylsilanes is applicable to the synthesis of substituted oxetanes from aldehydes and ketoesters (Equation (49)).176,193 The BF3-OEt2-promoted reaction of iV-tosylaldimines with allylsilanes gives [3 + 2]-cycloadducts, 3-silylpyrrolidines (vide supra).178 In contrast, iV-acylaldimines are converted into [2 + 21-cycloadducts under similar conditions.194... 

The BF3-catalyzed reaction of a-aminoaldehydes with 10 is valuable for highly stereoselective synthesis of 2,3,5-trisubstituted pyrrolidines with all -cis configurations (Equation (50)).197 The stereochemical outcome like chelation-controlled stereochemistry might result from the inherent conformational arrangement of the aldehyde-BF3 complex. />-Quinoneimines, o-quinones, and a-alkoxyhydroperoxides undergo similar types of [3 + 2]-cycloadditions with allylsilanes to afford dihydro indoles, dihydrobenzofurans, and 1,2-dioxolanes, respectively.164,175,198... 

Allylsilanes act as good acceptors of nitrones and oxyallyl cations. The 1,3-dipole species arising from electronically activated cyclopropanes can be trapped by allylsilanes.203 204 2043 Epoxides as well as aziridines act as 1,3-dipole precursors for inter- and intramolecular [3 + 2]-cycloadditions with allylsilanes.205 2053 206 2063... 

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