Acetals reaction with allylsilanes

A one-pot reaction between a tryptophan ester, benzotriazole, and 2,5-dimethoxytetrahydrofuran in acetic acid gives the diastereomeric benzotriazolyl tetracycles, 349, in good yield. Substitution of the benzotriazole by reaction with silyl enol ethers and boron trifluoride etherate gives the corresponding ketones 350 and 351, and reaction with allylsilanes gives the corresponding alkenes 352 and 353. If the boron trifluoride etherate is added to the mixture before the silane, elimination of benzotriazole from 349 is also observed (Scheme 83) <1999T3489>. 

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. 

Neighboring methoxy group assisted asymmetric induction is demonstrated in the TMSOTf-catalyzed reaction of y-alkoxy acetal 18 with allylsilane. A possible cyclic oxocarbenium ion intermediate 19 is proposed (equation 12)55. Siladioxanes 20 react with allylsilane in the presence of a catalytic amount of the Brpnsted superacid TfOH2+ B(OTf)4 to afford the allylated products 21 diastereoselectively (equation 13)56,57. 

Acetals and ketals 14 are the most often used precursors for the generation of oxonium species 16. Their main advantages lie in their resistance towards various basic reagents, their easy conversion into the desired oxonium intermediates in the presence of a suitable Lewis acid and in their ability to produce, by reaction with allylsilanes, enantiomerically pure homoallylic alcohols 26. For example, both enantiomers of 26 can be obtained starting from the enantiopure acetals 24 (Scheme 13.11). 

Under the influence of TMSOTf acyclic acetals react with allylsilanes, while cyclic counterparts remain unaffected. Formation of 5,6-dihydropyrans is interesting because new O-C and C-C bonds are created. The transformation of allylic acetals to mixed acetals and oxathioacetals is mediated by MeaS via 1,3-transposed sulfo-nium ions, which then undergo S 2 reactions with alkoxides or thiolates. On the other hand, oxathioacetals are cleaved when p-nitrobenzaldehyde is present. ... 

The derivatization method is compensated by the electrophihc addition reaction using organosilicon reagents [40-43]. Thus, the squaric acid family of derivatives, e.g., dichloride 17, methyl ester chloride 18, amide chloride 19, and diester 5 are the partners of the reactions with allylsilanes, silyl enol ethers, and silyl ketene acetals (Scheme 3). In this case, 1,2- and 1,4-addition to 20 and 21, respectively, are regulated by the substitution pattern of unsaturated organosilanes, kind of Lewis acid catalysts, and the reactivity of acid derivatives. The less congested is the reaction site, the more preferable is 1,2-... 

Using liquid sulfur dioxide as a Lewis-acid solvent for the alkoxyalkylation and alkylation of allylsilanes has been investigated50. The reaction of acetals with allylsilanes results in the formation of homoallyl ethers 11 in excellent yields (equation 8). 

More recently, allylic carbonates 115 have been found to couple with alkenylfluo-rosilanes in the presence of a palladium catalyst to yield the corresponding 1,4-dienes (equation 93)174. The corresponding palladium-catalyzed reaction of allylsilane with allyl acetate 116 has been explored (equation 94)175. 

Marko et al. initially employed allylsilane 171 during their study on the scope and limitations of the IMSC methodology in 1993 [65] and expected to obtain the exo-methylene tetrahydrofurans 175 (Scheme 13.60). However, none of the desired furan derivatives 175 was obtained when a mixture of 171 and aldehydes 174 was treated with a range of Lewis acids. Rather, the diastereomerically pure exo-methylene tetrahydropyrans 173 were isolated, albeit in modest yields (Scheme 13.61). In 1995, Oriyama et al. [79] published the IMSC reaction of acetals with allylsilane 171, yielding the desired tetrahydrofurans 175 in the presence of the SnC /AcCl system (See Chapter 13.4.2). Interestingly, product 173 was not formed when the corresponding acetals were used instead of the aldehydes in this coupling reaction and vice-versa [49, 65]. 

Tandem acetalization-allylation reaction of aldehydes with allylsilanes provides a convenient route to homoallyl ethers.82 Under catalysis by Sc(OTf)3, various aldehydes can be converted into homoallyl ethers by treatment with... 

Hie regioselective cycloadditions can generate useful synthetic intermediates. Conversion of adduct (n9) to (120) provides the equivalent of an a-phenylcyclopentanone annulation (equation 124). Hie allylsilane moiety generated by the bis(silyl) acetate (113) is a useful entity. For example, the cycloadduct (121) can be easily transformed to (122), which is a regioisomer of the adduct resulting from reaction with (112 R = OAc) (equation 125). ... 

Analogous to the allylation with allylsilanes and -stannanes, the transformations, vinylallylation, propargylation, allenylation, alkenylation, alkynylation, and arylation, are viable by the use of an appropriate reagent in the presence of a titanium Lewis acid these are surveyed in the review articles cited both in the Introduction and in this section. The stereochemistry of the reaction of a (vinylallyl)silane in the presence of TiCU has been reported [234]. Equation (113) shows that the major reaction of this silane and isobutyraldehyde occurred mainly in the anti sense with a ratio of anti to syn attack of 90 10 at the terminus remote from the silyl group. Essentially the same stereochemical outcome was observed for the same reaction with the corresponding trimethylsilyl derivative. The intramolecular reaction with an acetal, however, proceeded less selectively the anti syn ratio was 60 40 (Eq. 114) [234]. 

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