Allylsilanes iodotrimethylsilane

The acetal 14, activated by the iodotrimethylsilane 17, produces the oxonium cation 16 which can be intercepted by allylsilane 1 yielding homoallylic ether 15, one equivalent of methoxytrimethylsilane 18 and the catalyst 17. 

Allylsilane 59 was obtained by initial reaction of the succinimide 55 with the corresponding Grig-nard alkylsilane and a subsequent simple sequence of reactions. This allylsilane was readily cyclized in formic acid at room temperature to 60, which was subjected to a Wacker oxidation (Tsuji 1984) to afford the dihydroanatoxin-a derivative 28. The nitrogen deprotection step with iodotrimethylsilane (Melching et al. 1986) funushedthe ( )-dihydroanatoxin-a, which was reprotected with oc according to the previously discussed Rapoport methodology (Sardina et al. 1989) to furnish the racemic ( )-anatoxin-a. The overall yield of this synthesis was 7% from the succinimide. 

HomoallyUc ethers.1 Iodotrimethylsilane catalyzes the allylation of acetals and ketals by allylsilanes, with transposition of the allylic group (equation I). It does not catalyze allylation of aldehydes and ketones. Note that TiCI4 can catalyze both of these reactions (7, 370-371). In this respect, ISi(CH3)3 resembles (CH3)3SiOTf (this volume). 

Notes and discussion. Methyl a-D-gluco- and mannopyranosides and a-D-glycopyranosyl chlorides can react with allylsilanes via activation with trimethylsi-lyl trifluoromethanesulfonate or iodotrimethylsilane to stereoselectively afford the corresponding a-C-allylated glycopyranosides in excellent yields. The yields of the reactions depend on the solvent and the amount of catalyst employed. Acetonitrile is the most suitable solvent whereas dichloromethane, the most commonly used solvent for allylation reactions using allylsilanes, does not afford satisfactory results. The reactions proceed very slowly when promoted by less than 5 mol% of the catalyst, however, the use of 50 mol% of catalyst is sufficient to force the reactions to completion. 

The reaction of allylsilanes with acetals to give homoallylic ethers is usually conducted with stoicheiometric amounts of Lewis acids it has now been shown that a catalytic quantity of iodotrimethylsilane is equally effective. ... 

General Discussion. 2,4-Pentadienyltrimethylsilane is used as a stable (storable) nucleophilic substitute for pentadienyllithium. This compound reacts with Lewis acid (boron trifluoride etherate, iodotrimethylsilane, titanium(IV) chloride) activated electrophiles in the same manner as allylsilanes (eq 1). Generally only e-substitution is observed (cf. Table 1). ... 

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See also in sourсe #XX -- [ ]

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