ChemInform Abstract: Catalytic Asymmetric Aldol‐Type Reactions Using a Chiral (Acyloxy) borane Complex.

Ishihara, Kazuaki; Maruyama, Tadashi; Mouri, Makoto; Gao, Qingzhi; Furuta, Kyoji; Yamamoto, Hisashi

doi:10.1002/chin.199408124

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“…8 This type of reagent-controlled strategy has rarely been extended to achiral crotylsilanes and other γ-substituted allylsilanes, where both diastereoselectivity and enantioselectivity need to be considered. 9 This is probably a result of (i) the relatively poorly defined open transition states through which allyltrialkylsilanes (type II allyl metals 10 ) react, making the control of relative stereoselectivity difficult, and (ii) competition from type I allylating agents (crotylboranes 11 and crotylboronates 12 being the most important examples), which react with aldehydes through well-defined chair-like transition state conformations, and offer a straightforward route to both syn and anti homoallylic alcohol products through judicious choice of the appropriate crotylmetal stereoisomer. That said, crotylsilanes have been used successfully in enantio-and diastereoselective synthesis.…”

Section: Introductionmentioning

confidence: 99%

Temporary Silicon Connection Strategies in Intramolecular Allylation of Aldehydes with Allylsilanes

2008

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Three gamma-(amino)silyl-substituted allylsilanes 14a-c have been prepared in three steps from the corresponding dialkyldichlorosilane. The aminosilyl group has been used to link this allylsilane nucleophile to a series of beta-hydroxy aldehydes through a silyl ether temporary connection. The size of the alkyl substituents at the silyl ether tether governs the outcome of the reaction on exposure to acid. Thus, treatment of aldehyde (E)-9aa, which contains a dimethylsilyl ether connection between the aldehyde and allylsilane, with a range of Lewis and Brønsted acid activators provides an (E)-diene product. The mechanism of formation of this undesired product is discussed. Systems containing a sterically more bulky diethylsilyl ether connection react differently: thus in the presence of TMSOTf and a Brønsted acid scavenger, intramolecular allylation proceeds smoothly to provide two out of the possible four diastereoisomeric oxasilacycles, 23 (major) and 21 (minor). A diene product again accounts for the remaining mass balance in the reaction. This side product can be completely suppressed by using a sterically even more bulky diisopropylsilyl ether connection in the cyclization precursor, although this is now at the expense of a slight erosion in the 1,3-stereoinduction in the allylation products. The sense of 1,3-stereoinduction observed in these intramolecular allylations has been rationalized by using an electrostatic argument, which can also explain the stereochemical outcome of a number of related reactions. Levels of 1,4-stereoinduction in the intramolecular allylation are more modest but can be significantly improved in some cases by using a tethered (Z)-allylsilane in place of its (E)-stereoisomer. Oxidation of the major diastereoisomeric allylation product 23 under Tamao-Kumada conditions provides an entry into stereodefined 1,2-anti-2,4-syn triols 28.

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Section: Introductionmentioning

confidence: 99%