Temporary silicon connection strategies in intramolecular allylation of aldehydes with allylsilanes.
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.
Beignet
Julien
J
School of Chemistry, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom.
Jervis
Peter J
PJ
Cox
Liam R
LR
D000447:Aldehydes / Q000138:chemical synthesis* / Q000737:chemistry; D000475:Alkenes / Q000737:chemistry*; D003500:Cyclization; D004986:Ether / Q000737:chemistry; D007659:Ketones / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D015394:Molecular Structure; D012821:Silanes / Q000737:chemistry*; D012825:Silicon / Q000737:chemistry*; D055672:Static Electricity; D013237:Stereoisomerism
D000447:Aldehydes; D000475:Alkenes; D007659:Ketones; D012821:Silanes; C511847:allylsilane; D004986:Ether; D012825:Silicon