Using a Temporary Silicon Connection in a Highly Stereoselective Synthesis of a New Class of Cyclic Allenylsilanes

Abstract

Aldehyde 4 reacts in the presence of TMSOTf and an acid scavenger to provide the corresponding oxasilacycle 5 in very good yield and as a single diastereoisomer. The sense of 1,3-stereoinduction serves to introduce a 1,3-syn-diol relationship into the cyclic product. This is in accord with a previous study involving an analogous intramolecular allylation.Forming a tether between two reacting partners allows a subsequent reaction to proceed in an intramolecular fashion and benefit from the advantages associated with unimolecular processes. However, if the tether is then cleaved, the product obtained is the result of a net intermolecular reaction. 2 We recently employed such a strategy in the stereoselective allylation of a series of aldehydes 1, in which the carbinol centre provided the site for appending an allylsilane through its g-terminus (Scheme 1). 3 Aldehyde 1 reacted in the presence of TMSOTf and an acid scavenger, 4 providing the corresponding intramolecular allylation product, oxasilacycle 2, in good yield. While levels of 1,4-stereoinduction were modest and rather substrate-dependent, 1,3-induction was consistently excellent; thus of the four possible diastereoisomeric products, only two were ever observed. We postulate that the adoption of reactive conformations in which the dipoles across the polar C-O and C=O bonds are opposed, are responsible for the high levels of 1,3-induction, 5 and that more subtle steric effects likely govern the 1,4-induction. 6 Acyclic dienes, 3, were also obtained as side-products in these allylations. We believe that these products arise through a cascade process initiated by premature cleavage of the silyl ether tether in the collapse of the initially formed carbocationic intermediate. 7 This transformation provides an acyclic allylsilane which then undergoes a vinylogous silicon-mediated olefination 8 to provide the observed diene product. 9Replacing the allylsilane nucleophile in our cyclisation precursor with a propargylsilane would provide aldehyde 4. In analogy with the reaction of aldehyde 1, exposure of 4 to a Lewis acid should effect an intramolecular allenylation to provide a novel oxasilacycle, 5, containing one new stereogenic centre and an allenylsilane moiety, which is ripe for further elaboration (Scheme 2). 10 We reasoned that such a reaction should be highly selective for a single stereoisomer since the formation of the allene moiety in 5 removes the issue of 1,4-stereoinduction, which had proven to be only modest in our previous allylation study (see Scheme 1). 3 This leaves just the issue of 1,3-induction to be considered. Since the factors governing the formation of this new carbinol stereocentre should be the same as those in our allylation study, a similar level and sense of 1,3-induction ought to be observed. Furthermore, since the silyl tether should be less efficient in stabilising the positive charge in the oxasilacyclic intermediate 6, 11 the likelihood of premature tether collapse, resulting in the Scheme 1 Reaction of an allylsilane tethered thr...