When starting with 4-pentyn-2-ol, the aldehyde addition in Scheme 2 furnished mixtures of diastereomers. Fortunately, in all cases, these diastereomers were separable by chromatography on silica gel. We were pleased to see that the subsequent palladium catalyzed cyclization was perfectly stereospecific. The major syn diastereomer reliably formed anti product; the minor anti diastereomer formed syn product (Scheme 5). Determining the stereochemistry of the linear starting materials was a considerable challenge (Scheme 6A). After various failed crystallization attempts, we globally deprotected 31 (Scheme 5, see supporting information for full experimental details) and converted it into silocine 61. Based on observed nOe correlations, the stereochemistry of 61 and, by analogy, of 31 was assigned. To explain the stereospecificity of this reaction, we propose a mechanism shown in Scheme 6B. Insertion of palladium occurs anti to the ethyl chloroformate leaving group and is followed by coordination of the silanol nucleophile (inner-sphere process). Subsequent syn reductive elimination furnishes product (Scheme 5). It is interesting to note that this stereoselectivity stands in contrast to related reactions where alcohols or phenols are used as nucleophiles; in these reactions, there is an overall retention of stereochemistry through an anti-anti mechanism. The product alkenyl dioxasilinanes could be further elaborated (Scheme 7). Using the 2 nd generation Hoveyda-Grubbs catalyst, cross metathesis of 33 with ethyl acrylate formed 62 in a 59% yield (Scheme 7A). Dihydroxylation of 48 formed tetrols 63 and 64 as a separable mixture of diastereomers (Scheme 7B).