We were very interested in determining the mechanism of formation for these 1'-silanoxy-heterocycles (Scheme 6), and, for simplicity, we will focus on the formation of 1'-silanoxytetrahydrofurans. Which oxygen of the di-tert-butyl silanol is responsible for the initial epoxide attack? If the distal oxygen (silanol OH) attacks first, one may draw an 8-membered ring intermediate, which will immediately contract into a fivemembered ring upon a second nucleophilic attack (Scheme 6A, Pathway 1). If the proximal oxygen attacks first, a [5,5]mechanism is likely operative, comprised of tetrahydrofuran ring formation followed by a silyl migration 24-28 (Scheme 6A, Pathway 2). To distinguish between the two pathways, we designed a series of test substrates (Scheme 6B). With TBDPS substrate 75, rearrangement did occur, albeit in a much lower yield. This experiment established that the distal oxygen of the silanol was not required for a productive reaction, but its presence did promote product formation. With silanoxy methyl ether substrate 77, the yield of rearrangement product 78 was higher. Both substrates were outperformed by silanol 1, which rearranged in a 78% isolated yield. Thus, the performance of the rearrangement reaction was positively correlated to the electrophilicity of the silicon auxiliary, and we hypothesize that the [5,5]-mechanism (Scheme 6A, Pathway 2) is operative. Analogously, for the formation of 1'-silanoxytetrahydropyrans, a [6,5]-mechanism likely underlies product formation.