Definition of Several Control Elements Relevant to the Stereodefined Serial Elaboration of Belted Poly(spirotetrahydrofurans) Fitted with a Cyclohexane Core

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Abstract
The stereochemistry of the condensations of 2-cyclohexenones, α-arylidenecyclohexanones, and α-(tert-butyldimethylsiloxy)cyclohexanones carrying one or two (both syn and anti) spirotetrahydrofuran units adjacent to the carbonyl with allyl organometallics (especially indium) and with the Normant reagent (ClMgO(CH2)3MgCl) is described. Good levels of anti stereoselection are observed in the α-arylidene series. Subsequent cyclization generates a second (or third) tetrahydrofuran ring possessing trans vicinal oxygens. Useful levels of matched and mismatched diastereoselection are also attainable by prior α-oxygenation. The intrinsic differences in diastereomer production between indium and magnesium organometallics are highlighted. A clear distinction regarding the anticipated direction of stereoselectivity is made on the grounds of chelation capabilities and the intra- or intermolecularity of carbon−carbon bond formation. Finally, the two protocols that are described in detail are shown to be iterative, a feature that augurs well for ultimately accessing the eight possible hexaspirocyclohexanes in an efficient and stereocontrolled manner.

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