Thiopyran route to polypropionates: aldol diastereoselectivity of linear and two-directional iterative homologations

[formula: see text] Aldol reaction of tetrahydro-4H-thiopyran-4-one with racemic 1,4-dioxa-8-thiaspiro[4.5]decane-6-carboxaldehyde is easily controlled to give the 2,3-anti-3,4-syn or the 2,3-syn-3,4-syn adduct. Aldol homologations of these beta-hydroxy ketones with the same aldehyde occur with considerable mutual kinetic enantioselection (MKE) and, in each case, selectively give one of the eight possible diastereomers. Similar reactions of related beta-methoxy ketones are also very diastereoselective but proceed without significant MKE, resulting in two diastereomers. The adducts can be used for polypropionate synthesis.