The diastereoselectivity of the aldol reaction of tetrahydro-4H-thiopyran-4-one (3) with 1,4-dioxa-8-thiaspiro[4.5]decane-6-carboxaldehyde (9a) under a variety of conditions is examined. Under optimized conditions, three of the four possible diastereomers from this aldol reaction can be obtained selectively (3-16:1). Reactions of 9a with the Li, B, Mg(II), and Ti(IV) enolates of 3 and with the corresponding trimethylsilyl enol ether 4b in the presence of BF(3) x OEt(2), SnCl(4), or TiCl(4) as promoters gave the Felkin adducts exclusively (>95%) as mixtures of syn (11a) and anti (12a) diastereomers. Use of the "amine-free" Li enolate of 3 gave 12a with a much higher diastereoselectivity (9:1) and yield (70%) than that obtained using the lithium diisopropylamide-generated Li enolate of 3 (2-3:1; 15-40%). The TiCl(4)-promoted reaction of 4b with 9a gave 11a with excellent selectivity (16:1). In contrast, the MgBr(2) x OEt(2)-promoted reaction of 4b with 9a gave the anti-Felkin adducts exclusively as a 3:1 mixture of syn (13a)/anti (14a) diastereomers. Similar aldol reactions of 3 with the cis and trans isomers of 4-(methoxy)methoxytetrahydro-2H-thiopyran-3-carboxaldehyde (9b and 9c) were examined to probe the influence of the ketal protecting group in 9a on the observed aldol diastereoselectivity. The results are rationalized by applying Evans' stereochemical model for merged 1,2- and 1,3-asymmetric induction (non-chelation), with the exception of the MgBr(2) x OEt(2)-promoted reactions of 4b with 9a, 9b, and 9c, which are accommodated by assuming chelation control. Comparison of the reactions of 9a, 9b, and 9c suggests that the ketal group in 9a uniquely allows high levels of either Felkin or anti-Felkin selectivity to be achieved.