The facial selectivities of the Diels-Alder cycloadditions of several dienophiles to the title substrates were studied. The observed selectivities are interpreted as a consequence of the relative steric interactions offered by the substituents. The addition of dimethylacetylene dicarboxylate (DMAD) is influenced by the electrostatic repulsion arising from the interaction of an electron pair orbital on the acetal oxygen and the orthogonal pi-orbital of the acetylene unit in DMAD in the syn-to-oxygen addition of the latter. This repulsion is offset on coordination of Li+ to the said oxygen electron pair orbital, and the addition thus proceeds syn to oxygen. The enhanced and accelerated syn-to-oxygen addition in lithium perchlorate in nitromethane is interpreted as a consequence of the coordination of Li+ to both the acetal oxygen and a heteroatom in the dienophile that brings them in close proximity to facilitate a reaction. The Li+-oxygen combination, however, also exerts some steric effect that results in reduced syn-to-oxygen addition of dienophiles having large substituents such as N-phenylmaleimide.

The stereoisomers of 7-phenyl-1-oxa-4-thiaspiro[4.5]decan-7-ol, C(14)H(18)O(2)S, have the same stereochemistry at the C atom bearing an OH group, i.e. axial OH and equatorial phenyl groups. However, the acetal S and O atoms are axial and equatorial, respectively, in one isomer and reversed in the second. Furthermore, the crystals of one isomer are composed of hydrogen-bonded molecules involving the hydroxyl H atom and the O atom of the five-membered heterocyclic ring, with an O.O distance of 2.962 (3) A, forming a polymeric chain along the b axis. The asymmetric unit of the other isomer is composed of two molecules, wherein hydroxyl H atoms and the O atoms of the five-membered heterocyclic rings display intramolecular O-H.O hydrogen bonds with O.O separations of 2.820 (2) and 2.834 (2) A.

The crystal structures of two 1-oxa-4-thiaspiro[4.5]decane derivatives, ethyl 6-benzylidene-1-oxa-4-thiaspiro[4.5]decane-7-acetate, C19H24O3S, and (6-ethylidene-1-oxa-4-thiaspiro[4.5]decan-7-yl)ethyl 4-bromobenzoate, C19H23BrO3S, obtained under Johnson orthoester Claisen rearrangement conditions, exhibit different olefin geometry. Both structures are composed of independent molecules separated by normal van der Waals distances. The S-Csp(3) bond distances are significantly different from each other, as has been observed in similar structures; the remaining molecular dimensions are as expected.

6-Vinyl-1-oxa-4-thiaspiro[4.5]dec-6-ene has been reacted with dienophiles, such as N-phenylmaleimide (NPM), N-methyltriazoline-2,5-dione (MTAD) and dimethylacetylene dicarboxylate (DMAD), to assess the 1,3-diastereofacial selection caused by the acetal function. In each case, a mixture of two diastereoisomers was produced. The crystal structures of the products of the addition of NPM and MTAD syn to the acetal oxygen, 2-phenyl-2,3,3a,4,5,5a,6,7,8,9,9a,9b-dodecahydro-1H-benz[e]isoindole-6-spiro-2'-[1',3']oxathiolane-1,3-dione, C20H21NO3S, (IIa), and 2-methyl-5,7,8,9,10,10a-hexahydro-1H-1,2,4-triazolo[1,2-a]cinnoline-7-spiro-2'-[1',3']oxathiolane-1,3-dione, C13H17N3O3S, (IIIa), respectively, and the product of the addition of DMAD syn to the acetal sulfur, dimethyl 1,2,3,4,4a,7-hexahydronaphthalene-1-spiro-2'-[1',3']oxathiolane-5,6-dicarboxylate, C16H20O5S, (IVb), have been determined. All three structures are composed of independent molecules separated by normal van der Waals distances. The 1-oxa-4-thia heterocyclic ring has an envelope conformation in the three structures and the S-Csp(3) bond distances differ significantly from each other, as observed in comparable structures; the remaining molecular dimensions are as expected.