Stabilizing Effect by Geminal Dioxy Substitution and Anomeric Effect in 3,6-Dihydro-6-Methoxy-1,2-Oxathiin 2-Oxides

The hetero-Diels-Alder addition Of SO2 to (E)-hexa-1,3-diene (4) gives first 6-ethyl-3,6-dihydro-1,2-oxathiin 2-oxide (= 6-ethylsultine) with the Et group occupying a pseudoaxial position, and then the more stable stereoisomer 6 with the Et substituent in a pseudoequatorial position. The SO2 additions to 1-methoxybuta-1,3-diene (7) and to 1-methoxy-3-[(trimethylsilyl)oxy]buta-1,3-diene (8) give the 6-methoxysultines 9 and 10, respectively, with the MeO groups in pseudoaxial positions and which do not equilibrate with sultines having pseudoequatorial MeO substituents (anomeric effect). A lower limit of DELTADELTAG = 3.9 kcal/mol was evaluated at -60-degrees for the stabilizing effect arising from the geminal vicinity of a MeO and sulfinate moiety in 3,6-dihydro-6-methoxy-4-[(trimethylsilyl)oxy]-1,2-oxathiin 2-oxide.