At low temperature and in the presence of CF 3 COOH, SO 2 undergoes Diels-Alder additions with (E)-1-acetoxybutadiene (8d) giving a 1:10 mixture of diastereomeric 6-acetoxysultines (9d + 10d). The Van't Hoff plot for equilibria 8d + SO 2 a 9d + 10d led to ∆H r ) -7.0 ( 0.3 kcal/mol, ∆S r ) -42 ( 3 cal‚mol -1 ‚K -1 . At 20 °C, 8d underwent a slow cheletropic addition with SO 2 giving 2-acetoxysulfolene (11d, ∆H r = -11.5 kcal/mol), the structure of which was established by singlecrystal X-ray diffraction studies. (E)-Chloro (8e) and (E)-bromobutadiene (8f) did not undergo Diels-Alder additions with SO 2 , even in the presence of protic or Lewis acid promoters. Low yields of 2-chloro-(11e) and 2-bromosulfolene (11f) were obtained at 20 °C. The structure of 11e was confirmed by single-crystal X-ray diffraction. The potential energy hypersurfaces of the Diels-Alder and cheletropic additions of SO 2 to butadiene (8a), (E)-piperilene (8b), (E)-1-methoxy-(8c), (E)-1-acetoxy-(8d), and (E)-1-chlorobutadiene (8e) were studied by ab initio quantum calculations at the MP2/6-31G* level. In agreement with the experiment, 6-substituted sultines 9X and 10X were less stable than the corresponding 2-substituted sulfolenes 11X for X ) Me, OAc, Cl. With X ) OMe, the two diastereomeric 6-methoxysultines (9c, 10c) and 2-methoxysulfolene (11c) were calculated to have similar stabilities. This is attributed to a stabilizing thermodynamic anomeric effect or gem-sulfinate/methoxy disubstitution effect in 9c, 10c. Such effects were not detected for sulfinate/acetoxy (9d, 10d) and sulfinate/chloro (9e, 10e) disubstitutions. The relative instability of 2-acetoxy-(11d) and 2-chlorosulfolene (11e) compared with their cycloaddents is attributed to repulsive interactions between the SO 2 moieties and the 2-substituents. The Alder endo mode of [4 + 2] cycloaddition of SO 2 is predicted to be faster than the "anti-Alder mode" of additions for dienes 8X, X ) Me, OMe, OAc, Cl. The resulting diastereomeric sultines 9X and 10X, respectively, exist as equilibria (energy barrier: ca. 5-6 kcal/mol) of two conformers 9X a 9′X, 10X a 10′X. In general, the conformers 9X, 10X with pseudoaxial SdO group are preferred (conformational anomeric effect of the sulfinate moiety). Repulsive interactions between pseudoaxial SdO and polar cis-6substituents (e.g.: X ) OMe, OAc) in 9X may render conformers 9′X (with the SdO and 6-X groups in pseudoequatorial positions) as stable as conformers 9X. The calculations predict the existence of conformational anomeric effects of 2-3 kcal/mol for the gem-sulfinate/methoxy (9c, 10′c) and gem-sulfinate/acetoxy disubstitution (9d, 10′d).

The reactivity of sulfur dioxide toward variously substituted butadienes was explored in an effort to define the factors affecting the competition between the hetero-Diels-Alder and cheletropic additions. At low temperature (< À 708), 1-alkyl-substituted 1,3-dienes 1 that can adopt s-cis-conformations add to SO 2 in the hetero-Diels-Alder mode in the presence of CF 3 COOH as promoter. In the case of (E)-1-ethylidene-2-methylidenecyclohexane ((E)-4a), the [4 2] cycloaddition of SO 2 is fast at À 908 without acid catalyst. (E)-1-(Acyloxy)buta-1,3-dienes (E)-1c, (E)-1y, and (E)-1z with AcO, BzO, and naphthalene-2-(carbonyloxy) substituents, respectively also undergo the hetero-Diels-Alder addition with SO 2 CF 3 COOH at low temperatures, giving a 1 : 10 mixture of the corresponding cis-and trans-6-(acyloxy)sultines c-2c,y,z and t2c,y,z, respectively). Above À 508, the sultines undergo complete cycloreversion to the corresponding dienes and SO 2 , which that add in the cheletropic mode at higher temperature to give the corresponding 2-substituted sulfolenes ( 2,5-dihydrothiophene 1,1-dioxides) 3. The hetero-Diels-Alder additions of SO 2 follow the Alder endo rule, giving first the 6-substituted cis-sultines that equilibrate then with the more stable trans-isomers. This statement is based on the assumption that the SO group in the sultine prefers a pseudo-axial rather than a pseudo-equatorial position, as predicted by quantum calculations. The most striking observation is that electronrich dienes such as 1-cyclopropyl-, 1-phenyl-, 1-(4-methoxyphenyl)-, 1-(trimethylsilyl)-, 1-phenoxy-, 1-(4-chlorophenoxy)-, 1-(4-methoxyphenoxy)-, 1-(4-nitrophenoxy)-, 1-(naphthalen-2-yloxy)-, 1-(methylthio)-, 1-(phenylthio)-, 1-[(4-chlorophenyl)thio]-, 1-[(4-methoxyphenyl)thio]-, 1-[(4-nitrophenyl)thio]-, and 1-(phenylseleno)buta-1,3-diene, as well as 1-(methoxymethylidene)-2-methylidenecyclohexane (4f) do not equilibrate with the corresponding sultines between À 100 and À 108, in the presence of a large excess of SO 2 , with or without acidic promoter. The hetero-Diels-Alder additions of SO 2 to 1-substituted (E)-buta-1,3-dienes are highly regioselective, giving exclusively the corresponding 6-substituted sultines. The 1-substituted (Z)-buta-1,3-dienes do not undergo the hetero-Diels-Alder additions with sulfur dioxide.

The hetero-Diels-Alder addition of sulfur dioxide to (E)and (Z)-4-(fluoromethylidene)-3-methylene-2,3-dihydronaphthalene follows the endo Alder rule. The first example of a crystalline sultine has been obtained. In agreement with high level quantum calculations, (1SR,3SR)-1-fluoro-1,4,5,6-tetrahydronaphtho[2,1-d][1,2]oxathiine 3-oxide adopts a sofa conformation in its ground state.

The reactivity of (E)‐ and (Z)‐1‐fluorobuta‐1,3‐diene ((E)‐ and (Z)‐11), 2‐fluorobutadiene (12), (E)‐ and (Z)‐1‐(fluoromethylidene)‐2‐methylidenecyclohexane ((E)‐ and (Z)‐13) toward SO2 has been explored and compared with that of (Z)‐ and (E)‐1‐(fluoromethylidene)‐2‐methylidene‐3,4‐dihydronaphthalene ((Z)‐8 and (E)‐8). In agreement with quantum calculations, 12 is unreactive toward SO2 (no cycloaddition, only polymerization), whereas (E)‐1‐fluoro‐1,3‐dienes react more rapidly than their (Z)‐isomers to give the corresponding 6‐fluorosultines following the endo (Alder rule) mode of hetero‐Diels–Alder addition. No sulfolene has been observed following the cheletropic mode of addition with the fluorodienes, in contrast to other substituted dienes. In agreement with the calculations, cis‐2‐fluoro‐3,4‐oxathiabenzobicyclo[4.4.0]dec‐1(6),9‐diene‐4‐oxide (cis‐9, the sultine obtained by SO2 addition to (Z)‐8 under conditions of kinetic control) adopts a sofa conformation with the oxygen atom of the ring lying in the average plane of the four carbon atoms of its sultine moiety when it is in the crystalline state at −100 °C. A similar sofa conformation was found for its trans‐isomer, trans‐9, obtained by isomerization of cis‐9 or by hetero‐Diels–Alder addition of SO2 to (E)‐8. Experiments (equilibrium constant for hetero‐Diels–Alder additions, bond lengths, and bond angles in crystalline fluorosultines cis‐9 and trans‐9) and high‐level quantum calculations on cis‐ and trans‐6‐fluoro‐3,6‐dihydro‐1,2‐oxathiin‐2‐oxide (cis‐ and trans‐20) confirm the existence of a stabilizing, enthalpic, anomeric (gem‐disubstitution by sulfinyloxy and fluoro groups) effect, which is interpreted in terms of (lone pair) n(O1)→σ*(C−F) hyperconjugative interactions. This effect is strongest in the sofa conformers with a gauche arrangement of the σ(O1,S2) and σ(C6,F) bonds. The calculations suggest also that n(O1)→σ*(S2,O2′), π*(S=O), and n(S2)→σ*(O1,C6) interactions intervene and affect the relative stability of the conformers (sofa, boat, pseudo‐chair) found for 6‐fluorosultines cis‐ and trans‐20.