Regiochemical, Stereochemical, and Mechanistic Aspects

Kresze and Wagner examined the regioselectivity of the cycloaddition of IV-sulfinyl-p-toluenesulfonamide (9) with several unsymmetrical dienes [Eq. (9)].  

The cycloaddition of 9 with some 2-substituted dienes was found to yield only the 5-substituted dihydrothiazine oxides. Cycloadditions of 9 with 1-sulstituted 1,3-dienes [Eq. (10)] 

Dihydrothiazine oxide 15 was proposed to result from initial addition of the electrophilic sulfur atom of the N-sulfinyl compound to ( ,Z)-diene 11 to form a dipolar intermediate [Eq. (11)]. 

Addition of 9 to the Z olefinic bond of the diene would be favored since a transoid ( sickle ) allylic carbonium ion would be formed rather than the less stable cisoid ( U-form ) allylic moiety. Closure would afford dihy-drothiazine oxide 15. 

The rationalization for the formation of adduct 16 was somewhat more complex. Isomerization of (Z,Z)-hexadiene to ( ,Z)-hexadiene could be discounted since the cycloaddition of the ( ,Z)-diene and 9 led to the formation of sulfur epimer 15 exclusively. Assuming the reactive conformation of the (Z,Z)-diene is cisoid, only a nonplanar, helical skew arrangement can be achieved [Eq. (12)]. 

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