1.7- Dithiaspiro undecane

C NMR spectroscopy of 1,7-dioxa- (314), l-oxa-7-thia- (315), and 1,7-dithiaspiro[5.5]undecane (316) and many of their derivatives, with special emphasis on stereoelectronic and steric effects governing the configurational and conformational properties of these compounds (484-486). This work showed that chemical-shift parameters developed for six-membered carbocyclic compounds in chair conformations (149) could be applied successfully only to sulfur and not to oxygen heterocycles (486). 

The detailed analysis of these conformers showed that 66 exists in a conformation equivalent to 64A only. Similarly, 67 would also exist in a conformation equivalent to 64B only. This analysis further predicted that 66 is more stable than 67 by 2.4 kcal/mol and since these isomers can be inter-converted by equilibration under acidic conditions, isomer 66 should be the major isomer formed under thermodynamically controlled conditions. Cy-clization of 65 under equilibrating conditions gave 66 and 67 in a 98 2 ratio and 66 was shown to exist in a conformation equivalent to 64A. The fact that a small percentage of 67 was detected at room temperature shows that, at sufficiently low temperature, less than 0.1% of 67 would be present. This result confirms that, at low temperature, l,7-dithiaspiro[5.5]-undecane exists essentially in the conformation 64A only. The anomeric effect for a sulfur atom in the dithioacetal function must therefore be of the order of 1.4 kcal/mol. 

Dithiaspiro[5.5]undecane can exist in the three conformations 64A, 64B, and 64C (Fig. 11). Conformer 64A which has the two sulfurs axially oriented can have two anomeric effects, conformer 64C which has the two sulfurs equa-torially oriented has no anomeric effects and conformer 64B with one sulfur axially oriented has only one anomeric effect. Taking Into account the ster-1c effects (0.9 kcal/mol for a gauche form of butane and 0.4 kcal/raol for a gauche form of S-CH2 X-CH2) and an anomeric effect of 1.4 kcal/mol. the relative stabilities 64A, 64B, and 64C are 0, 2.4, and 4.8 kcal/mol. On that basis, this compound must be essentially conformationally rigid existing in the conformation 64A only. This prediction was verified experimentally (25, 62), and further confirmation was found by the study of the acid catalyzed cyclization of ketone dithiol 65. This compound can give either isomer 66 or 67 which can take two different conformations each. 

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