Steric decompression

Curiously enough, bulky substituents on nitrogen increase this reactivity towards methyl iodide (119). This has been related to a steric decompression of the thiocarbonyl group in the transition state. Furthermore, knowledge of the ratio of conformers in the starting 4-alkyl-3-i-Pr-A-4-thiazoline-2-thiones and in the resulting 4-alkyl-3-i-Pr-2-methylthiothi-azolium iodides combined with a Winstein-Holness treatment of the kinetic data indicates that in the transition state, the thiocarbonyl bond is approximately 65% along the reaction coordinate from the initial state... 

The importance of this steric effect was verified by carrying out the oxono-lysis of acetal 103. Oxidation of 103 by ozone gave the ester 105 exclusively (33). In this reaction, the hydrotrioxide intermediate 104 has proper electron pair orientation to favor the opening in both directions, but cleavage occurs in only one, yielding 105 because of this steric decompression factor. 

In agreement with the hypothesis that similar intermediates are involved in p-dicarbonyl ar in which the steric decompression resulting from the ligand coupling step favours the arylatioi more substituted p-dicarbonyl substrates, the thermal instability of the (p-methoxyphenyl)ph lead diacetates increased in the series ... 

Figure 5.36 Effect of steric decompression in crowded amides on the mecheuiism of their reactions with...

A bridging allylic derivative, such as the one on cyclodextrin 82, can also create a similar steric decompression on the preceding glucose unit. This orientates the second de-O-benzylation toward the clockwise positions (Scheme 9.39) [77], leading to cyclodextrin 83 in good yields, with no traces of the other regioisomer 84. 

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