ELECTROSTATIC MODULATION OF HYDROXYL GROUP IONIZATION IN ACIDIC MEDIA - EVIDENCE FOR THE COMPETITIVE OPERATION OF INTRAMOLECULAR SN2 REACTIONS

The acid-catalyzed cyclodehydration of the cis and trans isomers of 2-substituted 1-(3-hydroxypropyl)cyclohexanols results in the formation of spirocyclic tetrahydrofurans. The stereochemical course of these reactions is highly varied, ranging from a dominant preference for retention when R = OCH3 to modestly favored inversion when R = CH3. Experiments with O-18-labeled diols show that in the methoxyl series most of the isotope is retained irrespective of relative stereochemistry. On the other hand, the pair of phenyl-substituted isomers responds by losing approximately 50% of the label. The isotopic level in the product erodes further when R = CH3. The stereochemical and isotopic labeling results are interpreted in terms of competing intramolecular S(N)2 and classical S(N)1 pathways. The extent to which cooperative nucleophilic attack with loss of the primary hydroxyl is facilitated reaches a maximum in the methoxyl-substituted diols, as a consequence of electrostatic inhibition of tertiary carbocation formation. As this effect is progressively lessened, the percentage of S(N)1 response rises. At no time, however, do the stereoisomeric carbocations interconvert conformationally prior to cyclization.