ACID-CATALYZED CYCLIZATION OF 1,4-DIOLS TETHERED TO (BUTADIENE)IRON TRICARBONYL SEGMENTS - ISOTOPIC LABELING AS A MECHANISTIC PROBE OF STEREOCHEMICAL RETENTION DURING TETRAHYDROFURAN FORMATION

1,4-Butanediols can undergo acid-catalyzed dehydrative cyclization by either of two pathways. Selected 1-(3-hydroxypropyl) cyclohexanols have previously been shown to prefer the intramolecular S(N)2 option where displacement of water by the more highly substituted carbinol oxygen atom operates. All four O-18-labeled 1,4-diols prepared in this study, constructed so as to carry the secondary hydroxyl immediately adjacent to a tricarbonyliron-complexed diene, choose the alternative S(N)1 option. As a consequence, all of the isotopic content is absent in the tetrahydrofuran products, and stereochemical integrity is not preserved. Control experiments performed under the same mild conditions reveal product equilibration to be facile. Consequently, the opposite mechanistic extreme is followed by these systems. Other ground-state and transition-state considerations are discussed.