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Communication | Special issue | Vol 15, No. 2, 1981, pp.719-734
Published online, 1st January, 1970
DOI: 10.3987/S-1981-02-0719
Reductive Transformation of a Hydroxyiodoketone of 8-Azabicyclo[3.2.1]octane into 7-Azabicyclo[2.2.1]heptane with Lithium Aluminium Hydride

Shalom Sarel* and Edmund Dykman

*Department of Pharmaceutical Chemistry, The Hebrew University, P.O.Box 12065 Jerusalem, Isarel, Israel


1α,4β-Diodo-2α-acetoxytropan-3-one (2) and 1α-iodo-2α-acetoxytropan-3-one (3),which comprise 64% of products emerging from the lead tetraacetate oxidation of tropan-3α-ol in presence of iodine, were subjected to lithium aluminium hydride reduction. The expected 2.3-dihydroxytropane was not obtained. Instead, a diol which was not prone to periodate oxidation, containing one primary and one secondary alcoholic groups was obtained. It was assigned the 1-hydroxymethyl-2α-hydroxy-7-methyl-7-azabicylo[2.2.1]heptane structure (8), on the basis of spectroscopic evidences (ir, nmr and mass spectrometry) and some chemical transformations [formations of a cyclic sulfite (9), an epichlorohydrin (11) and 1-methyl-2α-hydroxy-7-methyl-7-azabicyclo[2.2.1]heptane (12)]. The reaction is understood as an initial β-elimination process, triggered by the delivery of hydride ion to the carbonyl function in the sequence 32122, yielding a highly reactive enolate aldehyde (22) which is prone to an immediate internal aldol condensation (22) → (23) (24), affording the observed diol (8) on further reduction of the aldehydie group by LiAIH4. The reductive deiodination of (3) into 2α-acetoxytropine (2b) or into 2α-hydroxytropan-3-one (25) was smoothly achieved by means of hydrogenation over Raney Nickel on mild basic conditions.
Unlike the tropane derivatives (1, 26-28) which under electron impact enter into two distinctly different modes of fragmentation, the 7-azabicyclo[2.2.1]heptane framework in (8)-(12) characteristically favor one mode by which carbon-2 and carbon-3 are lost and the methylene pyrrolenium ion emerges as the most prominent ion in their spectra.