The utility of zeolites as promoters for the
cyclization of an epoxide-containing polyene has been
investigated. Reaction of polyene 1 with oven-dried 4
Å molecular sieves (type A zeolite) proceeds
efficiently to generate bicyclic alcohol 2 in a 90%
isolated yield. The reaction is sensitive to a
variety
of factors, including solvent type, water content, and zeolite acidity.
Reactivity is apparently due
to the zeolite lattice, since alumina and silica either are unreactive
or generate a complex mixture
of epoxide ring-opened products. Compared to the aluminum-based
Lewis acid Me2AlCl, the zeolite-promoted cyclization of 1 was a more facile reaction,
providing excellent product recovery after
filtration. These results indicate that zeolites represent a new
class of promoters in biomimetic
polyene cyclizations.
A new approach for the biomimetic polyene cyclization of
epoxyolefins is demonstrated by the zeolite-promoted tricyclization of 1. Reaction of 1
with a variety of zeolitic materials, including Zeolite A,
Mordenite, Faujasite, and ZSM-5, yielded variable amounts of cyclic and
acyclic epoxide-rearranged
materials. Small-pore Zeolite A, in either the Na or Li form,
caused the exclusive formation of
ketone 3, while both H−Mordenite and H−ZSM-5 were
sufficiently reactive to provide tricyclic
products in modest yields. Low recovery of polyene 1
was observed when larger-pore zeolites of
type Beta and H−Faujasite were utilized as cyclization promoters,
suggesting that decomposition
products had been included within the zeolite cavity. Dramatic
improvement in reaction selectivity
was achieved after surface dealumination of H−ZSM-5, which gave 52%
(isolated yield) of tricycle
2. Compared to the Lewis acid cyclization of
1 by Ti(OiPr)Cl3 or
MeAlCl2, the reaction is highly
stereoselective, producing the all-trans tricycle as the predominant
product. These results, in
conjunction with surface and pore deactivation studies using pyridine
bases, suggest that the
cyclization is initiated at the opening of the zeolite pores, with
concurrent adsorption of the polyene
onto the aluminosilicate surface.
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