Starting from bromo/iodobenzaldehyde
derivatives, the corresponding
(Z)- and (E)-(2-stilbenyl)methanols
could be prepared in 2–5 steps via Pd-catalyzed cross-coupling
reactions (Sonogashira and Heck reactions) followed by aryllithium/aryl
Grignard addition. For the (E)-stilbenes, subsequent
acid-mediated cyclization using p-TsOH immobilized
on silica (PTS-Si) at low temperatures furnished the 2,3-trans-1-indanols with complete stereocontrol at the C2–C3. Further
oxidization of the alcohol provided the indanones, which are structurally
related to the natural product paucifloral F. At higher temperatures,
1,2- and 2,3-disubstituted indenes could be selectively prepared in
good to excellent yields. On the other hand, the (Z)-stilbenes, under similar conditions (PTS-Si), did not give the
indanols; only the 1,2-disubstituted indenes could be obtained. To
gain further insights into the stereochemistry at C2–C3 for
the (Z)-stilbenes, hydride or azide was employed
as a nucleophile; the corresponding indane products were obtained
with the cis stereochemistry at the C2–C3.
Thus, the (Z)- or (E)-olefin geometry
of the substrate directed the stereoselective indanyl cyclization
to furnish the cis or trans at the
C2–C3 ring junction, respectively, while reaction conditions
controlled the selectivity of the product types.