Abstract
Simple enantioselective synthesis of 6,6-disubstituted pentafulvenes bearing chiral pendant hydroxy groups are attained by cascade reactivity using commercially available proline-based organocatalysts. Condensation of cyclopentadiene with the acetyl function of a 1,2-formylacetophenone, followed by cyclization of a resulting fulvene-stabilized carbanion with the formyl group, generates bicyclic chiral alcohols with initial er values up to 94:6. Exceptional enantio-enrichment of the resultant alcohols results upon crystallization-even near racemic samples spontaneously de-racemize. This enables new families of substituted cyclopentadienes that are both enantiomerically and diastereomerically pure to be rapidly attained.
Highlights
To develop a route to new chiral pentafulvenes bearing pendant hydroxy groups, we investigated the reaction between 2-acetyl-benzaldehyde and cyclopentadiene in the presence of organocatalysts (Table 1)
Synthetic methodology for pentafulvene formation has not altered significantly since these were first prepared by Thiele in 1900 by sodium ethoxide facilitated condensation of cyclopentadiene with ketones (Scheme 1).[1]
Frequent applications include: cycloadditions to generate complex, polycyclic scaffolds[5][6] and their use as intermediates in the synthesis of substituted cyclopentadienyl derivatives via nucleophilic addition to the exocyclic C=C bond as a route to cyclopentadiene units.[7][8][9] we describe a simple approach to families of asymmetric 6,6-disubstituted pentafulvenes bearing chiral pendant hydroxy groups by straightforward organocatalytic methodology
Summary
To develop a route to new chiral pentafulvenes bearing pendant hydroxy groups, we investigated the reaction between 2-acetyl-benzaldehyde and cyclopentadiene in the presence of organocatalysts (Table 1). Synthetic methodology for pentafulvene formation has not altered significantly since these were first prepared by Thiele in 1900 by sodium ethoxide facilitated condensation of cyclopentadiene with ketones (Scheme 1).[1] improved by Little[2] and Ottosson[3] among others none of these allows access to chiral fulvenes.
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