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Updated: May 21, 2026

Preparation of a Corannulene-functionalized Hexahelicene by Copper(I)-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
Published on: September 18, 2016
Regiodivergent 1,3-Annulation of Valylene with 4-Hydroxycoumarins
Yong-Kang Mei1,2, Zhi-Yuan Ding1,2, Yilitabaier Julaiti1,2
1Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China.
Abstract:
Pyranocoumarins are widely distributed in natural products and exhibit intriguing biological activities, making their efficient synthesis highly desirable. Herein, we developed an efficient method for synthesizing two new types dimethylallyl-decorated pyranocoumarins in efficiency through catalytic formal [3 + 3] annulation of 4-hydroxycoumarins with valylene. Under acid catalysis, exclusive 1,3-alkenoxylation afforded natural product-like angular pyranocoumarins, while Rh catalysis switched regioselectivity to 3,1-alkenoxylation. The protocol features high atom-economy, readily available substrates, and excellent regio- and chemo-selectivity. Mechanistic studies indicated the reaction proceeds via regioselective hydroalkoxylation to form diene or allene intermediates, followed by intramolecular cyclization. This work not only represents a good paradigm in addressing selectivity challenges in reactions involving substrates with multireactive-site but also offers a robust platform for rapid construction of skeletal variants beyond naturally occurring structures.
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Structure of Conjugated Dienes
Conjugated dienes are compounds characterized by the presence of alternating double and single bonds. In a conjugated system like 1,3-butadiene, the unhybridized 2p orbital on each carbon overlaps continuously, allowing the π electrons to be delocalized across the entire molecule. In contrast, this type of overlap does not occur in cumulated and isolated dienes, such as 2,3-pentadiene and 1,4-pentadiene, respectively. Instead, the π electrons remain localized between the double...
Regioselectivity and Stereochemistry of Hydroboration
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
Preparation of Diols and Pinacol Rearrangement
The reaction begins with transferring a proton from the acid catalyst to one of the hydroxyl groups, producing an oxonium ion.
Stability of Conjugated Dienes
A comparison of the enthalpies of hydrogenation of dienes reveals that conjugated dienes release less heat on hydrogenation, rendering them more stable than their nonconjugated analogs.
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