The enantioselective Michael reaction catalyzed by a bifunctional tertiary amine/squaramide has been used to trigger a Michael/transannular aldol cascade process that leads to densely substituted bicyclo[5.4.0]undecanes and in which three...
The organocatalytic
synthesis of densely substituted mono- and
bis-γ-lactams involving the Mukaiyama Mannich addition of 2,5-bis(trimethylsilyloxy)furan
to imines is described. Use of a ditoluenesulfonylimide catalyst
produces γ-lactams from monoaddition, whereas a more acidic
catalyst (triflic acid) produces fused bis-lactams from double addition.
Optimized organocatalytic conditions allow for the selective synthesis
of either desired core as well as the one-pot, multicomponent assembly
of the trisubstituted monolactams from aldehydes, amines, and bis-trimethylsilyloxyfuran.
An examination of chiral acids found these organocatalysts to be highly
active and diastereoselective in the monoaddition reaction, albeit
with no enantioselectivity.
Catalytic and enantioselective approaches to transannular reactions are very limited and mostly are based on chiral Lewis acid catalyzed pericyclic reactions. In this report, we present an efficient and straightforward methodology to access bicyclic carbo-and heterocyclic scaffolds combining different ring sizes through transannular Morita−Baylis−Hillman reaction catalyzed by a chiral enantiopure bifunctional phosphine. The reaction is remarkably wide in scope and enables the use of a variety of medium and large size ketoenone substrates leading to the final products in high yields and providing excellent stereocontrol in the formation of a quaternary stereogenic center at the ring fusion. Moreover, its potential as a general tool in organic synthesis has been highlighted through the accomplishment of the first enantioselective total synthesis of (−)-γ-gurjunene, a sesquiterpene natural product.
An approximation to the synthesis of several sesquiterpenes from the Guaiane family is described in which the core structure was obtained through a transannular Morita-Baylis-Hillman reaction performed under kinetic resolution. Several manipulations of the obtained MBH adduct have been carried out directed towards the total synthesis of γ-Gurjunene, to the formal synthesis of Clavukerin A, to the synthesis of a non-natural isomer of isoguaiane and to the synthesis of an advanced intermediate in the total synthesis of Palustrol.
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