Studies toward the Total Synthesis of Parvifolals A/B: An Intramolecular o-Quinone Methide [4 + 2]-Cycloaddition To Construct the Central Tetracyclic Core

All-Carbon Quaternary Center Containing Tetrahydro-5H-benzo[c]fluorenes via Highly Stereoselective Transannular Carbocation Cyclization

Transannular carbocation cyclization of the achiral benzannulated cyclononene tertiary alcohols under acid catalysis furnished the [5/6]-fused tetracycles with three contiguous stereogenic centers, one of which is an all-carbon C11b quaternary center, in good-to-excellent yields and diastereoselectivity. Hydride, water, azide, allyl, and electron-rich (hetero)arenes could be employed as nucleophiles. The reaction is under both substrate- and condition-control. Further, functionalization of some selected substrates was also demonstrated. Computational chemistry delineated different competing reaction pathways such as direct substitution at the bisbenzylic position for other related systems-the benzannulated cyclooctene/cyclononene secondary alcohols and provided further evidence for the observed stereoselectivity.

Enantioselective access to tricyclic tetrahydropyran derivatives by a remote hydrogen bonding mediated intramolecular IEDHDA reaction

Inverse-electron-demand-hetero-Diels-Alder reactions of alkenes with α,β-unsaturated keto compounds allow rapid access to the tetrahydropyran ring found in numerous natural products and bioactive molecules. Despite its synthetic interest, catalytic asymmetric versions of this process remain underdeveloped, especially regarding the use of non-activated alkenes reacting with α,β-unsaturated ketone or aldehyde, for which no report can be found in the literature. Herein, we describe the catalytic inverse-electron-demand-hetero-Diels-Alder reactions between neutral alkenes and an α,β-unsaturated ketones or aldehydes to produce a variety of trans-fused [5,6,8] tricyclic structures containing a central, chiral tetrahydropyran ring. This complex transformation, which is achieved using a chiral phosphoric acid, allows for the formation of four stereogenic centers in a single step with high regio-, diastereo- and enantioselectivity (up to 99% ee). Such level of stereocontrol could be achieved by a key remote double hydrogen atom bonding interaction between the linear substrate and the catalyst.

Although the hetero-Diels–Alder reaction is a staple of organic chemistry, catalytic asymmetric versions of the inverse-electron demand variant often require specially engineered substrates for the reaction to work. Here the authors cyclize non-activated alkenes with α,β-unsaturated ketones or aldehydes to form chiral fused heterocycles using a chiral phosphoric acid catalyst.