The combination of an in situ formed MnL complex (HL = Hacac or R(C═O)CHCOR) and a chiral phosphoric acid HX* allows for a fully catalytic, asymmetric synthesis of 4H-chromenes starting from 2-alkyl-substituted phenols. The aerobic oxidation toward a transient ortho-quinone methide was efficiently catalyzed by a manganese(III) species MnL while the ensuing Michael addition of β-dicarbonyl compounds proved to be catalyzed by a chiral manganese phosphate MnLX*.
Synthetic strategies for the ibophyllidines, indole alkaloids with a characteristic pyrroloindoline substructure, are discussed and compared with their proposed biosynthesis.
A divergent approach to the pyrroloquinoline
scaffold as present
in the class of Aspidosperma alkaloids was developed.
As a case study, abundant and renewable nicotinic acid was transformed
via pericyclic framework reconstruction into aspidodispermine, a unique
member of pyrroloquinoline alkaloids. The sequence comprises a [2
+ 2]-photocycloaddition, a Ramberg–Bäcklund contraction,
and a strain-promoted formal electrocyclic rearrangement of a bicyclo[2.2.0]hexene
and is potentially extendable to pyrroloindole scaffolds as present
in the ibophyllidine alkaloids.
The features of two iconic chemical classes are united in the structure of the highly complex diterpene canataxpropellane and set a daunting challenge that has been met by the Gaich group. Their daring strategy and its benefit to the field of terpene chemistry is presented and discussed in this Highlight.
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