A Fused Hexacyclic Ring System: Diastereoselective Polycyclization of 2,4-Dienals through an Interrupted iso-Nazarov Reaction.

We report an unprecedented domino polycyclization from readily available 2,4-dienals and cyclic α,ß-unsaturated imines that is initiated by an iso-Nazarov reaction. This Brønsted acid promoted reaction enables the concomitant formation of four bonds, three cycles, and four contiguous stereogenic centers to yield elaborated structures in a single operation. A range of fused hexacyclic molecules is obtained in a highly diastereoselective manner.

Rearrangements Induced by Hypervalent Iodine.

This chapter describes advances in hypervalent iodine(III)-induced rearrangements reported between 2004 and 2015, beginning with Hofmann-type rearrangements and aliphatic aryl transpositions. In both reactions the iodine(III) reagent may be off-the-shelf or catalytically generated in situ. A number of stereoselective transformations are discussed, followed by transpositions triggered through phenol dearomatization, including Wagner-Meerwein-type rearrangements, Prins-pinacol transpositions, and a tandem polycylization-pinacol process. Other rearrangements such as an iodonio-Claisen rearrangement, an ipso-rearrangement, and rearrangements performed using iodine(V) are also described.

Entropy is key to the formation of pentacyclic terpenoids by enzyme-catalyzed polycyclization.

Polycyclizations constitute a cornerstone of chemistry and biology. Multicyclic scaffolds are generated by terpene cyclase enzymes in nature through a carbocationic polycyclization cascade of a prefolded polyisoprene backbone, for which electrostatic stabilization of transient carbocationic species is believed to drive catalysis. Computational studies and site-directed mutagenesis were used to assess the contribution of entropy to the polycyclization cascade catalyzed by the triterpene cyclase from A. acidocaldarius. Our results show that entropy contributes significantly to the rate enhancement through the release of water molecules through specific channels. A single rational point mutation that results in the disruption of one of these water channels decreased the entropic contribution to catalysis by 60 kcal mol(-1) . This work demonstrates that entropy is the key to enzyme-catalyzed polycyclizations, which are highly relevant in biology since 90 % of all natural products contain a cyclic subunit.