Enantioselective formation of quaternary carbon stereocenters in natural product synthesis: a recent update

Covering: 2013-2018 Natural products bearing quaternary carbon stereocenters have attracted tremendous interest from the synthetic community due to their diverse biological activities and fascinating molecular architectures. However, the construction of these molecules in an enantioselective fashion remains a long-standing challenge because of the lack of efficient asymmetric catalytic methods for installing these motifs. The rapid progress in the development of new-generation efficient chiral catalysts has opened the door for several asymmetric reactions, such as Michael addition, dearomative cyclization, polyene cyclization, α-arylation, cycloaddition, allylation, for the construction of quaternary carbon stereocenters in a highly enantioselective fashion. These asymmetric catalytic methods have greatly facilitated the synthesis of complex natural products with improved output and overall efficiency. In this concise review, we highlight the progress in the last six years in complex natural product synthesis, in which at least one quaternary carbon stereocenter has been constructed via asymmetric catalytic technologies, with particular emphasis on the analysis of the stereochemical model of each enantioselective transformation.

Coupling of sterically hindered trisubstituted olefins and benzocyclobutenones by C-C activation: total synthesis and structural revision of cycloinumakiol

The first total syntheses of the proposed structure of cycloinumakiol (1) and its C5 epimer (18) are achieved in a concise and efficient fashion. Starting from the known 3-hydroxybenzocyclobutenone, 1 and 18 are obtained in nine and five steps with overall yields of 15% and 33%, respectively. The key for the success of this approach is the use of a catalytic C-C activation strategy for constructing the tetracyclic core of 1 through carboacylation of a sterically hindered trisubstituted olefin with benzocyclobutenone. In addition, the structure of the natural cycloinumakiol was reassigned to 19-hydroxytotarol (7) through X-ray diffraction analysis. This work demonstrates the potential of C-C activation for streamlining complex natural product synthesis.

Synthesis of antimicrobial natural products targeting FtsZ: (+)-totarol and related totarane diterpenes

An efficient, convergent synthesis of totarol by a diastereoselective epoxide/alkene/arene bicyclization is described. The reported synthesis enables the preparation of related diterpenes totaradiol and totarolone as well as previously unavailable derivatives that exhibit comparable inhibition of the bacterial cell division protein FtsZ.

Syntheses of carnosic acid and carnosol, anti-oxidants in Rosemary, from pisiferic acid, the major constituent of Sawara

Carnosic acid (2), a major anti-oxidant in rosemary (Rosmarinus officinalis), was synthesized from pisiferic acid (1), the major constituent of Sawara (Chamaecyparis pisifera), via ortho-oxidation of the phenol using meta-chlorobenzoyl peroxide (mCBPO), chloroacetyl meta-chlorobenzoyl peroxide (CAMCBPO) or 2-iodoxybenzoic acid (IBX). Carnosol (3), another anti-oxidant in rosemary, was synthesized from carnosic acid by oxidation with silver oxide. Potent antibacterial activities against Propionibacterium acnes (ATCC 6919) (minimum inhibitory concentration (MIC) mug/ml) and Staphylococcus aureus ME/GM/TC Resistant (ATCC 33592) (MIC mug/ml) of carnosic acid and carnosol were reported.