Asymmetric Synthesis of the Main Core of Kaurane Family Members Triggered by an Oxidative Polycyclization–Pinacol Tandem Process

Lactonization with concomitant 1,2-aryl migration and alkoxylation mediated by dialkoxyphenyl iodides generated in situ

A series of alkoxylated isobenzofuranones were conveniently synthesized from the reaction of 2-(1-arylvinyl)benzoic acids with PhI(OR)2, generated in situ from the reaction of iodosobenzene (PhIO) with alkyl alcohols. This hypervalent iodine mediated one-pot transformation is postulated to undergo a cascade reaction involving lactonization, 1,2-aryl migration and alkoxylation processes. The organocatalytic and chiral organoiodine-catalyzed asymmetric reactions of the current transformation were also probed.

Hypervalent iodine reagent-mediated reactions involving rearrangement processes

Hypervalent iodine reagents have been extensively employed in various types of oxidative organic reactions including oxidative coupling/cyclization, bifunctionalization of olefins and cyclopropane, C-H functionalization, and oxidative rearrangement reactions. In this review, the developments of the exclusive hypervalent iodine-mediated reactions involving oxidative rearrangement processes, including [1,2]-migration, Hofmann rearrangement, Beckmann rearrangement, ring contraction, ring expansion, [3,3]-sigmatropic/iodonium-Claisen rearrangement and some miscellaneous rearrangements, have been summarized.

Oxetane Intermediate during a Direct Aldol Reaction: Stereoselective [5 + 1] Annulation Affording Tetralines

An oxetane intermediate during a direct aldol reaction was trapped with an internal aryl group to yield trans-tetraline products. The contribution of the oxetane intermediate was confirmed by ¹⁸O-isotope labeling experiments.

Elaboration of Functionalized Organophosphates

In this study, the rapid transformation of inexpensive phenols into polyfunctionalized cyclohexenones containing a phosphonate in one pot is described. Such systems readily obtained from simple aromatic compounds could open up a multitude of synthetic possibilities. For example, this scaffold was easily and stereoselectively transformed into the corresponding enol functionality in the same pot by the addition of sodium borohydride.

Noncanonical Cation-π Cyclizations of Alkylidene β-Ketoesters: Synthesis of Spiro-fused and Bridged Bicyclic Ring Systems

Three cation-π cyclization cascades initiated at alkylidene β-ketoesters bearing pendent alkenes are described. Depending upon the alkene substitution pattern and the reaction conditions employed, it is possible to achieve selective synthesis of the three different types of products, including 1-halo-3-carbomethoxycyclohexanes, spiro-fused tricyclic systems, and [4.3.1] bridged bicyclic ring systems. All three reactions begin with 6- endo addition of an olefin to the alkylidene β-ketoester electrophile, followed by one of three different cation capture events.

Total Synthesis of (+)-Gracilamine Based on an Oxidative Phenolic Coupling Reaction and Determination of Its Absolute Configuration

The enantioselective total synthesis of (+)-gracilamine (1) is described. The strategy features a diastereoselective phenolic coupling reaction followed by a regioselective intramolecular aza-Michael reaction to construct the ABCE ring system. The configuration at C3a in 1 was controlled by the stereocenter at C9a, which was selectively generated (91 % ee) by an organocatalytic enantioselective aza-Friedel-Crafts reaction developed by our research group. This synthesis revealed that the absolute configuration of (+)-gracilamine is 3aR, 4S, 5S, 6R, 7aS, 8R, 9aS.

Asymmetric synthesis of (+)-17-epi-methoxy-kauran-3-one through tandem oxidative polycyclization-pinacol process

A synthesis of (+)-17-epi-methoxy-kauran-3-one, an O-methylated isomer of the natural diterpene 17-hydroxy-kauran-3-one, has been achieved. The strategy is based on a diastereoselective oxidative polycyclization-pinacol tandem process consisting of transforming a functionalized phenol into a compact and complex tetracycle, which represents the main core of kaurane family members. The synthesis also includes an enantioselective Yamamoto's allylation, a diastereoselective Ru-catalyzed hydrocyanation, a ring-closing metathesis and a reductive isomerization process as key steps. The structure of our synthetic substrate was determined through comparison with an O-methylated derivative of the natural compound.

Advances in Synthetic Applications of Hypervalent Iodine Compounds

The preparation, structure, and chemistry of hypervalent iodine compounds are reviewed with emphasis on their synthetic application. Compounds of iodine possess reactivity similar to that of transition metals, but have the advantage of environmental sustainability and efficient utilization of natural resources. These compounds are widely used in organic synthesis as selective oxidants and environmentally friendly reagents. Synthetic uses of hypervalent iodine reagents in halogenation reactions, various oxidations, rearrangements, aminations, C-C bond-forming reactions, and transition metal-catalyzed reactions are summarized and discussed. Recent discovery of hypervalent catalytic systems and recyclable reagents, and the development of new enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important achievement in the field of hypervalent iodine chemistry. One of the goals of this Review is to attract the attention of the scientific community as to the benefits of using hypervalent iodine compounds as an environmentally sustainable alternative to heavy metals.

Formation of the Main Cores Present in Natural Products by Tandem Additions

A rapid route to 5,5- and 5,6- bicyclic systems is provided by an 1,3-alkyl-shift process mediated by a hypervalent iodine reagent on aromatics. The structures obtained contain several unsaturations with different behaviors and reactivities. Such diversity allows further elaborations for the rapid formation of compact systems present in a variety of natural products. The potential for further transformations has been demonstrated by performing a double Michael addition. This cyclization process is regio- and stereoselective due to the presence of a former benzylic substituent. Furthermore, an extension of this approach has been accomplished on indole derivatives.

Total synthesis of natural products using hypervalent iodine reagents

We present a review of natural product syntheses accomplished in our laboratory during the last 5 years. Each synthetic route features a phenol dearomatization promoted by an environmentally benign hypervalent iodine reagent. The dearomatizations demonstrate the "aromatic ring umpolung" concept, and involve stereoselective remodeling of the inert unsaturations of a phenol into a highly functionalized key intermediate that may contain a quaternary carbon center and a prochiral dienone system. Several new oxidative strategies were employed, including transpositions (1,3-alkyl shift and Prins-pinacol), a polycyclization, an ipso rearrangement, and direct nucleophilic additions at the phenol para position. Several alkaloids, heterocyclic compounds, and a polycyclic core have been achieved, including sceletenone (a serotonin reuptake inhibitor), acetylaspidoalbidine (an antitumor agent), fortucine (antiviral and antitumor), erysotramidine (curare-like effect), platensimycin (an antibiotic), and the main core of a kaurane diterpene (immunosuppressive agent and stimulator of apoptosis). These concise and in some cases enantioselective syntheses effectively demonstrate the importance of hypervalent iodine reagents in the total synthesis of bioactive natural products.