The Ferrier carbocyclization reaction is one of the most powerful transformations of carbohydrates. This reaction provides enantiomerically pure cyclohexanone derivatives from aldohexoses, and is particularly useful in the chiral pool synthesis of cyclohexane-containing natural products from carbohydrates. We have investigated the synthesis of natural products utilizing the Ferrier carbocyclization reaction. This account provides a brief overview of the Ferrier carbocyclization and its application to natural product synthesis. The utility and versatility of the Ferrier carbocyclization reaction are showcased with the syntheses of hygromycin A, lycoricidine, actinobolin, galanthamine, and morphine starting from carbohydrates.

The development of a two-step synthesis of multi-substituted N-methoxyamines from N-methoxyamides is reported. Utilization of the N-methoxy group as a reactivity control element was the key to success in this two-step synthesis. The first reaction involves a N-methoxyamide/aldehyde coupling reaction. Whereas ordinary amides cannot condense with aldehydes intermolecularly due to the poor nucleophilicity of the amide nitrogen, the N-methoxy group enhances the nucleophilicity of the nitrogen, enabling the direct coupling reaction. The second reaction in the two-step process was nucleophilic addition to the N-methoxyamides. Incorporation of the N-methoxy group into the amides increased the electrophilicity of the amide carbonyls and promoted the chelation effect. This nucleophilic addition enabled quick diversification of the products derived from the first step. The developed strategy was applicable to a variety of substrates, resulting in the elaboration of multi-substituted piperidines and acyclic amines, as well as a substructure of a complex natural alkaloid.

Invited for the cover of this issue is the group of Prof. Noritaka Chida and Dr. Takaaki Sato at Keio University, Japan. The cover illustrates the­ N-methoxy group as the reactivity control element to allow for beautiful chemical transformations to blossom in organic synthesis. Read the full text of the article at 10.1002/chem.201402231.

As the complexity of targeted molecules increases in modern organic synthesis, chemoselectivity is recognized as an important factor in the development of new methodologies. Chemoselective nucleophilic addition to amide carbonyl centers is a challenge because classical methods require harsh reaction conditions to overcome the poor electrophilicity of the amide carbonyl group. We have successfully developed a reductive nucleophilic addition of mild nucleophiles to tertiary amides, secondary amides, and N-methoxyamides that uses the Schwartz reagent [Cp₂ZrHCl]. The reaction took place in a highly chemoselective fashion in the presence of a variety of sensitive functional groups, such as methyl esters, which conventionally require protection prior to nucleophilic addition. The reaction will be applicable to the concise synthesis of complex natural alkaloids from readily available amide groups.

A chemoselective approach for the total synthesis of (±)-gephyrotoxin has been developed. The key to success was the utilization of N-methoxyamides, which enabled the direct coupling of the amide with an aldehyde and selective reductive nucleophilic addition to the amide in the presence of a variety of sensitive and electrophilic functional groups, such as a methyl ester. This chemoselective approach minimized the use of protecting-group manipulations and redox reactions, which resulted in the most concise and efficient total synthesis of (±)-gephyrotoxin described to date.

A detailed exploration of the synthesis of (−)-morphine based on sequential [3,3]-sigmatropic rearrangements is described. The sequential Claisen/Claisen rearrangements of an allylic vicinal diol resulted in the stereoselective formation of the two contiguous carbon centers, including a sterically encumbered quaternary carbon, in a single operation. The two ethyl esters generated in this reaction were successfully differentiated during a subsequent Friedel–Crafts-type cyclization. The (−)-morphine double bond was introduced at a late stage in our first-generation synthesis, but was formed at an earlier stage in the second-generation synthesis, resulting in a more efficient route to the end product.

While the synthesis of amide bonds is now one of the most reliable organic reactions, functionalization of amide carbonyl groups has been a long-standing issue due to their high stability. As an ongoing program aimed at practical transformation of amides, we developed a direct nucleophilic addition to N-alkoxyamides to access multisubstituted amines. The reaction enabled installation of two different functional groups to amide carbonyl groups in one pot. The N-alkoxy group played important roles in this reaction. First, it removed the requirement for an extra preactivation step prior to nucleophilic addition to activate inert amide carbonyl groups. Second, the N-alkoxy group formed a five-membered chelated complex after the first nucleophilic addition, resulting in suppression of an extra addition of the first nucleophile. While diisobutylaluminum hydride (DIBAL-H) and organolithium reagents were suitable as the first nucleophile, allylation, cyanation, and vinylation were possible in the second addition including inter- and intramolecular reactions. The yields were generally high, even in the synthesis of sterically hindered α-trisubstituted amines. The reaction exhibited wide substrate scope, including acyclic amides, five- and six-membered lactams, and macrolactams.

This article describes the details of two new types of Overman rearrangement from allylic vicinal diols. Starting from identical diols, both bis(imidate)s and cyclic orthoamides were selectively synthesized by simply changing the reaction conditions. Whilst exposure of the bis(imidate)s to thermal conditions initiated the double Overman rearrangement to introduce two identical nitrogen groups in a single operation (the cascade-type Overman rearrangement), the reaction of cyclic orthoamides resulted in a single rearrangement (the orthoamide-type Overman rearrangement). The newly generated allylic alcohols from the orthoamide-type reaction can potentially undergo a variety of further transformations. For instance, we demonstrated an Overman/Claisen sequence in one pot. The most conspicuous feature of this method is that it offers precise control over the number of Overman rearrangements from the same allylic vicinal diols. This method also excludes the tedious protecting-group manipulations of the homoallylic alcohols, which are necessary in conventional Overman rearrangements. All of the performed rearrangements proceeded in a completely diastereoselective fashion through a chair-like transition state.

A detailed description is presented of two sequential sigmatropic rearrangements starting from enantiopure allylic vicinal diols. Starting from the same allylic diol, the sequential Claisen/Claisen rearrangement can install two identical functional groups in a one-pot reaction, whereas, the sequential Claisen/Overman rearrangement can introduce two different functional groups, both occurring without protecting group manipulation. Both sequential reactions proceeded with complete stereoselectivity, which was easily predictable by the judicious choice of two factors regarding the allylic diols: (1) the stereochemistry of the hydroxy groups and (2) the geometry of the olefin. To demonstrate this sequential rearrangement methodology, we accomplished the total synthesis of (–)-kainic acid, whose three contiguous stereocenters were completely established by three chirality transfer reactions (S_N2′ and sequential Claisen/Overman reactions) of flexible acyclic intermediates derived from D-arabinose.