Cyclopentannelation Reaction for Rapid Assembly of Multifunctional Products. (d,l)-Desepoxymethylenomycin a Methyl Ester

Anion-accelerated asymmetric Nazarov cyclization: access to vicinal all-carbon quaternary stereocenters

We have developed a catalytic asymmetric Nazarov cyclization that results in the formation of two contiguous all-carbon quaternary stereocenters in high yield with excellent levels of asymmetric induction. This method requires no catalyst recognition elements in the starting materials that are simple diketoesters. Geometrically pure E or Z isomers of the starting material lead to diastereomerically pure products with high enantioselectivity because the species that undergoes cyclization is a rhodium enolate that is configurationally stable.

Total Syntheses of Calyciphylline A-Type Alkaloids (-)-10-Deoxydaphnipaxianine A, (+)-Daphlongamine E and (+)-Calyciphylline R via Late-Stage Divinyl Carbinol Rearrangements

Among the famous Daphniphyllum alkaloids family, the calyciphylline A-type subfamily has triggered particular interest from the organic synthesis community in recent years. Here, we report divergent total syntheses of three calyciphylline A-type alkaloids, namely, (-)-10-deoxydaphnipaxianine A, (+)-daphlongamine E, and (+)-calyciphylline R. Our work highlights an efficient, divergent strategy via late-stage divinyl carbinol rearrangements, including an unprecedented oxidative Nazarov electrocyclization using an unfunctionalized tertiary divinyl carbinol and an unusual allylic alcohol rearrangement. A highly efficient "donor-acceptor" platinum catalyst was used for a critical nitrile hydration step. Moreover, the power of selective amide reductions has also been showcased by novel and classic tactics.

Synthesis of Fluorenes and Dibenzo[g,p]chrysenes through an Oxidative Cascade

We have developed robust, operationally simple syntheses of fluorenes and of dibenzo[g,p]chrysenes through oxidative cascade processes. These structures that are commonly encountered in optoelectronic materials, dyes, and pharmaceutical products are accessible from 1,4-dioxaspiro[4.5]decan-8-one. The reactions are conducted open to air with inexpensive, safe CuBr₂ or CuCl₂.

New Twists in Nazarov Cyclization Chemistry

The defining feature of the Nazarov cyclization is a 4π-conrotatory electrocyclization, resulting in the stereospecific formation of functionalized cyclopentanones. The reaction provides access to structural motifs that are found in many natural products and drug targets. Harnessing the full potential of the Nazarov cyclization broadens its utility by enabling the development of new methodologies and synthetic strategies. To achieve these goals through efficient cyclization design, it is helpful to think of the reaction as a two-stage process. The first stage involves a 4π-electrocyclization leading to the formation of an allylic cation, and the second stage corresponds to the fate of this cationic intermediate. With a complete understanding of the discrete events that characterize the overall process, one can optimize reactivity and control the selectivity of the different Stage 2 pathways.In this Account, we describe the development of methods that render the Nazarov cyclization catalytic and chemoselective, focusing specifically on advances made in our lab between 2002 and 2015. The initial discovery made in our lab involved reactions of electronically asymmetric ("polarized") substrates, which cyclize efficiently in the catalytic regime using mild Lewis acidic reagents. These cyclizations also exhibit selective eliminative behavior, increasing their synthetic utility. Research directed toward catalytic asymmetric Nazarov cyclization led to the serendipitous discovery of a 4π-cyclization coupled to a well-behaved Wagner-Meerwein rearrangement, representing an underexplored Stage 2 process. With careful choice of promoter and loading, it is possible to access either the rearrangement or the elimination pathway. Additional experimental and computational studies provided an effective model for anticipating the migratory behavior of substiutents in the rearrangements. Problem-solving efforts prompted investigation of alternative methods for generating pentadienyl cation intermediates, including oxidation of allenol ethers and addition of nucleophiles to dienyl diketones. These Nazarov cyclization variants afford cyclopentenone products with vicinal stereogenic centers and a different arrangement of substituents around the ring. A nucleophilic addition/cyclization/elimination sequence can be executed enantioselectively using catalytic amounts of a nonracemic chiral tertiary amine.In summary, the discovery and development of several new variations on the Nazarov electrocyclization are described, along with synthetic applications. This work illustrates how strongly substitution patterns can impact the efficiency of the 4π-electrocyclization (Stage 1), allowing for mild Lewis acid catalysis. Over the course of these studies, we have also identified new ways to access the critical pentadienyl cation intermediates and demonstrated strategies that exploit and control the different cationic pathways available post-electrocyclization (Stage 2 processes). These advances in Nazarov chemistry were subsequently employed in the synthesis of natural product targets such as (±)-merrilactone A, (±)-rocaglamide, and (±)-enokipodin B.

Versatile One-Pot Synthesis of Polysubstituted Cyclopent-2-enimines from α,β-Unsaturated Amides: Imino-Nazarov Reaction

The imino-Nazarov cyclization of the polysubstituted pentan-1,4-diene-3-imines was realized. To this aim, a one-pot procedure involving reductive alkenyliminylation of α,β-unsaturated secondary amides with potassium organotrifluoroborates, followed by acid-catalyzed imino-Nazarov cyclization of the polysubstituted pentan-1,4-diene-3-imine intermediates, was studied systematically. This mild, operationally simple, flexible, and high-yielding protocol efficiently affords polysubstituted pentan-1,4-diene-3-imines, cyclopentenimines, and α-amino cyclopentenones, which are useful scaffolds in organic synthesis. The substituent effect at the C2 position of the polysubstituted pentan-1,4-diene-3-imines was studied by means of density-functional theory calculations. Results suggested that the electron-donating group facilitates the imino-Nazarov cyclization process.

Water freezing as a regiocontrol element in the multicomponent assembly of cyclic enones

Regioselective synthesis of dialkoxy 2-cyclopentenones and 2-cyclohexenones with novel substitution patterns has been accomplished by the one-pot combination of three simple starting materials (chromium carbene complex, Weinreb acetamide lithium enolate and 1-alkoxyallenyllithium) under either anhydrous conditions or water-promoted solidification of the reaction mixture. These results revealed an unprecedented water-freezing effect that plays a key role to completely reverse the regioselectivity of the intramolecular carbometalation of an allene moiety.

Diastereospecific nazarov cyclization of fully substituted dienones: generation of vicinal all-carbon-atom quaternary stereocenters

No vacancy: Fully substituted dienones that are highly polarized by a vinylogous carbonate group were found to undergo a remarkably rapid and diastereospecific Nazarov cyclization that led to cyclopentenones with vicinal all-carbon-atom quaternary centers (see example; SEM=2-(trimethylsilyl)ethoxymethyl, Tf=trifluoromethanesulfonyl).

An organocatalytic asymmetric Nazarov cyclization

An organocatalytic asymmetric Nazarov cyclization of diketoesters that proceeds by means of a dual activation mechanism has been developed. Screening of a number of catalysts led to a new thiourea that incorporates a primary amino group. The method gives rise to cyclic products with two adjacent quaternary asymmetric carbon atoms, one of which is an all-carbon stereocenter, with complete or nearly complete diastereoselectivity and in high or very high enantiomeric excess. A brief and very convenient synthesis of the acyclic diketoester starting materials through nucleophilic addition to a ketene is also described.

Nazarov cyclizations of an allenyl vinyl ketone with interception of the oxyallyl cation intermediate for the formation of carbon-carbon bonds

Treatment of an allenyl vinyl ketone with BF(3) x Et(2)O leads to a cyclic oxyallyl cation by a Nazarov reaction, and when this reaction is conducted in the presence of an acyclic diene, [4 + 3] and [3 + 2] products are obtained efficiently with high regio- and stereoselectivity. The proportion of [4 + 3] to [3 + 2] product depends on the substitution on the diene. Cyclic dienes react with the oxyallyl cation by forming only one carbon-carbon bond, but the site of bond formation can be affected by steric hindrance. Electron-rich alkenes intercept the allyl cation by forming one carbon-carbon bond, or two carbon-carbon bonds through [3 + 2] cyclization. In some instances, further treatment of the initial products with BF(3) x Et(2)O leads to equilibrated products in good yield.

Enamine-iminium ion Nazarov cyclization of alpha-ketoenones

The mono-triflate salts of some chiral nonracemic 1,2-diamines react with alpha-ketoenones in a stoichiometric reaction to form products of the Nazarov cyclization in high enantiomeric ratios. The mechanism appears to involve rearrangement of an enamine-iminium ion.

Synthesis of 5-hydroxycyclopent-2-enones from allenyl vinyl ketones via an interrupted Nazarov cyclization

Treatment of an allenyl vinyl ketone with trifluoroacetic acid leads to Nazarov cyclization, and the intermediate carbocation is trapped efficiently by trifluoroacetate. Hydrolysis of the ester with methanol and basic alumina provides, in good to excellent overall yield, a 5-hydroxycyclopent-2-enone in which the alcohol is predominantly trans to a substituent at C-4.

Traceless chiral auxiliaries for the allene ether Nazarov cyclization

The key stereochemical factors that determine transfer of asymmetry from the chiral auxiliary to the cyclopentenone in the allene ether version of the Nazarov reaction have been elucidated. On the basis of the new insights into the mechanism, two highly effective chiral auxiliaries were designed and prepared.

Interrupting the Nazarov cyclization with indoles

A carbon-terminated interrupted Nazarov reaction is described. The trimethylsilyl enol ethers derived from propargyl vinyl ketones undergo selective proton transfer at the distal acetylenic carbon atom to provide allenyl vinyl ketones as transient intermediates. In the presence of indoles, a cascade of reactions is initiated that converts the initially formed pentadienyl cations to cyclopentenones bearing a quaternary alpha carbon atom.

Polarizing the Nazarov cyclization: the impact of dienone substitution pattern on reactivity and selectivity

The impact of dienone substitution on the Nazarov cyclization has been examined in detail. Substrates bearing different substituents at each of four positions on the dienone backbone were systematically probed in order to identify trends leading to higher reactivity and better selectivity. Desymmetrization of the pentadienyl cation and oxyallyl cation intermediates through placement of polarizing groups at both the C-2 and C-4 positions was found to be particularly effective. These modifications allowed cyclizations to occur in the presence of catalytic amounts of mild Lewis acids. It was also found that stereoconvergent cyclization of mixtures of E and Z isomers of alkylidene beta-ketoesters occurred via an efficient isomerization process that occurred under the reaction conditions.

Synthesis of (+)-madindoline A and (+)-madindoline B

The allene ether version of the Nazarov cyclization was used to construct the cyclopentane dione portion of madindolines A and B. The racemic cyclopentane dione from the Nazarov cyclization was converted to an enol ether that was combined with the chiral, nonracemic hydroxyfuroindoline in a Mannich reaction. Deprotection and oxidation led to (+)-madindoline A and (+)-madindoline B. [reaction: see text].

Polarizing the Nazarov cyclization: efficient catalysis under mild conditions

Substituted divinyl ketones were studied in the Nazarov cyclization. alpha-Carbomethoxy divinyl ketones underwent efficient Nazarov cyclization with catalytic copper triflate (2 mol %) to give a single cyclopentenone regio- and stereoisomer. The efficiency of the cyclizations correlated with the ability of the substituents to favorably polarize the pi-system of the cationic intermediate.