An alkoxide-directed intermolecular [2+2+1] annulation: a three-component coupling reaction for the synthesis of tetrasubstituted alpha,beta-unsaturated gamma-lactams

Titanium isopropoxide-mediated cis-selective synthesis of 3,4-substituted butyrolactones from CO2

We report a Ti(OiPr)₄-mediated multicomponent reaction, which produces 3,4-substituted cis-δ-lactones from alkyl magnesium chloride, benzaldehyde and CO₂. The key intermediate, titanacyclopropane, is formed in situ from Ti(OiPr)₄ and a Grignard reagent, which enables 1,2-dinucleophilic reactivity that is used to insert carbon dioxide and an aldehyde. An alternative reaction route is also described where a primary alkene is used to create the titanacyclopropane. A computational analysis of the elementary steps shows that the carbon dioxide and the aldehyde insertion proceeds through an inner-sphere mechanism. A variety of cis-butyrolactones can be synthesized with up to 7 : 1 diastereoselectivity and 77% yield.

One-Pot Synthesis of α,β-Unsaturated γ-Lactones and Lactams via a Sequential trans-Hydroalumination and Catalytic Carboxylation of Propargyl Alcohols and Amines with Carbon Dioxide

A one-pot, sequential process that combines a trans-selective hydroalumination of propargyl alcohols and amines with a copper- or silver-catalyzed carboxylation reaction using carbon dioxide, followed by an acid-mediated intramolecular condensation step, led to the formation of a wide range of α,β-unsaturated γ-butyrolactones and lactams.

Dative Directing Group Effects in Ti-Catalyzed [2+2+1] Pyrrole Synthesis: Chemo- and Regioselective Alkyne Heterocoupling

Transient dative substrate-Ti interactions have been found to play a key role in controlling the regioselectivity of alkyne insertion and [2+2] cycloaddition in Ti-catalyzed [2+2+1] pyrrole synthesis and Ti-catalyzed alkyne hydroamination. TMS-protected alkynes with pendent Lewis basic groups can invert the regioselectivity of TMS-protected alkyne insertion, leading to the selective formation of highly substituted 3-TMS pyrroles. The competency of various potential directing groups was investigated, and it was found that the directing-group effect can be tuned by modifying the catalyst Lewis acidity, the directing-group basicity, or the directing-group tether length. Dative directing-group effects are unexplored with Ti catalysts, and this study demonstrates the potential power of dative substrate-Ti interactions in tuning selectivity.

Recent advances in the synthesis of analogues of phytohormones strigolactones with ring-closing metathesis as a key step

In this paper, we synthesized and evaluated the biological activity of structural analogues of natural strigolactones in which the butenolide D-ring has been replaced with a γ-lactam. The key step to obtain the α,β-unsaturated-γ-lactam was an RCM on suitably substituted amides. Strigolactones (SLs) are plant hormones with various developmental functions. As soil signaling chemicals, they are required for establishing beneficial mycorrhizal plant/fungus symbiosis. Beside these auxinic roles, recently SLs have been successfully investigated as antitumoral agents. Peculiar to the SL perception system is the enzymatic activity of the hormone receptor. SARs data have shown that the presence of the butenolide D-ring is crucial to retain the biological activity. The substitution of the butenolide with a lactam might shed light on the mechanism of perception. In the following, a dedicated in silico study suggested the binding modes of the synthesized compounds to the receptor of SLs in plants.

Cyclopropenes in Metallacycle-Mediated Cross-Coupling with Alkynes: Convergent Synthesis of Highly Substituted Vinylcyclopropanes

Stereodivergent metallacycle-mediated cross-coupling reactions are described for the synthesis of densely functionalized vinylcyclopropanes from the union of alkynes with cyclopropenes. Strategies explored include hydroxyl-directed and nondirected processes, with the latter of these delivering vinylcyclopropanes with exquisite levels of regio- and stereoselectivity. Challenges inherent to these coupling reactions include diastereoselectivity (with respect to the cyclopropene) and regioselectivity (with respect to both coupling partners).

Enantioselective synthesis of β-substituted chiral allylic amines via Rh-catalyzed asymmetric hydrogenation

Rh–DuanPhos complex-catalyzed asymmetric hydrogenation of α-alkenyl substituted enamides has been developed, which provides a readily accessible approach to chiral allylic amines with excellent enantioselectivities and high regioselectivities.

Reaction design, discovery, and development as a foundation to function-oriented synthesis

Convergent C-C bond-forming reactions define the fabric of organic synthesis and, when applied in complex molecule synthesis, can have a profound impact on efficiency by decreasing the longest linear sequence of transformations required to convert simple starting materials to complex targets. Despite their well-appreciated strategic significance, campaigns in natural product synthesis typically embrace only a small suite of reactivity to achieve such bond construction (i.e., nucleophilic addition to polarized π-bonds, nucleophilic substitution, cycloaddition, and metal-catalyzed "cross-coupling"), therefore limiting the sites at which convergent coupling chemistry can be strategically employed. In our opinion, it is far too often that triumphs in the field are defined by chemical sequences that do not address the challenges associated with discovery, development, and production of natural product-inspired agents. We speculated that advancing an area of chemical reactivity not represented in the few well-established strategies for convergent C-C bond formation may lead to powerful new retrosynthetic relationships that could simplify approaches to the syntheses of a variety of different classes of natural products. Our studies ultimately embraced the pursuit of strategies to control the course of metallacycle-mediated "cross-coupling" between substrates containing sites of simple π-unsaturation (ubiquitous functionality in organic chemistry including alkenes, alkynes, allenes, aldehydes, and imines, among others). In just eight years since our initial publication in this area, we have defined over 20 stereoselective intermolecular C-C bond-forming reactions that provide access to structural motifs of relevance for the synthesis of polyketides, fatty acids, alkaloids, and terpenes, while doing so in a direct and stereoselective fashion. These achievements continue to serve as the foundation of my group's activity in natural product and function-oriented synthesis, where our achievements in reaction development are challenged in the context of complex targets. Among our early efforts, we achieved the most concise synthesis of a benzoquinone ansamycin ever described (macbecin I), and moved beyond this achievement to explore the role of our chemistry in function-oriented synthesis targeting the discovery of natural product-inspired Hsp90 inhibitors. These later efforts have led to the discovery of a uniquely selective benzoquinone ansamycin-inspired Hsp90 inhibitor that lacks the problematic quinone present in the natural series. This achievement was made possible by a concise chemical synthesis pathway that had at its core the application of metallacycle-mediated cross-coupling chemistry.

Titanium-mediated reductive cross-coupling reactions of imines with terminal alkynes: An efficient route for the synthesis of stereodefined allylic amines

Low-valency titanium species, generated in situ by using Ti(OiPr)₄/2 c-C₅H₉MgCl reagent, react with imines to give a titanium-imine complex that can couple with terminal alkynes to provide azatitanacyclopentenes with excellent regioselectivity. Stereodefined allylic amines are obtained in good yields after hydrolysis or iodonolysis of the corresponding azatitanacyclopentenes. When ethynylcyclopropane is used as the coupling partner to react with imines in this reaction, the initially generated allylic amine undergoes an unexpected 1,3-amino migration on silica gel during the column chromatography.

The regio- and stereochemical course of reductive cross-coupling reactions between 1,3-disubstituted allenes and vinylsilanes: Synthesis of (Z)-dienes

In investigations aimed at exploring the potential of disubstituted allenes in stereoselective synthesis, we report studies that explore the reductive cross-coupling reaction of vinylsilanes with a range of substituted allenes. Regiochemical control is attained by employing allenic alkoxides, where the proximal heteroatom dictates the site-selectivity in a process that proceeds by net formal metallo-[3,3] rearrangement (directed carbometalation/elimination). Stereoselectivity in these reactions is complex, with both the nature of allene substitution and relative stereochemistry of the substrate impacting the stereoselective generation of each alkene of a substituted 1,3-diene. 2009 Elsevier Ltd. All rights reserved.

Regioselective Reductive Cross-Coupling Reactions of Unsymmetrical Alkynes

The present microreview summarizes our progress over the last few years in defining regioselective reductive cross-coupling reactions of unsymmetrical alkynes with terminal- and internal alkynes, aldehydes, and imines. We begin with a brief historical perspective of metal-mediated reductive dimerization reactions of aromatic alkynes and discuss the challenges associated with "crossed" versions of this mode of reactivity. Next, a collection of available methods that allow for regioselective reductive cross-coupling of internal alkynes with terminal and internal alkynes, aldehydes, and imines is summarized. After an examination of the requirements for regioselectivity in these cases, the logic behind our design of alkoxide-directed titanium-mediated reductive cross-coupling reactions is presented. A nomenclature is introduced to delineate the presumed mechanistic origin of regioselection associated with each reaction design, and a presentation of alkoxide-directed regioselective reductive cross-coupling reactions of alkynes follows. Throughout, principal issues related to reactivity and selectivity are discussed to assess scope and limitations of available methods and to describe the broad challenges that exist for defining complex fragment union reactions based on reductive cross-coupling chemistry.

Convergent synthesis of piperidines by the union of conjugated alkynes with imines: a unique regioselective bond construction for heterocycle synthesis

A two-step process is described for the union of aromatic imines, conjugated alkynes, and aldehydes that results in a stereoselective synthesis of highly substituted piperidines. This synthetic process has been made possible by defining a unique regioselective functionalization of conjugated alkynes that establishes a suitably functionalized substrate for subsequent heterocycle-forming cationic annulation. Given the flexibility of the coupling process, heterocycles can be accessed through a process that establishes up to four stereogenic centers and four fused rings.

Aliphatic imines in titanium-mediated reductive cross-coupling: unique reactivity of Ti(O-i-Pr)4/n-BuLi

A procedure for the coupling of aliphatic imines with allylic and allenic alkoxides is described. The success of these studies was enabled by a unique reactivity profile of Ti(IV) isopropoxide/n-BuLi compared to well-known Ti(IV) isopropoxide/RMgX systems.

Stereoselective synthesis of trisubstituted (E,E)-1,3-dienes by the site-selective reductive cross-coupling of internal alkynes with terminal alkynes: a fragment coupling reaction for natural product synthesis

A highly selective convergent coupling reaction is described between alkynes for the synthesis of stereodefined trisubstituted (E,E)-1,3-dienes-structural motifs commonly found embedded in the skeletons of bioactive polyketide-derived natural products. While numerous multistep processes for the synthesis of this stereodefined functional group exist, the current method represents a significant advance as it does not require stereodefined olefinic coupling partners (vinyl halide or vinyl organometallic); it proceeds by a single convergent C-C bond-forming event (avoiding multistep methods based on carbonyl olefination) and is tolerant of a diverse array of functional groups including free hydroxyls. Through a systematic study of titanium-mediated reductive cross-coupling reactions of internal alkynes with terminal alkynes, a fragment coupling reaction of great utility in natural product synthesis has emerged. Here, use of a proximal hydroxy group to control regioselection in the functionalization of a preformed titanacyclopropene has led to the establishment of a highly selective bimolecular coupling process, where C-C bond formation occurs in concert with the establishment of two stereodefined alkenes. Compared to the body of literature known for related metal-mediated coupling reactions, the current work defines a powerful advance, achieving site-selective bimolecular C-C bond formation without the need for using TMS-alkynes or conjugated alkynes. Overall, complex 1,3-dienes relevant for the synthesis of polyketide-derived natural products of varying stereochemistry were prepared with typically >or=20:1 selectivity, defining the important role of an alkoxide directing group located delta to preformed titanacyclopropenes.

Convergent synthesis of stereodefined exo-alkylidene-gamma-lactams from beta-halo allylic alcohols

A convergent process for the assembly of stereodefined mono- and bicyclic exo-alkylidene-gamma-lactams is described that proceeds through the union of beta-halo allylic alcohols, aromatic imines, and CO. Overall, regio- and stereoselective Ti-mediated reductive cross-coupling, followed by Pd-catalyzed carbonylation, can be performed in a one- or two-pot procedure, defining a highly selective three-component coupling process for heterocycle synthesis.

Complex allylation by the direct cross-coupling of imines with unactivated allylic alcohols

Regioselective, stereoselective: The convergent coupling of allylic alcohols with imines to deliver stereodefined homoallylic amines is described (see scheme). The process proceeds with net allylic transposition without the intermediacy of allylic organometallic reagents. Two stereodefined centers and a geometrically defined di- or trisubstituted alkene are forged with high selectivity.

Allene-alkyne cross-coupling for stereoselective synthesis of substituted 1,4-dienes and cross-conjugated trienes

Titanium-mediated cross-coupling of allenic alcohols with alkynes has been investigated. Divergent reaction pathways were discovered that provide either stereodefined 1,4-dienes or substituted cross-conjugated trienes. In short, allene substitution plays a critical role in the determination of reaction pathway.

Regio- and Stereoselective Direct Cross-Coupling of Aromatic Imines with Allenic Alcohols

We describe a titanium-mediated reaction for the convergent coupling of allenic alcohols with aromatic imines. Overall, the bond formation occurs at the central carbon of the allene, proceeds with net allylic transposition, and provides substituted 1,3-dienes bearing allylic amine functionality in a regio- and stereoselective fashion.