Enantioselective Allylic Hydroxylation of ω‐Alkenoic Acids and Esters by P450 BM3 Monooxygenase

Chiral allylic alcohols of ω‐alkenoic acids and derivatives thereof are highly important building blocks for the synthesis of biologically active compounds. The direct enantioselective C? H oxidation of linear terminal olefins offers the shortest route toward these compounds, but known synthetic methods are limited and suffer from low selectivities. Described herein is an enzymatic approach using the P450 BM3 monooxygenase mutant A74G/L188Q, which catalyzes allylic hydroxylation with high to excellent chemo‐ and enantioselectivities providing the desirable secondary alcohols.     

A Bioinspired Catalytic Oxygenase Cascade to Generate Complex Oxindoles

Catalytic, selective, and controlled oxidative functionalization of C? H bonds using molecular oxygen as an oxidant remains highly desirable and equally challenging in the development of synthetic methodologies. Presented herein is a one‐pot oxygenase cascade reaction wherein a copper(I)‐catalyzed oxygenase reaction transforms the allylic methyl group in 3‐methylidene oxindoles into an aldehyde, which then undergoes an aldol–oxa‐Michael addition sequence with β‐ketoesters to yield dihydrofuran‐bearing oxindoles.     

Construction of Quaternary Stereogenic Carbon Centers through Copper‐Catalyzed Enantioselective Allylic Cross‐Coupling with Alkylboranes

A combination of an in situ generated chiral Cu^I/DTBM‐MeO‐BIPHEP catalyst system and EtOK enabled the enantioselective S_N2′‐type allylic cross‐coupling between alkylborane reagents and γ,γ‐disubstituted primary allyl chlorides with enantiocontrol at a useful level. The reaction generates a stereogenic quaternary carbon center having three sp³‐alkyl groups and a vinyl group. This protocol allowed the use of terminal alkenes as nucleophile precursors, thus representing a formal reductive allylic cross‐coupling of terminal alkenes. A reaction pathway involving addition/elimination of a neutral alkylcopper(I) species with the allyl chloride substrate is proposed.     

Repairing the Thiol‐Ene Coupling Reaction

Thiol‐ene coupling (TEC) reactions emerged as one of the most useful processes for coupling different molecular units under reaction mild conditions. However, TEC reactions involving weak C? H bonds (allylic and benzylic fragments) are difficult to run and often low yielding. Mechanistic studies demonstrate that hydrogen‐atom transfer processes at allylic and benzylic positions are responsible for the lack of efficiency of the radical‐chain process. These competing reactions cannot be prevented, but reported herein is a method to repair the chain process by running the reaction in the presence of triethylborane and catechol. Under these reaction conditions, a unique repair mechanism leads to an efficient chain reaction, which is demonstrated with a broad range of anomeric O‐allyl sugar derivatives including mono‐, di‐, and tetrasaccharides bearing various functionalities and protecting groups.     

Synthesis of Cyclobakuchiols A,B, and C by Using Conformation‐Controlled Stereoselective Reactions

Cyclohexanone with the pMeOC₆H₄ and CH₂?C(Me) substituents at the C3 and C4‐positions was prepared from (+)‐β‐pinene and converted to the allylic picolinate by a Masamune–Wittig reaction followed by reduction and esterification. Allylic substitution of this picolinate with Me₂CuMgBr ? MgBr₂ in the presence of ZnI₂ proceeded with γ regio‐ and stereoselectively to afford the quaternary carbon center on the cyclohexane ring with the CH₂?CH and Me groups in axial and equatorial positions, respectively. This product was converted to cyclobakuchiol A by demethylation and to cyclobakuchiol C by epoxidation of the CH₂?C(Me) group. For the synthesis of cyclobakuchiol B, the enantiomer of the above cyclohexanone derived from (?)‐β‐pinene was converted to the cyclohexane‐carboxylate, and the derived enolate was subjected to the reaction with CH₂?CHSOPh followed by sulfoxide elimination to afford the intermediate with the quaternary carbon center with MeOC(?O) and CH₂?CH groups in axial and equatorial positions. The MeOC(?O) group was transformed to the Me group to complete the synthesis of cyclobakuchiol B.     

Palladium‐Catalyzed Chemoselective Allylic Substitution,Suzuki–Miyaura Cross‐Coupling,and Allene Formation of Bifunctional 2‐B(pin)‐Substituted Allylic Acetate Derivatives

A formidable challenge at the forefront of organic synthesis is the control of chemoselectivity to enable the selective formation of diverse structural motifs from a readily available substrate class. Presented herein is a detailed study of chemoselectivity with palladium‐based phosphane catalysts and readily available 2‐B(pin)‐substituted allylic acetates, benzoates, and carbonates. Depending on the choice of reagents, catalysts, and reaction conditions, 2‐B(pin)‐substituted allylic acetates and derivatives can be steered into one of three reaction manifolds: allylic substitution, Suzuki–Miyaura cross‐coupling, or elimination to form allenes, all with excellent chemoselectivity. Studies on the chemoselectivity of Pd catalysts in their reactivity with boron‐bearing allylic acetate derivatives led to the development of diverse and practical reactions with potential utility in synthetic organic chemistry.     

Gold(I)‐Catalysed Direct Thioetherifications Using Allylic Alcohols: an Experimental and Computational Study

A gold(I)‐catalysed direct thioetherification reaction between allylic alcohols and thiols is presented. The reaction is generally highly regioselective (S_N2′). This dehydrative allylation procedure is very mild and atom economical, producing only water as the by‐product and avoiding any unnecessary waste/steps associated with installing a leaving or activating group on the substrate. Computational studies are presented to gain insight into the mechanism of the reaction. Calculations indicate that the regioselectivity is under equilibrium control and is ultimately dictated by the thermodynamic stability of the products.     

An Examination of the Scope and Stereochemistry of the Ireland–Claisen Rearrangement of Boron Ketene Acetals

The Ireland–Claisen rearrangement of boron ketene acetals is described. The boron ketene acetal intermediates are formed through a soft enolization that obviates the use of strong bases and the intermediacy of alkali metal enolates. Yields and diastereoselectivities of these rearrangements are very sensitive to the choice of boron reagent, even among those that have been shown to effect quantitative formation of boron ketene acetals from esters. The rearrangement occurs at room temperature for all substrates with generally high levels of stereoselectivity. In contrast to previous reports using boron triflates, the use of a commercially available boron iodide reagent allows for a wider substrate scope that extends to propionates and arylacetates, as well as the previously described α‐oxygenated esters. This work also provides insight into the dynamic nature of boron ketene acetals and the ramifications of this behavior for reactions in which they are intermediates.     

Insights into the Synthesis of Steroidal A‐Ring Olefins

The classical synthesis, followed by purification of the steroidal A‐ring Δ¹‐olefin, 5α‐androst‐1‐en‐17‐one ( 5 ), from the Δ¹‐3‐keto enone, (5α,17β)‐3‐oxo‐5‐androst‐1‐en‐17‐yl acetate ( 1 ), through a strategy involving the reaction of Δ¹‐3‐hydroxy allylic alcohol, 3β‐hydroxy‐5α‐androst‐1‐en‐17β‐yl acetate ( 2 ), with SOCl₂, was revisited in order to prepare and biologically evaluate 5 as aromatase inhibitor for breast cancer treatment. Surprisingly, the followed strategy also afforded the isomeric Δ²‐olefin 6 as a by‐product, which could only be detected on the basis of NMR analysis. Optimization of the purification and detection procedures allowed us to reach 96% purity required for biological assays of compound 5 . The same synthetic strategy was applied, using the Δ⁴‐3‐keto enone, 3‐oxoandrost‐4‐en‐17β‐yl acetate ( 8 ), as starting material, to prepare the potent aromatase inhibitor Δ⁴‐olefin, androst‐4‐en‐17‐one ( 15 ). Unexpectedly, a different aromatase inhibitor, the Δ^3,5‐diene, androst‐3,5‐dien‐17‐one ( 12 ), was formed. To overcome this drawback, another strategy was developed for the preparation of 15 from 8 . The data now presented show the unequal reactivity of the two steroidal A‐ring Δ¹‐ and Δ⁴‐3‐hydroxy allylic alcohol intermediates, 3β‐hydroxy‐5α‐androst‐1‐en‐17β‐yl acetate ( 2 ) and 3β‐hydroxyandrost‐4‐en‐17β‐yl acetate ( 9 ), towards SOCl₂, and provides a new strategy for the preparation of the aromatase inhibitor 12 . Additionally, a new pathway to prepare compound 15 was achieved, which avoids the formation of undesirable by‐products.     

Palladium(II)‐Catalyzed Allylic CH Oxidation of Hindered Substrates Featuring Tunable Selectivity Over Extent of Oxidation

The use of Oxone and a palladium(II) catalyst enables the efficient allylic C? H oxidation of sterically hindered α‐quaternary lactams which are unreactive under known conditions for similar transformations. This simple, safe, and effective system for C? H activation allows for unusual tunable selectivity between a two‐electron oxidation to the allylic acetates and a four‐electron oxidation to the corresponding enals, with the dominant product depending on the presence or absence of water. The versatile synthetic utility of both the allylic acetate and enal products accessible through this methodology is also demonstrated.