Direct, Mild, and General n-Bu4NBr-Catalyzed Aldehyde Allylsilylation with Allyl Chlorides

Trimethylene Methane Dianion Equivalent for the Asymmetric Consecutive Allylation of Aldehydes: Applications to Prins-Driven Macrocyclizations for the Synthesis of Bryostatin 1 and Analogues

We report a one-step (one-flask) generation and reaction of a bifunctional allylating reagent, a trimethylene methane dianion equivalent, that provides a route for the asymmetric 2-(trimethylsilylmethyl) allylation of aldehydes. The product of the first aldehyde allylation process is then set to engage in a second separate aldehyde allylation, providing an improved Prins macrocyclization strategy both for the scalable synthesis of bryostatin 1 and for the total synthesis of a new potent bryostatin analogue.

Diastereo- and Enantioselective Ruthenium-Catalyzed C-C Coupling of 1-Arylpropynes and Alcohols: Alkynes as Chiral Allylmetal Precursors in Carbonyl anti-(α-Aryl)allylation

Highly tractable 1-aryl-1-propynes, which are readily accessible via Sonogashira coupling, serve as chiral allylmetal pronucleophiles in ruthenium-JOSIPHOS-catalyzed anti-diastereo- and enantioselective aldehyde (α-aryl)allylations with primary aliphatic or benzylic alcohol proelectrophiles. This method enables convergent construction of homoallylic sec-phenethyl alcohols bearing tertiary benzylic stereocenters. Both steric and electronic features of aryl sulfonic acid additives were shown to contribute to the efficiency with which a more selective and productive iodide-bound ruthenium catalyst is formed. As corroborated by isotopic labeling studies, a dual catalytic process is operative in which alkyne-to-allene isomerization is followed by allene-carbonyl reductive coupling via hydrogen auto-transfer. Crossover of ruthenium hydrides emanating from these two discrete catalytic events is observed. The utility of this method is illustrated by conversion of selected reaction products to the corresponding phenethylamines and the first total syntheses of the neolignan natural products (-)-crataegusanoids A-D.

A Catalytic Dual Isomerization/Allylboration Sequence for the Stereoselective Construction of Congested Secondary Homoallylic Alcohols

A catalytic sequence for the diastereo- and enantioselective preparation of homoallylic alcohols with an adjacent quaternary (stereo)center is reported. The one-pot process relies on the use of a single (achiral or chiral) iridium complex to catalyze the concomitant isomerization of primary allylic alcohols and homoallylboronates into (chiral) aldehydes and allylboronates, respectively. In the same flask, a chiral Brønsted acid is added next to engage the isomerization products into a stereocontrolled allylboration reaction. Structural variations have been performed on both the allylic alcohols and the homoallylboronates. This mild process affords an array of stereochemically congested and complex chiral secondary homoallylic alcohols in high yield, excellent diastereoselectivity, and usually high enantioselectivity.

Successive Nucleophilic and Electrophilic Allylation for the Catalytic Enantioselective Synthesis of 2,4-Disubstituted Pyrrolidines

Successive nucleophilic and electrophilic allylation mediated by the bis-Boc-carbonate derived from 2-methylene-1,3-propane diol enables formation of enantiomerically enriched 2,4-disubstituted pyrrolidines. An initial enantioselective iridium-catalyzed transfer hydrogenative carbonyl C-allylation is followed by Tsuji-Trost N-allylation using 2-nitrobenzenesulfonamide. Subsequent Mitsunobu cyclization provides the N-protected 2,4-disubstituted pyrrolidines.

Enantioselective Ir-Catalyzed Bidirectional Reductive Coupling

In the presence of a chiral iridium complex, commercially available 3-chloro-2-chloromethyl-1-propene (1) was selectively activated for various reductive couplings. Depending on the reaction conditions it allows a selective mono- or bidirectional condensation with one or two external aldehydes with excellent enantiocontrol (>90% ee). This approach occurring simply under mild conditions and avoiding premetalated reagents constructs rapidly chiral homoallylic alcohols, key precursors of important molecular fragments such as furans, pyrans, ketodiols, or 1,3,5-polyols.

Design and 22-step synthesis of highly potent D-ring modified and linker-equipped analogs of spongistatin 1

Spongistatin 1 is among the most potent anti-proliferative agents ever discovered rendering it an attractive candidate for development as a payload for antibody-drug conjugates and other targeted delivery approaches. Unfortunately, it is unavailable from natural sources and its size and complex stereostructure render chemical synthesis highly time- and resource-intensive. As a result, the design and synthesis of more acid-stable and linker functional group-equipped analogs that retain the low picomolar potency of the parent natural product requires more efficient and step-economical synthetic access. Using uniquely enabling direct complex fragment coupling crotyl- and alkallylsilylation reactions, we report a 22-step synthesis of a rationally designed D-ring modified analog of spongistatin 1 that is characterized by GI50 values in the low picomolar range, and a proof-of-concept result that the C(15) acetate may be replaced with linker functional group-bearing esters with only minimal reductions in potency.

Selection between Diastereomeric Kinetic vs Thermodynamic Carbonyl Binding Modes Enables Enantioselective Iridium-Catalyzed anti-(α-Aryl)allylation of Aqueous Fluoral Hydrate and Difluoroacetaldehyde Ethyl Hemiacetal

Enantioselectivity increases with increasing carbonyl electrophilicity in 2-propanol-mediated reductive couplings of aldehydes with branched aryl-substituted allylic acetates to form products of carbonyl anti-(α-aryl)allylation. This unusual phenomenon is caused by aldehyde coordination to diastereomeric kinetic vs thermodynamic carbonyl binding sites that deliver enantiomeric products. Exploiting this effect, anti-diastereo- and enantioselective (α-aryl)allylations of fluoral hydrate and difluoroacetaldehyde ethyl hemiacetal were developed.