New General Method for Regio- and Stereoselective Allylic Substitution with Aryl and Alkenyl Coppers Derived from Grignard Reagents

Coupling Reactions on Secondary Allylic, Propargylic, and Alkyl Carbons Using Organoborates/Ni and RMgX/Cu Reagents

In the first part of this review, secondary carbon-carbon bond formation by using allylic coupling reactions with aryl and alkenyl borates is presented. Early investigations have revealed the suitability of a nickel catalyst and [RTB(OMe)3]Li (RT: transferable group). Due to their low reactivity, the borates were converted to more reactive congeners possessing an alkanediol ligand, such as 2,3-butanediol and 2,2-dimethyl-1,3-propanediol. Borates with such diol ligands were used to install aryl and alkenyl groups on the monoacetate of 4-cyclopentenyl-1,3-diol. Furthermore, alkenyl borates showed sufficient reactivity toward less reactive allylic alcohol derivatives with bromine atoms at the cis position, producing dieneyl alcohols. In the second part, copper-based and/or copper-catalyzed substitutions of secondary allylic picolinates, propargylic phosphonates, and alkyl (2-pyridine)sulfonates with RMgX are briefly summarized. The application of these reactions to the synthesis of biologically active compounds is also discussed.

Studies directed toward the synthesis of hedycoropyrans: total synthesis of des-hydroxy (-)-hedycoropyran B (ent-rhoiptelol B)

A full account of our efforts directed towards the synthesis of the diarylheptanoid-derived natural products hedycoropyrans that led to the total synthesis of ent-rhoiptelol B is described. In this endeavor, we have attempted two distinct synthetic strategies to access hedycoropyrans A and B, which led us to establish a facile synthetic route for des-hydroxy (-)-hedycoropyran B (ent-rhoiptelol B) from simple and readily accessible building blocks of 4-allylanisole and vanillin, employing Sharpless asymmetric epoxidation, CBS reduction, and an intramolecular AgOTf-catalyzed oxa-Michael reaction of suitably functionalized hydroxy-ynone as key transformations. The investigations disclosed herein will provide insights into designing novel synthetic routes for THP-DAH-derived natural products.

Synthesis of a TNF inhibitor, flurbiprofen and an i-Pr analogue in enantioenriched forms by copper-catalyzed propargylic substitution with Grignard reagents

The copper-catalyzed substitution reaction of diethyl phosphate derived from TMSCCCH(OH)CH₂CH₂OTBDPS with 3-c-C₅H₉-4-MeOC₆H₃MgBr, followed by several transformations, afforded a tumor necrosis factor inhibitor possessing a Ph-acetylene moiety. The inhibitor was also synthesized from phenylacetylene phosphate PhCCCH(OP(O)(OEt)₂)CH₂CH₂OTBDPS. Furthermore, the substitution of phosphates derived from TMSCCCH(OH)CH₃ and TMSCCCH(OH)-i-Pr with 3-F-4-PhC₆H₃MgBr gave the corresponding substitution products, which were transformed to flurbiprofen and its i-Pr analogue, respectively. The copper-catalyzed substitutions in these syntheses proceeded in a regio- and stereoselective manner.

α- and γ-Regiocontrol and Enantiospecificity in the Copper-Catalyzed Substitution Reaction of Propargylic Phosphates with Grignard Reagents

The regioselectivity (r.s.) and enantiospecificity (e.s.) of the substitution reactions of secondary propargylic alcohol derivatives using reagents derived from ArMgBr and Cu salts were studied. First, the picolinate, 3-methylpicolinate, and diethylphosphonate derivatives of Ph(CH₂ )₂ CH(OH)C≡CTMS were reacted with PhMgBr/CuCN in ratios of 2.5:2.7-2.5:0.25. The use of 2.5:0.25 ratio in THF/DME (6:1) at 0 °C for 1 h afforded the α-substitution product from the phosphate with ≥98 % r.s. and 99 % e.s. CuBr⋅Me₂ S gave similar selectivity. The reaction system was then applied to phosphates derived from R¹ CH(OH)C≡CR² and ArMgBr to obtain synthetically sufficient r.s. and e.s. values with R² =TMS, Ph, whereas iPr was borderline in terms of size as an R¹ substituent. The presence of a substituent at the o-position of Ar marginally affected the selectivity. We also found that the use of PhMgBr/Cu(acac)₂ in a 2:1 ratio in THF produced the γ-substitution products (allenes) with high r.s. and e.s.

Organocatalytic Michael Addition to (D)-Mannitol-Derived Enantiopure Nitroalkenes: A Valuable Strategy for the Synthesis of Densely Functionalized Chiral Molecules

Carbohydrates are abundant renewable resources and are a feedstock for green chemistry and sustainable synthesis of the future. Among the hexoses and the pentoses present in biomass, mannitol was selected in the present project as a valuable platform, directly available from the chiral pool, to build highly functionalized molecules. Starting from (R)-2,3-O-cyclohexylidene glyceraldehyde, which is easily prepared in a large scale from D-mannitol, an enantiopure chiral nitro alkene was prepared by reaction with nitromethane, and its reactivity studied. Organocatalytic Michael addition of dimethyl malonate, β-keto esters, and other nucleophiles on the nitro alkene afforded high stereoselectivity and densely functionalized chiral molecules, which were further synthetically developed, leading to five-membered lactones and bicyclic lactams. Preliminary studies showed that the metal-free catalytic reaction on the chiral nitro alkene can be performed under continuous flow conditions, thus enabling the use of (micro)mesofluidic systems for the preparation of enantiomerically pure organic molecules from the chiral pool.

Total Synthesis of Callyspongiolide: An Anticancer Marine Natural Product

The stereoselective total synthesis of cytotoxic marine macrolide callyspongiolide has been reported. The 14-membered macrolactone ring along with Z-olefin in the molecule was constructed via an intramolecular Horner-Wadsworth-Emmons olefination in a Z-selective fashion. The other E-olefinic moiety as well as the C9 stereocenter was introduced via stereoselective addition of the methyl group in an S_N2' fashion. The C5 stereocenter was installed via Sakurai allylation, whereas the C7 center was fixed by Jacobsen hydrolytic kinetic resolution. The C12 methyl and C13 hydroxy centers were fixed via Macmillan coupling reaction. The macrolactone core with a vinyl iodide side chain was coupled with the known alkyne fragment to complete the synthesis.

Stereoselective Synthesis of Fused Vinylcyclopropanes by Intramolecular Tsuji-Trost Cascade Cyclization

A stereoselective intramolecular Tsuji–Trost cascade cyclization of (homo)allylic vicinal diacetates with a pendant β-ketoamide or related carbon nucleophile to give γ-lactam-fused vinylcyclopropanes is reported. In addition to two new rings, the products contain three new C–C stereocenters (two of which are quaternary) with a 9:1 dr. Moreover, the reaction proceeds in >94% enantiospecificity with optically enriched starting materials, using an inexpensive carbohydrate as the source of chirality.

Ir-Catalyzed reactions in natural product synthesis

This review highlights the recent applications of Ir-catalyzed reactions in the total synthesis of natural products.

Stereoselective Total Synthesis of Macrophage-Produced Prohealing 14,21-Dihydroxy Docosahexaenoic Acids

Synthesis of 14S,21R- and 14S,21S-dihydroxy-DHA (diHDHA) among the four possible stereoisomers of 14,21-diHDHA was studied. Methyl (R)-lactate (>97% ee), selected as a C20-C22 fragment (DHA numbering), was converted to the C17-C22 phosphonium salt, which was subjected to a Wittig reaction with racemic C16-aldehyde of the C12-C16 part with the TMS and TBS-oxy groups at C12 and C14, yielding the C12-C22 derivative with 14R/S and 21R chirality. Kinetic resolution using Sharpless asymmetric epoxidation of the TBS-deprotected allylic alcohol with l-(+)-DIPT/Ti(O-i-Pr)₄ afforded 14S-epoxy alcohol and 14R-allylic alcohol with >99% diastereomeric excess (de) for both. The CN group was introduced to the epoxy alcohol by reaction with Et₂AlCN. The 14R-allylic alcohol was also converted to the nitrile via Mitsunobu inversion. Reduction of the nitrile with DIBAL afforded the key aldehyde corresponding to the C11-C22 moiety. The Wittig reaction of this aldehyde with a phosphonium salt of the remaining C1-C10 part followed by functional group manipulation gave 14S,21R-diHDHA. Similarly, ethyl (S)-lactate (>99% ee) was converted to 14S,21S-diHDHA. The chiral LC-UV-MS/MS analysis demonstrated that each of these two 14,21-diHDHAs synthesized using the presented total organic synthesis was highly stereoselective and identical to the macrophage-produced counterpart.

Synthetic studies of callyspongiolide: synthesis of the macrolactone core of the molecule

Synthesis of the fully functionalized macrolactone core of the highly cytotoxic marine natural product callyspongiolide has been achievedviaaZ-selective intramolecular H–W–E reaction and allylic alkylation of an activatedZ-allylic alcoholviaan S_N2′ fashion as key steps.

Copper-free arylation of 3,3-disubstituted allylic halides with triazene-softened aryl Grignard reagents

A copper-free allylic arylation reaction between 3,3-disubstituted allylic halides and triazene-softened aryl Grignard reagents has been developed. This protocol presents a direct and efficient way to construct both α- or γ-isomers with high regioselectivity under environmentally benign conditions. Various functional groups can be tolerated in the reaction and the products are of high value for multiple synthetic applications. The α- and γ-isomers can be converted to the corresponding 3H-indole and indole derivatives in multigram scale respectively.

Synthesis of prostaglandin analogues, latanoprost and bimatoprost, using organocatalysis via a key bicyclic enal intermediate

Two antiglaucoma drugs, bimatoprost and latanoprost, which are analogues of the prostaglandin, PGF2α, have been synthesized in just 7 and 8 steps, respectively. The syntheses employ an organocatalytic aldol reaction that converts succinaldehyde into a key bicyclic enal intermediate, which is primed for attachment of the required lower and upper side chains. By utilizing the crystalline lactone, the drug molecules were prepared in >99% ee.

Reversing the stereoselectivity of a palladium-catalyzed O-glycosylation through an inner-sphere or outer-sphere pathway

An efficient and concise method for the construction of various O-glycosidic bonds by a palladium-catalyzed reaction with a 3-O-picoloyl glucal has been developed. The stereochemistry of the anomeric center derives from either an inner-sphere or outer-sphere pathway. Harder nucleophiles, such as aliphatic alcohols and sodium phenoxides give β-products, and α products result from using softer nucleophiles, such as phenol.

Synthesis of trans-2,6-disubstituted cyclohexanones through allylic substitution

trans-2,6-Disubstituted cyclohexanones were synthesized with high regio- and stereoselectivity by allylic substitution followed by ozonolysis. Both alkyl and aryl groups were successfully installed to the cyclohexane ring. The stereochemistry of the SN2' products was determined to be controlled by the pre-existing chirality on the ring. The present method is highlighted by the synthesis of enantiomerically enriched cyclohexanones.

Allylic substitution for construction of a chiral quaternary carbon possessing an aryl group

Phenylcopper reagents derived from 2:1 PhMgBr/Cu(acac)2 and 3:1:1 PhMgBr/Cu(acac)2/ZnI2 were found to be highly reactive and regioselective in the allylic substitution of γ,γ-disubstituted secondary allylic picolinates designed for construction of a quaternary carbon, whereas the previous 2:1 ArMgBr/CuBr·Me2S reagent and that with ZnX2 were unsuccessful. The generality of the ArMgBr/Cu(acac)2 reagent was examined with enantiomerically enriched allylic picolinates, which furnished quaternary carbons with high efficiency in >92% regioselectivity and >97% chirality transfer. Two cyclohexanes with a quaternary carbon were synthesized by using these reagents.

Total synthesis of the marine alkaloid halichlorine: development and use of a general route to chiral piperidines

The total synthesis of the marine alkaloid halichlorine is described, based on an approach that involves constructing the fully substituted asymmetric center at an early stage. The five-membered ring is formed by 5-exo-trig radical cyclization and the unsaturated six-membered ring by a process that formally represents a sequential combination of conjugate addition and S(N)2' displacement-a method that is general for making bicyclic compounds with nitrogen at a ring fusion position. A formal synthesis of (+)-halichlorine is also reported, based on the development of a general method for preparing optically pure piperidines. The key step of this method, which was used to make one of our intermediates, is the Claisen rearrangement of a 4-vinyloxy-3,4-dihydro-2H-pyridine-1-carboxylic acid benzyl ester. Such O-vinyl compounds are easily generated in situ from the corresponding alcohols, which are themselves readily assembled from serine and terminal acetylenes.