Palladium-catalyzed DYKAT of butadiene monoepoxide: enantioselective total synthesis of (+)-DMDP, (-)-bulgecinine, and (+)-broussonetine G

Palladium catalyzed asymmetric allylic alkylation reaction of an amine with two equivalents of butadiene monoxide allows for the expedient synthesis of trans- and cis-2,5-dihydropyrroles. The versatility of these chiral synthons towards the synthesis of a wide variety of iminosugar natural products was demonstrated with the short and high yielding asymmetric syntheses of (+)-DMDP, and (-)-bulgecinine. In addition, the first total synthesis of (+)-broussonetine G, a potent glycosidase inhibitor, is described along with the assignment of its relative and absolute stereochemical configuration.     

Substrate‐Controlled Asymmetric Total Syntheses of Microcladallenes A,B, and C Based on the Proposed Structures

Substrate‐controlled asymmetric total syntheses of (+)‐microcladallenes A, B, and C have been accomplished based on the proposed structures. The syntheses of microcladallenes A and B confirmed the structures and absolute configurations of both natural products. However, the synthesis of microcladallene C, which includes seven stereogenic centers and an (R)‐bromoallene in its compact C₁₅ framework, brought the realization that its proposed structure must be revised. The introduction of C12‐bromine into these natural products with retention of configuration relied on TiBr₄‐mediated nucleophile‐assisting leaving group brominations, the stereochemical outcome of which could be attributed, at least in part, to an oxonium or halonium ion formation–fragmentation sequence through intricate neighboring group participation. In addition, the pivotal β‐oriented vicinal cis‐dichloride function in microcladallene C was elaborated through a novel tandem Cl₂‐induced electrophilic cyclization/imidate chlorination process. The positive rotations of these natural products with an (R)‐bromoallene constitute exceptions to Lowe’s rule for reasons yet to be determined.     

Catalytic Asymmetric Total Synthesis of (−)‐Galanthamine and (−)‐Lycoramine

The catalytic asymmetric total syntheses of (?)‐galanthamine ( 1 ) and (?)‐lycoramine ( 2 ) have been achieved by using a conceptually new strategy featuring two metal‐catalyzed reactions as the key steps. A new method for the construction of 3,4‐fused benzofurans has been developed through a palladium‐catalyzed intramolecular Larock annulation reaction, which was successfully applied to the construction of the ABD tricyclic skeleton of 1 and 2 . To achieve the asymmetric synthesis of 1 and 2 , a Sc^III/N,N′‐dioxide complex was used to catalyze the enantioselective conjugate addition of 3‐alkyl‐substituted benzofuranone to methyl vinyl ketone for the construction of a chiral quaternary carbon center.     

A convergent Pd-catalyzed asymmetric allylic alkylation of dl- and meso-divinylethylene carbonate: enantioselective synthesis of (+)-australine hydrochloride and formal synthesis of isoaltholactone

The use of a mixture of dl- and meso-divinylethylene carbonate as an electrophile in palladium-catalyzed asymmetric allylic alkylation reactions is reported. From the diastereomeric mixture of meso and chiral racemic starting materials, a single product is obtained in high optical purity employing either oxygen or nitrogen nucleophiles. The resulting dienes have proven to be versatile synthetic intermediates as each carbon is functionalized for further transformation and differentiated by virtue of the reaction. A mechanism for this intriguing transformation is proposed and a concise enantioselective total synthesis of (+)-australine hydrochloride is reported as well as a formal synthesis of isoaltholactone.     

A Unified Approach to the Daucane and Sphenolobane Bicyclo[5.3.0]decane Core: Enantioselective Total Syntheses of Daucene,Daucenal, Epoxydaucenal B,and 14‐para‐Anisoyloxydauc‐4,8‐diene

Access to the bicyclo[5.3.0]decane core found in the daucane and sphenolobane terpenoids via a key enone intermediate enables the enantioselective total syntheses of daucene, daucenal, epoxydaucenal B, and 14‐para‐anisoyloxydauc‐4,8‐diene. Central aspects include a catalytic asymmetric alkylation followed by a ring contraction and ring‐closing metathesis to generate the five‐ and seven‐membered rings, respectively.     

Concise Total Syntheses of Amphidinolides C and F

The marine natural products amphidinolide C ( 1 ) and F ( 4 ) differ in their side chains but share a common macrolide core with a signature 1,4‐diketone substructure. This particular motif inspired a synthesis plan predicating a late‐stage formation of this non‐consonant (“umpoled”) pattern by a platinum‐catalyzed transannular hydroalkoxylation of a cycloalkyne precursor. This key intermediate was assembled from three building blocks ( 29 , 41 and 47 (or 65 )) by Yamaguchi esterification, Stille cross‐coupling and a macrocyclization by ring‐closing alkyne metathesis (RCAM). This approach illustrates the exquisite alkynophilicity of the catalysts chosen for the RCAM and alkyne hydroalkoxylation steps, which activate triple bonds with remarkable ease but left up to five other π‐systems in the respective substrates intact. Interestingly, the inverse chemoselectivity pattern was exploited for the preparation of the tetrahydrofuran building blocks 47 and 65 carrying the different side chains of the two target macrolides. These fragments derive from a common aldehyde precursor 46 formed by an exquisitely alkene‐selective cobalt‐catalyzed oxidative cyclization of the diunsaturated alcohol 44 , which left an adjacent acetylene group untouched. The northern sector 29 was prepared by a two‐directional Marshall propargylation strategy, whereas the highly adorned acid subunit 41 derives from D ‐glutamic acid by an intramolecular oxa‐Michael addition and a proline‐mediated hydroxyacetone aldol reaction as the key steps; the necessary Me₃Sn‐group on the terminus of 41 for use in the Stille coupling was installed via enol triflate 39 , which was obtained by selective deprotonation/triflation of the ketone site of the precursor 38 without competing enolization of the ester also present in this particular substrate.     

A short and concise asymmetric synthesis of hamigeran B

The interesting biological properties of the hamigerans wherein hamigeran B is a potent antiviral agent with low cytotoxicity to host cells make these deceptively simple looking structures challenging synthetic targets. A strategy to hamigeran B evolved wherein the three contiguous stereocenters are established ultimately from a Pd catalyzed asymmetric allylic alkylation (AAA). The latter involves an asymmetric allylation of a non-stabilized ketone enolate in 77 % yield and 93 % ee. By using this process, (S)-5-allyl-2-isopropyl-5-methyl-1-trifluoromethanesulfonyloxycyclopentene becomes available in four steps from 2-methylcyclopentanone. Introduction of the aryl unit by cross-coupling proceeded intermolecularly but failed intramolecularly. On the other hand, reductive removal of the triflate permitted a Heck reaction to effect intramolecular introduction of the aryl ring. The unusual conformational properties of this molecular architecture are revealed by the regioselectivity of the beta-hydrogen elimination in the Heck reaction and the diastereoselectivity of the reduction establishing the stereochemistry of the carbon bearing the isopropyl group. The successful route consists of 15 steps from 2-methylcyclopentanone and dimethylorcinol illustrating the efficiency of the route based upon the Pd AAA.     

Total Syntheses of Sesterterpenoid Ansellones A and B,and Phorbadione

Ansellane‐type sesterterpenoids including, ansellones A‐G and (+)‐phorbadione are structurally novel marine secondary metabolites which exhibit anticancer and anti‐HIV activity. The first, asymmetric total syntheses of three structurally representative members, (−)‐ansellones A and B and (+)‐phorbadione, were accomplished in 16–23 steps from (+)‐sclareolide. The route features the first regioselective cyclization of vinyl epoxides with internal alcohol nucleophiles in a 1,4‐addition manner (S_N2′). Additionally, the allylic C−H oxidation was exploited at a late stage of the synthesis of (−)‐ansellone A and (+)‐phorbadione. This strategy is expected to be applicable to the synthesis of other ansellane sesterterpenoids.     

Enantioselective Total Synthesis of (−)‐Diversonol

For the synthesis of (?)‐diversonol (ent‐ 1 ), an enantioselective domino‐Wacker/carbonylation/methoxylation reaction and an enantioselective Wacker oxidation were used to give the chroman in high yield and 96 % and 93 % ee, respectively. Dihydroxylation at the vinyl moiety using the Sharpless procedure and a Wittig–Horner reaction followed by hydrogenation, benzylic oxidation, and an intramolecular acylation provided the tetrahydroxanthenone, from which ent‐ 1 is accessible in a few steps. Furthermore, the synthesis of the diastereomeric diversonol rac‐1,9 a‐epi‐diversonol (rac‐ 41 ) is also described.