Total synthesis of (−)-martinellic acid

An enantioselective formal total synthesis of the pyrrolo[3,2-c]quinoline natural product martinellic acid has been achieved. The key steps involve a Pd-catalyzed aryl amidation reaction of a pyrroglutamate derivative, an intramolecular [3+2] azomethine ylide-alkene cycloaddition and a reductive ring opening reaction.     

Formal total synthesis of (±)-martinellic acid

A concise formal total synthesis of the pyrrolo[3,2-c]quinoline containing natural product, martinellic acid, has been achieved from a pyrroloquinol-2-one through addition of a copper acetylide to an iminium ion. Global deprotection and alkyne reduction then provides the tricyclic triamine, which can be converted to martinelline and martinellic acid.     

Development of radical addition-cyclization-elimination reaction of oxime ether and its application to formal synthesis of (±)-martinelline

Radical addition-cyclization-elimination (RACE) reaction of oxime ether carrying unsaturated ester provides a novel method for the construction of pyrroloquinoline. Treatment of oxime ethers with Bu₃SnH and AIBN gave N-norpyrroloquinoline as a major product, which was also obtained by the radical reaction of the corresponding hydrazone and imine. The radical reaction of aldehyde and ketone carrying unsaturated ester proceeded stereoselectively to give cis-furoquinolines and cis-hydroxyesters. The RACE reactions by using each of Bu₃SnNMe₂, Bu₃SnD, and/or D₂O were also examined in order to propose a reaction pathway to N-norpyrroloquinoline. Furthermore, the synthetic utility of RACE reaction is demonstrated by preparation of a key intermediate for the synthesis of (±)-martinelline.     

Total synthesis of (-)-(alpha)-kainic acid via a diastereoselective methylenecyclopropane ring expansion

[reaction: see text] A concise and enantioselective synthesis of (-)-(alpha)-kainic acid in 13 steps with an overall yield of 15% is reported. The pyrrolidine kainoid precursor with the required C2/C3 trans stereochemistry was prepared with excellent diastereoselectivity (>20:1) via a MgI(2)-mediated ring expansion of a tertiary methylenecyclopropyl amide. A selective hydroboration was then employed to set the remaining stereochemistry at the C4 position en route to (-)-(alpha)-kainic acid.     

Total synthesis of (-)-pseudolaric acid B

We report a full account of our work toward the total synthesis of pseudolaric acid B (1a), a diterpene acid isolated from the bark of Pseudolarix kaempferi Gordon (pinaceae). Compound 1a is an antifungal and antifertility agent. Furthermore, its capacity for inhibiting tubulin polymerization makes it a potential lead for cancer therapy. Herein, we describe the use of a Ru- or Rh-catalyzed [5 + 2] intramolecular cycloaddition reaction of an alkyne and a vinylcyclopropane for the construction of the polyhydroazulene core of the molecule. Our first unsuccessful strategy for the introduction of the quaternary center based on an epoxide opening with cyanide led to the discovery of a new TBAF-mediated isomerization of a 1,4-diene to a 1,3-diene and a vinylogous eliminative opening of an epoxide to form a dienol. Our second strategy, based on the cyclization of an alkoxycarbonyl radical upon a diene system, succeeded in forming the quaternary center. Detailed studies showed the dependence of this underutilized approach for the synthesis of lactones on substrate structure and reaction conditions. In the late stage of the synthesis, the unique capacity of cerium organometallic reagents to add to a sensitive, sterically hindered ketone was demonstrated. The easy formation of an oxo-bridged derivative was the major hurdle to the completion of the synthesis and showcased the intriguing reactivity of the complex core of the pseudolaric acids.     

Total synthesis of (-)-pseudolaric acid B

We report the enantioselective synthesis of pseudolaric acid B (1a), a diterpene acid isolated from the bark of Pseudolarix kaempferi Gordon, which displays interesting antifungal, antifertility, and cytotoxic activity against multidrug resistant cell lines. Our synthesis utilizes a highly efficient metal-catalyzed [5 + 2] vinylcyclopropane-alkyne intramolecular cycloaddition to construct the polyhydroazulene core of the natural product. Elaboration to the tricyclic scaffold of the pseudolaric acids was completed with an intramolecular alkoxycarbonyl radical cyclization to form the quaternary center and a highly diastereoselective cerium acetylide addition to a methyl ketone for introduction of the acid side chain.     

Total synthesis of the marine alkaloid (+/-)-lepadiformine via a radical carboazidation

[reaction: see text] The total synthesis of lepadiformine has been achieved in 10 steps and 15% overall yield from cyclohexanone. The amino-substituted quaternary carbon center is created through a radical carboazidation reaction. The tricyclic core of lepadiformine is built via an efficient hydrogenation process, involving reduction of the azide and intramolecular reductive amination of a ketone, followed by lactamization of the intermediate gamma-aminoester. The hydroxymethyl side chain is introduced according to a modified Takahata procedure after conversion of the lactam into a thiolactam.     

Scope of the radical addition-cyclization-elimination reaction of oxime ether towards the synthesis of tricyclic lactam derivatives

The synthesis of tricyclic lactam building blocks by the radical addition-cyclization-elimination (RACE) reaction is presented. A range of oxime ethers carrying unsaturated ester part have been tested for the radical reaction. A variety of substituents were incorporated around the aromatic backbones and their effect on the RACE reaction has been examined. In addition, the power of RACE reaction is demonstrated by preparation of a key intermediate for the synthesis of constrained analogue of methoctramine.     

Total synthesis of (-)-kainic acid via intramolecular Michael addition: a second-generation route

A total synthesis of (-)-kainic acid starting from the commercially available 2-azetidinone is described. The key delta-lactone intermediate was concisely prepared from the commercially available azetidinone through the Reformatsky-type reaction and an introduction of a glycine moiety. The construction of the functionalized pyrrolidine ring was executed by a one-pot sequential elimination-Michael addition protocol of a beta-amino-delta-lactone intermediate with high diastereoselectivity.     

Total synthesis of (-)-alpha-kainic acid by (-)-sparteine-mediated asymmetric deprotonation-cycloalkylation

[reaction: see text] We report a new enantioselective synthesis of (-)-alpha-kainic acid from d-serine methyl ester hydrochloride, based on a (-)-sparteine-mediated asymmetric deprotonation of an intermediate carbamate that, by stereospecific anti S(N)'S(E)' intramolecular cycloalkylation, leads to the pyrrolidine ring precursor of (-)-alpha-kainic acid, in high yield and diastereoselectivity. The intermediate pyrrolidine was further transformed to (-)-alpha-kainic acid in three steps.     

Total synthesis of (-)-20-epiuleine via stereocontrolled one-pot asymmetric azaelectrocyclization followed by novel 1,4-addition reaction

The first asymmetric total synthesis of an indole alkaloid, (-)-20-epiuleine, containing the 2,3,4-trisubstituted piperidine core, was achieved using a stereocontrolled one-pot asymmetric 6π-azaelectrocyclization followed by a stereoselective 1,4-addition reaction of the unsaturated ester with a Grignard reagent resulting from the novel neighboring participation of the hydroxyl group in cis-aminoindanol as a chiral nitrogen source.     

Total synthesis of anthocyanidins via Heck reaction

Anthocyanidin moiety was obtained by cyclization of a 2-hydroxychalcone prepared by the Heck approach to the synthesis of flavonoids. 1-(3,4-dimethoxyphenyl)-prop-2-en-1-one has been used as a coupling partner along with o-acetoxyiodobenzene in the Heck reaction.     

Total synthesis of (+/-)-pentacycloanammoxic acid

The first total synthesis of (+/-)-pentacycloanammoxic acid/methyl ester (1), an unusual naturally occurring fatty acid from Candidatus Brocadia anammoxidans, has been accomplished by the sequence of reactions shown in Scheme 1. The C20-structure of 1 was assembled with stereocontrol from three building blocks: cyclooctatetraene, 2-cyclopentenone, and 7-bromoheptanoic acid. The synthesis of 1 confirms the proposed structure of 1. The mode of biosynthesis and absolute configuration have yet to be clarified.     

Total synthesis of (+/-)-actinophyllic acid

The first total synthesis of (+/-)-actinophyllic acid (1) is reported. Key steps of this synthesis include an intramolecular oxidative coupling of ketone and malonic ester enolates and an aza-Cope-Mannich rearrangement that assembled the core structure of the natural product's unique ring system. The synthesis was accomplished from di-tert-butyl malonate in 8% overall yield by a concise sequence that proceeds by way of only seven isolated intermediates.     

Total synthesis of (+/-)-mitorubrinic acid

[reaction: see text] (+/-)-Mitorubrinic acid, a member of the azaphilone family of natural products, has been constructed in 12 steps. Key aspects of the synthesis include elaboration and oxidative dearomatization of an isocoumarin intermediate to provide the azaphilone nucleus with a disubstituted, unsaturated carboxylic acid side chain.     

Total synthesis of (-)-aspidospermine via diastereoselective ring-closing olefin metathesis

An enantiocontrolled total synthesis of (-)-aspidospermine has been achieved. The key element of the strategy is the diastereoselective construction of the quaternary stereogenic center employing 1,4-asymmetric induction during the ring-closing olefin metathesis.     

Total synthesis of (-)-dactylolide and formal synthesis of (-)-zampanolide via target oriented beta-C-glycoside formation

The total synthesis of (-)-dactylolide and formal synthesis of (-)-zampanolide via target oriented beta-C-glycoside formation is described. The two key reactions involved a stereoselective reduction of the appropriate oxocarbenium cation and a highly chemo- and diastereoselective ring-closing metathesis protocol for the formation of the macrocyclic core. In addition to the described chemistry, in vitro screening of the antipode of natural dactylolide against the NCI's 60 cancer cell line helped to illuminate the critical importance of the N-acyl hemiaminal side chain of natural zampanolide for its reported potent nanomolar cytotoxicities. Furthermore, by means of the in vitro screen of (-)-dactylolide, a promising cancer therapeutic lead has now emerged for a variety of carcinomas. More specifically, (-)-dactylolide exhibited GI50 values in the nanomolar (25-99 ng/mL) range against the four cell lines HL-60, K-562, HCC-2998, and SF-539, while displaying modest LC50 values.     

Total synthesis of (-)-ascochlorin via a cyclobutenone-based benzannulation strategy

The application of a convergent benzannulation strategy in an efficient synthesis of (-)-ascochlorin is described.     

Total synthesis of (-)-colchicine via a Rh-triggered cycloaddition cascade

[reaction: see text] A synthesis of the antimitotic alkaloids (-)-colchicine and (-)-isocolchicine is reported. Important steps are (a) enantioselective transfer-hydrogenation of an alkynone, (b) iodine/magnesium exchange with subsequent aromatic acylation, (c) Rh-catalyzed transformation of an alpha-diazoketone into an oxatetracyclic key intermediate through intramolecular [3 + 2]-cycloaddition of an in situ generated carbonyl ylide, and (d) regioselective conversion of the cycloadduct into a tropolone derivative. The new synthetic strategy opens an efficient enantioselective access to colchicine and structural analogues.