The catalytic asymmetric total synthesis of elatol

Described in this report is the first total synthesis of elatol, a halogenated sesquiterpene in the chamigrene natural product family. The key disconnections in our synthetic approach include an enantioselective decarboxylative allylation to form the all-carbon quaternary stereocenter and a ring-closing olefin metathesis to concomitantly form the spirocyclic core as well as the fully substituted chlorinated olefin. This strategy represents a general platform for accessing the chamigrene natural product family, as demonstrated by the synthesis of (+)-laurencenone B as an intermediate in our route.     

Enantioselective synthesis of 10-epi-anamarine via an iterative dihydroxylation sequence

[reaction: see text] The enantioselective syntheses of 10-epi-anamarine and 5,10-epi,epi-anamarine have been achieved in 13 to 14 steps. The route relies upon an enantio- and regioselective Sharpless dihydroxylation of either dienoates or trienoates to establish the C-8 to C-11 stereochemistry. A diastereoselective Leighton allylation established the desired C-5 stereochemistry. The route also relies upon a ring-closing metathesis to establish the alpha,beta-unsaturated lactones.     

Concise asymmetric total synthesis of obolactone

The first efficient asymmetric synthesis of obolactone 1 has been accomplished in 11 steps and with a 15% overall yield in which Brown's enantioselective allylation reactions and ring-closing metathesis reaction are key steps.     

A convergent synthesis of the right-hand fragment of ciguatoxin CTX3C

A convergent synthesis of the right-hand fragment of ciguatoxin CTX3C was investigated. The first and second generation stereocontrolled syntheses of the LM ring fragment were achieved via spiroacetalization as a key step, respectively. The polyether framework of the HIJKLM ring fragment was constructed in a convergent manner by using intramolecular allylation, ring-closing metathesis, and stereoselective hydrogenation to form the 36-methyl substituent as the key transformations.     

Enantioselective formal total synthesis of roseophilin

[formula: see text] An enantioselective formal total synthesis of roseophilin 3 is presented. The 13-membered ring of macrotricycle 1 was formed via an efficient ring-closing metathesis reaction of bicycle 4. A palladium-catalyzed methoxycarbonylation reaction of enol triflate 5 was utilized to functionalize the right-hand ring of bicycle 2. The allyl substituent was introduced by a radical allylation of alpha-bromoketone 17.     

Asymmetric synthesis of cyclic β-hydroxyallylsilanes via sequential allyltitanation-ring closing metathesis

Highly enantioenriched cyclic β-hydroxyallylsilanes have been prepared via enantioselective allylation of unsaturated aldehydes using a chiral allyltitanium reagent, followed by a ring-closing metathesis. Functionalized rings of various sizes have been synthesized and the electronic effect of the silicon group in the RCM reaction has been studied. The resulting cyclic β-hydroxyallylsilanes reacted stereoselectively with a variety of electrophilic reagents. A first application of this method to the synthesis of a highly functionalized dihydropyrane is reported.     

Concise synthesis of the bacterial DNA primase inhibitor (+) -Sch 642305

[structure: see text]. A highly convergent, enantioselective synthesis of (+) -Sch 642305 is presented, which features a Mukaiyama-Michael addition followed by allylation to establish the syn-anti relationship of the three contiguous stereocenters. The 10-membered macrolactone was formed through ring-closing metathesis.     

Concise total syntheses of heliannuols B and D

Concise and efficient enantioselective total syntheses of heliannuols B and D have been accomplished using chirality transfer through a Lewis acid-promoted Claisen rearrangement for the construction of the C7 tertiary stereogenic center and a relay ring-closing metathesis for assembling the dihydrobenzo[b]oxepine backbone of the natural products as the key steps.     

Organocatalytic α-amination-allylation-RCM strategy: enantioselective synthesis of cyclic hydrazines

A highly enantioselective method for the synthesis of cyclic hydrazines by using organocatalytic α-amination-allylation-RCM strategy is described. Proline-catalyzed α-amination of aldehydes followed by indium-mediated one-pot allylation of the crude α-hydrazino aldehydes produces 1,2-aminoalcohols in high enantio- and diastereoselectivities. The 1,2-aminoalcohols are further converted into cyclic hydrazines by using ring-closing metathesis (RCM) reaction.     

An intramolecular Diels-Alder strategy for the asbestinins: enantioselective total syntheses of 11-acetoxy-4-deoxyasbestinin D and asbestinin-12

The enantioselective total syntheses of 11-acetoxy-4-deoxyasbestinin D and asbestinin-12 have been completed. A glycolate aldol reaction provided a diene useful for ring-closing metathesis to form an oxonene, which was ultimately employed as a template to execute a highly stereoselective intramolecular Diels-Alder cycloaddition, forming the hydroisobenzofuran moiety. The absolute configuration of the asbestinin subclass was confirmed via these synthetic efforts.     

Asymmetric synthesis of (-)-adaline

[reaction: see text] An enantioselective total synthesis of (-)-adaline has been achieved starting from a chiral 6,6-disubstituted piperidone derivative previously prepared by diastereoselective allylation of a chiral tricyclic N-acyl-N,O-acetal. The key steps include lithium ion-activated SN2-type alkynylation of the tricyclic N,O-acetal leading to exclusive formation of the (6S)-ethynylpiperidine and ring-closing olefin metathesis of the (2R,6S)-cis-2,6-dialkenylpiperidine for constructing the bridged azabicyclononane.     

Total synthesis of antifungal gamahonolide A

The first stereoselective total synthesis of gamahonolide A (1) has been accomplished using aminoxylation, Keck allylation, and ring-closing metathesis (RCM) reactions as key steps.     

Enantioselective imidazole-directed allylation of aldimines and ketimines

A new chiral allylchlorosilane has been developed that allows the highly enantioselective allylation and crotylation of a range of 2-imidazolylaldimines and ketimines. The method may be exploited for the protecting group-free synthesis of a diverse array of imidazole-bearing chiral carbinamines and, when coupled with ring-closing metathesis reactions, allows the one-pot synthesis of unusual heterocyclic motifs with potential relevance in medicinal chemistry.     

A short synthetic route to the calystegine alkaloids

An efficient strategy is described for the synthesis of enantiopure calystegine alkaloids. The key step employs a zinc-mediated fragmentation of benzyl-protected methyl 6-iodo-glycosides followed by in situ formation of the benzyl imine and Barbier-type allylation with zinc, magnesium, or indium metal. Stereochemistry in the pivotal allylation is controlled by the choice of the metal. The functionalized 1,8-nonadienes, thus formed, are converted into cycloheptenes by ring-closing olefin metathesis. Regioselective hydroboration and oxidation give the corresponding cycloheptanones, which are deprotected to afford the desired calystegines. Hereby, calystegine B(2), B(3), and B(4) are prepared from D-glucose, D-galactose, and D-mannose, respectively. This route constitutes the shortest synthesis of calystegine B(2) and gives rise to the first total syntheses of calystegine B(3) and B(4).     

Stereoselective synthesis of spicigerolide

The first total synthesis of the naturally occurring, cytotoxic lactone spicigerolide is described. The commercially available sugar l-rhamnose was the chiral starting materal. Key steps in the synthesis were an aldehyde two-carbon homologation via the Corey-Fuchs protocol, an asymmetric Brown-type aldehyde allylation and a ring-closing metathesis.     

Total synthesis and selective activity of a new class of conformationally restrained epothilones

Stereoselective total syntheses of two novel conformationally restrained epothilone analogues are described. Evans asymmetric alkylation, Brown allylation, and a diastereoselective aldol reaction served as the key steps in the stereoselective synthesis of one of the two key fragments of the convergent synthetic approach. Enzyme resolution was employed to obtain the second fragment as a single enantiomer. The molecules were assembled by esterification, followed by ring-closing metathesis. In preliminary cytotoxicity studies, one of the analogues showed strong and selective growth inhibitory activity against two leukemia cell lines over solid human tumor cell lines. The precise biological mechanism of action and high degree of selectivity of this analogue remain to be examined.     

First stereoselective total synthesis of seimatopolide A

The first stereoselective total synthesis of seimatopolide A (1) has been achieved. The key steps are Keck allylation, Sharpless asymmetric dihydroxylation, cross-, and ring-closing metathesis reactions.     

Total synthesis of (−)-diospongin A and (+)-cryptofolione via asymmetric aldol reaction

Stereoselective synthesis of two distinctive pyranone skeletons diospongin A and cryptofolione has been described based on an asymmetric aldol reaction starting from Chan’s diene. The synthetic strategy involves the enantioselective Mukaiyama aldol, diastereoselective reduction of δ-hydroxy-β-keto ester, a tandem sequence of deprotection, and intramolecular oxa-Michael reaction to obtain diospongin A and an asymmetric allylation and lactone formation using ring-closing metathesis reaction to obtain cryptofolione.     

Efficient and enantioselective total syntheses of heliannuols A and K

The second-generation enantioselective synthesis of heliannuol A and the first enantioselective total synthesis of heliannuol K (via two routes) have both been accomplished efficiently; (heliannuol A, nine steps and 25% yield; heliannuol K, seven steps and 47% yield). Highlights of our synthetic strategy include a substrate-controlled chirality transfer in the Lewis acid mediated Claisen rearrangement of the allyl aryl ether for the key construction of a tertiary stereogenic center at the benzylic position followed by, for heliannuol A, ring-closing metathesis, diastereoselective epoxidation, and regioselective cleavage of the epoxide; and for heliannuol K, ring-closing metathesis and conjugate reduction of the eight-membered enone.     

Development of a catalytic enantioselective synthesis of the guanacastepene and heptemerone tricyclic core

For nearly two decades, synthetic chemists have been fascinated by the structural complexity and synthetic challenges afforded by the guanacastepene and heptemerone diterpenoids. Numerous synthetic approaches to these compounds have been reported, but to date the application of enantioselective catalysis to this problem has not been realized. Herein we report an enantioselective synthesis of an advanced intermediate corresponding to the tricyclic core common to the guanacastepenes and heptemerones. Highlights of this work include sequential Pd-catalyzed decarboxylative allylic alkylation reactions to generate the two all carbon quaternary stereocenters, the use of ring-closing metathesis to close the A ring in the presence of a distal allyl sidechain, and a regio- and diastereoselective oxidation of a trienol ether to introduce oxygenation on the A ring.