A short access to the skeleton of Elisabethin A and formal syntheses of Elisapterosin B and Colombiasin A.

A short stereoselective synthesis of the Elisabethin A skeleton 4 is described, which opens a formal access to the diterpenes Elisapterosin B and Colombiasin A as well. Key reactions were an intermolecular endo-selective Diels-Alder reaction to generate the decalin part of the molecule, a chemo- and diastereoselective allylation of an aldehyde with allylzinc, a palladium ene annulation of the cyclopentane ring, and a novel sulfonium ylide induced fragmentation of a polycyclic ketone. Additional insights have been gained for the crucial epimerization at C-2.

Light-mediated total synthesis of 17-deoxyprovidencin.

An asymmetric synthesis of the diterpenoid 17-deoxyprovidencin is described. Key steps include an aldol addition, a base-catalyzed Wipf-type furan formation, a Z-selective ring-closing metathesis for macrocyclization, a photochemical E/Z isomerization to a highly strained and conformationally restricted ring system, and the stereoselective formation of two epoxides on the ring.

Trying to rationalize total synthesis.

Total synthesis has been held in high esteem for many decades. In recent years, however, public opinion has turned more and more skeptical. In particular, long and uneventful sequences towards complex yet useless targets are considered a waste of resources and are refused funding. There is only one way to remedy this unpleasant situation: total synthesis must be efficient and present attractive chemistry. This article tries to show some aspects on how this increase in efficiency might be achieved.

A palladium-catalyzed carbo-oxygenation: the bielschowskysin case.

An asymmetric synthesis of an advanced tetracyclic intermediate toward the synthesis of bielschowskysin (1) is described. A biomimetic [2 + 2]-photocyclization was used to establish the cyclobutane core of bielschowskysin. Macrocyclization under Heck conditions led to an unprecedented carbo-oxygenation of a 1,1-disubstituted double bond.

Total synthesis of branimycin: an evolutionary approach.

The first total synthesis of the macrolactone antibiotic branimycin (4) has been described. The key disconnection leads to a cis-dehydrodecalone core and a polyketide side chain which are connected via organometallic addition. The dehydrodecalone core was targeted via altogether five different approaches featuring various kinds of chiral elements and ring-closing methodology. In the end the most successful method starting from diepoxynaphthalene 109 was chosen to carry on with the synthesis. Thus the oxygen functions and carbon appendages were introduced via organometallic desymmetrization reactions to generate epoxy ketone 107, to which vinyl iodide 11 was added after conversion into the organolithium species. The synthesis was completed by introducing the ester side chain via Michael addition and subsequent macrolactonization.

Application of the Rodriguez-Pattenden photo-ring contraction: total synthesis and configurational reassignment of 11-gorgiacerol and 11-epigorgiacerol.

A stereospecific photochemical ring contraction was used as the key step in the first total synthesis of the marine pseudopteranyl diterpene 11-gorgiacerol and its 11-epimer. The synthesis allowed the correction of the configurations that had been misassigned in the literature. In addition, some novel pseudopteranyl derivatives have been made.

A non-photochemical approach to the bicyclo[3.2.0]heptane core of bielschowskysin.

An asymmetric synthesis of the tricyclic core (-)-1 of the marine diterpene bielschowskysin is described. In particular, a methodology was developed to introduce the crucial quaternary center at C-12.

Efficient and scalable one-pot synthesis of 2,4-dienols from cycloalkenones: optimized total synthesis of valerenic acid.

A mild and selective one-pot procedure to provide 2,4-dienols from simple cycloalkenones in high yields is described. This transformation is based on the in situ formation of acid-labile allylic alcohols, which on treatment with trifluoroacetic acid undergo a formal [1,3]-hydroxy migration to form diastereo- and enantiomerically enriched 2,4-dienols. The usefulness of this protocol is demonstrated in a short synthesis of valerenic acid.

In pursuit of a competitive target: total synthesis of the antibiotic kendomycin.

Kendomycin is a novel polyketide having a unique quinone methide ansa structure and an impressive biological profile. Herein we provide a chronological overview of the synthetic work towards the title compound. Thus far, over a period of about eight years, eight groups worldwide have published on their synthetic efforts resulting in five total syntheses, one formal synthesis, and a number of fragment syntheses. Most approaches roughly mimic the biogenetic pathway, starting with an aromatic polyphenol subunit to which a polyketide chain is attached. Subsequent key steps include macrocyclization and the formation of the densely substituted tetrahydropyran ring, and then a late-stage oxidation and lactol formation.

Synthesis of the lycopodium alkaloid (+)-lycoflexine.

The first total synthesis of (+)-lycoflexine (1), a constituent of Lycopodium clavatum var. inflexum , has been accomplished in eight steps with 13% overall yield. Our synthesis covers four one-pot reactions, including a tandem Sakurai/aldol sequence, a novel hydroboration/oxidation procedure, a deprotection/transannular Mannich reaction, and as a highlight, a tandem catalysis cascade combining an enynene ring-closing metathesis and a selective hydrogenation.

Syntheses and antibacterial properties of iso-platencin, Cl-iso-platencin and Cl-platencin: identification of a new lead structure.

Platencin is a novel antibiotic which is active against multiresistant pathogens. We describe efficient syntheses of three platencin analogues of varying activities which allow further conclusions about the pharmacophoric part of the molecule. The unnatural antibiotic iso-platencin, which is about as active as natural platencin, but much more selective, was identified as a new lead structure.

Ring-closing metathesis and photo-fries reaction for the construction of the ansamycin antibiotic kendomycin: development of a protecting group free oxidative endgame.

Two convergent total syntheses of the ansa-polyketide (-)-kendomycin (1) are described. The syntheses benefit from the use of readily available and cheap starting materials. Highly complex diastereoselective Claisen-Ireland rearrangements were used to introduce the (E)-double bond and the C16-Me group. The ring closure of the strained ansa macrocycle was achieved by ring-closing metathesis and a highly efficient combination of macrolactonization and photo-Fries reaction. A protecting group free endgame via an unstable o-quinone is presented. Additionally some unsuccessful synthetic efforts towards the total synthesis of 1 are described.

Synthesis of (Z)-trisubstituted olefins by decarboxylative grob-type fragmentations: epothilone D, discodermolide, and peloruside A.

Methyl-branched (Z)-trisubstituted olefins are found in many polyketides with interesting biological activity, such as epothilone D (1), discodermolide (3), and peloruside A (2). Despite the employment of numerous different strategies, this motif has often been the weak point in total synthesis. Thus, we present a novel hydroxide- induced Grob-type fragmentation as an easy access to trisubstituted olefins. In our case, beta-mesyloxy delta-lactones with three stereogenic centers were chosen whose fragmentation underlies a high stereoelectronic control. Major challenges in the syntheses were the installation of quaternary stereocenters, achieved by enzymatic desymmetrization of meso-diesters and by aluminium-promoted stereoselective rearrangement of chiral epoxides, respectively. Different aldol strategies were developed for the formation of the fragmentation precursors. Additionally a short survey about nucleophilic additions to aldehydes with quaternary alpha-centers is presented.

From silphinenes to penifulvins: a biomimetic approach to penifulvins B and C.

The biomimetic total synthesis of penifulvins B and C uses a meta-photocycloaddition as a key step which gives rapid access to the fenestrane-type carboskeleton with control of an onring quaternary stereocenter. The route is concise, stereocontrolled, scaleable, and flexibile and requires only one protecting group.

Total syntheses of (+)-echinopine A and B: determination of absolute stereochemistry.

The first total syntheses of the novel 3,5,5,7-sesquiterpenoids (+)-Echinopine A (1) and B (2) were achieved. Thereby the proposed structures were confirmed, and the absolute stereochemistry was determined. The key features are (1) the stereoselective installation of the vinyl-moiety on the concave side of the bicyclo[3.3.0]octane core via Myers' [3,3]-sigmatropic rearrangement, (2) the finding that the substituent on the C7 position next to the ketone can be epimerized to the desired concave face under basic conditions, (3) the closure of the highly strained seven-membered ring via RCM, and (4) the unusual C2-homologation of a vinyltriflate with a ketene silyl acetal.

A desymmetrization approach toward highly oxygenated cis-decalins.

The cis-decalin core 2 of the antibiotic branimycin has been prepared by desymmetrization of diepoxynaphthalene 4. The key steps involve two successive S(N)' opening of the oxa-bridges. An improved procedure for the synthesis of 4 is also described.

Decarboxylative Grob-type fragmentations in the synthesis of trisubstituted Z olefins: application to peloruside A, discodermolide, and epothilone D.

Discriminating elimination: A new method for the synthesis of methyl-branched trisubstituted Z olefins, a structural motif in many polyketides with anticancer activity, relies on an (-)OH-induced decarboxylative Grob-type fragmentation (see scheme; Ms = mesyl). The starting materials are beta-mesyloxy lactones with a quaternary alpha center, which are prepared by aldol reactions in a diastereo- and enantioselective manner.