Total Synthesis of (±)-Communesin F via a Cycloaddition with Indol-2-one

Johannes Belmar and Raymond L. Funk

 DOI: http:// dx.doi.org/10.1021/ja307277w

It has been some time since my last entry. I was very busy with moving to the US and starting my master’s thesis. But as you can see after about 12 weeks I am back. I chose a rather short synthesis but there is still some work to be covered within the next weeks which should result in much more detailed write-ups.

The communesins are known to the synthetic community for quite a while and were the targets of extensive research and synthetic studies. The current paper utilized some nice methodology developed by the group and published in an earlier synthesis of perophoramidine. [1]

The synthesis begins with the union of azide 1 and bromooxindole 2. Because their initially reported conditions did not give any product at all it was found that the reaction proceeded smoothly in the presence of substoichiometric amounts of silver carbonate yielding 3. After tosylation the mixture was exposed to methanolysis to produce the backbone structure 4 of communesin F. Methylation with Meerweins’s salt, hydrogenolysis of the azide with subsequent Boc-protection and detosylation furnished amide 5.

 Scheme 1

The bromine was then used as a handle to introduce the prenyl sidechain with a Heck reaction to give an intermediate allylic alcohol. In the presence of mercury salts a cyclization took place constructing the crucial seven-membered amine ring. Deprotection of the amine under mild conditions using TBSOTf was followed by amide formation with some help from trimethylaluminium. The second side-chain was introduced in the usual sequence of deprotonation and subsequent alkylation with iodoacetonitrile.

 Scheme 2

The last ring was closed in a straightforward manner. Reduction of the nitrile to the aldehyde and the amide to the hemiaminal gave tetrahydrofuran 9. Reductive amination and acetylation finally produced communesin F in an overall yield of 6.7 %.

Scheme 3

The mechanism of the key step is pretty straightforward but nevertheless a nice one. After tosylating the oxindole nitrogen the resulting amide can be cleaved by methoxide to give an anilide anion which undergoes intramolecular attack on the indolenine 2 position to close the ring.

Scheme 4

[1] J. Am. Chem. Soc. 2004, 126, 5068

Yeah… that is it for the moment. I found some new interesting stuff to write-up, so I promise I will be up to date from now on J

Scalable Total Synthesis of (-)-Berkelic Acid Using a Protecting-Group-Free Strategy

Scalable Total Synthesis of (-)-Berkelic Acid Using a Protecting-Group-Free Strategy[1]

Francisco J. Fananás, Abraham Mendoza, Tamara Arto, Baris Temelli, and Felix Rodriguez

 DOI: dx.doi.org/10.1002/anie.201109076

Berkelic acid is a rather old target to the synthetic community and three total syntheses have been published to date. Interestingly the material provided by synthesis produced contradictory biological results compared to earlier studies. So besides showing the power of their methodology the group planned to provide enough material for refined studies.

 Scheme 1

As can be seen from scheme 1 the group planned to construct almost the whole framework in one single step after disconnection of the side. It should be noted that the group has some experience with this kind of cascade transformations of which they can rely on. Nevertheless instead of employing palladium catalysts the group turned their attention to silver catalysis. With this cascade reaction in mind they hoped that the stereogenic methyl group would control the stereoselectivity of the whole transformation.

The three key building blocks were prepared in a straightforward manner. Starting from commercially available butynol 1 the hydroxy functionality was mesylated and replaced by diethylmalonate to give after complete reduction diol 2. Starting from ester 3 the second fragment was prepared by triflation of the least hindered hydroxy group followed by Suzuki cross coupling with the trifluoroborate of heptyne. Hydroxy-directed reaction with formaldehyde and subsequent oxidation produced ester 4. The last building block stems from dimethyl malate which was doubly alkylated in the first place. Then the a-hydroxy ester was used for a periodate cleavage followed by cyanohydrin formation which was catalyzed by PNPCl.[2]

 Scheme 2

 

Combination of the red fragment 2 and orange fragment 4 was accomplished in the presence of 5 mol% silver(II). Subsequent hydrogenation of the resulting double bond yielded 7 in good yield and diastereoselectivity favoring the desired one. Appel reaction under standard conditions was followed by cyanohydrin alkylation and unmasking of the ketone to give protected Berkelic acid 9. Small amounts of Berkelic acid can be produced in good yield by selective saponification of the more active ester. This was only done when material was needed for testing or analysis as the natural product is a short-lived compound.

 Scheme 3

 

The mechanism of the cool key step is presented below. On one hand the red fragment underwent a 5-exo-dig cyclization thus desymmetrizing the propanediol moiety to give after protodemetallation a tetrahydrofuran ring. On the other hand the carbonyl of the orange fragment underwent a 6-endo-dig cyclization. Supported by keto-enol tautomerism of the hydroxy functionality an ortho-quinone methide is formed. Michael addition of the enol ether from the red fragment onto the quinone methide was followed by acetal formation by the phenol. Hydrogenation of the newly formed double bond then gave intermediate 7.[3]

 Scheme 4

 

[1] I was pointed to the title which says “[...] protecting-group-FREE strategy”… I am not particularly sure how they got the title but I see almost two protecting groups: the TES-cyanohydrin and one of the methyl esters. Maybe the title refers to the neat cascade reaction in which no protecting groups are necessary…

[2] It is the first time I ever saw this reagent in action. It is usually used for halogenation reactions. The cited paper in this step found that in the presence of PNPCl the cyanohydrin formation is much faster which was ascribed to an activation of the carbonyl oxygen by the high oxophilicity of phosphorous.

[3] At first sight one might think of a Diels-Alder reaction. But brief examination of the stereochemistry on the newly formed pyran ring shows that only a stepwise mechanism can form this particular anti-substitution pattern.

Addendum: If you are interested in earlier studies of the group you should have a look into these two papers

Big THX to Bobby for proofreading.

Total Synthesis of (-)-Dendrobine

Lukas M. Kreis and Erick M. Carreira

DOI: http://dx.doi.org/10.1002/anie.201108564

Dendrobine is the most abundant alkaloid isolated from an orchid which is used in traditional chinese medicine. The caged structure of this natural product is responsible for the interest of organic chemists in its synthesis. Retrosynthetically the synthesis is almost straightforward. Opening of the lactone and intramolecular amination give a precursor which is easily built up through an Ireland-Claisen rearrangement and enamine induced Michael addition.

 Scheme 1

Ester 1 which is easily accessible from commercially available material underwent a nice Michael addition with iPrNO₂ to give after removal of the nitro group the cis-configured ester 2. The stereochemical outcome can be explained by using the Cornforth model. Excessive reduction with LiAlH₄ was followed by benzoylation, acetonide cleavage, double TBS protection, selective mono-deprotection, and Swern oxidation of the primary alcohol to give aldehyde 3. Parallel to the latter synthesis the second fragment commenced with alcohol 4. Silylation, methylation of the alkyne, and iodination after hydrozirconation employing Schwartz’s reagent yielded iodide 5. Both fragments were combined after halogen—metal exchange with tBuLi and one-pot deprotection of the benzoyl protecting group with ethyl Grignard to furnish advanced intermediate 6.

 Scheme 2

 

Selective oxidation of the primary alcohol produced lactone 7 most likely through transitional lactol formation. After converting the ester group into the TMS-ester enolate the mixture was refluxed and underwent the crucial Ireland-Claisen rearrangement. The naked acid which resulted after work-up was protected as the methyl ester 8. Global desilylation was accomplished with HF in pyridine and followed by PCC oxidation. Aldehyde 9 was then condensed with benzylmethylamine and the resulting Michael adduct reduced with palladium on charcoal and hydrogen to give 10. N-C bond formation was accomplished by bromination/S_N2 displacement and stereoselective reduction of the ketone then formed in situ dendrobine. [1]

 Scheme 3

The mechanistic rational of the enamine induced Michael addition is shown below. After formation of the enamine the unsaturated ketone is attacked from the bottom face to give presumably after some proton shifts another enamine. Reduction from the Re face delivered amine 10 while the benzyl group is cleaved off at the end of this sequence.

Scheme 4

The C-N bond formation was induced by PHT, a commercially available mild brominating reagent. It was hypothesized that the nitrogen is brominated first and delivers the bromine to the a-position of the ketone. DMAP was essential in this step because it epimerized this position and left the bromine in an ideal position for a S_N2 displacement by the nearby nitrogen.

 Scheme 5

 Luckily BRSM took the Indoxamycin B synthesis from Carreira. Check it out…

[1] Big thanks to Bobby for correcting the presumed structure of PHT: it is believed known that the tribromide ion forms an ion pair with a protonated pyrrolidinone. Makes sense compared to pyridinium tribromide. Here is the corrected link to the crystal structure: ftp://ftp.oldenbourg.de/pub/download/frei/ncs/224-4/1267-2622.pdf

Big THX to Bobby for proofreading and corrections.