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

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

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Total Synthesis of (+)-Condylocarpine, (+)-Isocondylocarpine, (+)-Tubotaiwine and (-)-Actinophyllic Acid

Total Synthesis of (+)-Condylocarpine, (+)-Isocondylocarpine and (+)-Tubotaiwine

Connor L. Martin, Seiichi Nakamura, Ralf Otte, and Larry E. Overman

DOI: http://dx.doi.org/10.1021/ol102709s


Total Synthesis of (±)- and (-)-Actinophyllic Acid

Connor L. Martin, Larry E. Overman, and Jason M. Rohde

DOI: http://dx.doi.org/10.1021/ja100178u

This review summarizes two very interesting papers published more or less recently by the Overman group (he is still my favourite… ). I decided to combine both papers because a common intermediate was used to make all  four natural products and its synthesis makes use of some uncommon in situ Umpolung chemistry.

The first schemes were reproduced from the JACS paper while the last two schemes came from the OL paper.

Scheme 1

Starting with Boc-protected GABA 1 the free acid was transformed into the Weinreb amide and alkylated with a vinyl-Grignard to get 2. Enantioselective reduction of the resulting ketone with high ee was accomplished by using catalyst A in the presence of hydrogen (Noyori’s catalyst). Ozonolysis of the double bond and trapping of the alcohol and resulting aminal with acetic anhydride furnished piperidine 4.

The second main fragment was obtained in two steps from di-tert-butylmalonate. Deprotonation and acylation gave compound 6 which formed indole 7 after reduction of the nitro group with Pd on charcoal in the presence of vanadate.

Scheme 2

Then it is getting more interesting: the blue and red fragment were combined by using a bit of scandium triflate to form 8 with great diastereoselectivity. Reductive removal of the acetyl protecting group and Swern oxidation of the resulting free alcohol produced ketone 9. Next my favourite reaction of the whole paper was employed: first a double deprotonation of the ketone and the malonate and then combination of the two carbanions to form the critical bicyclic ring system. Although the yield is moderate it proceeds with high dr. [Fe(DMF)3Cl2][FeCl4] was prepared from dehydrated iron(III)chloride and DMF by simply mixing the reagents. Finally addition of vinyl-Grignard under Luche conditions to the ketone forms lactone 11.

Scheme 3

Going on with the synthesis the lactone and the remaining ester group were reduced to get bis-alcohol 12. At this stage Overman makes use of his almighty aza-Cope/Mannich reaction.

The t-Bu- and Boc-groups were cleaved off in the presence of dilute acid before formalin was added. For clarity I added the main stages of the following events:

First a Schiff base formed from formaldehyde and the secondary amine. This underwent an aza-Cope rearrangement (or some sort of Prins-reaction) with the allyl alcohol to form 13b. The newly formed enol then attacks the rearranged Schiff base in a Mannich reaction to give (-)-Actinophyllic acid 14 as its hydrochloride.

Scheme 4

Finally to the paper mentioned first. Starting from key intermediate 10 the ketone was reduced, the Boc group removed and the malonate decarboxylated/transesterified to give amine 15. Reductive amination with the dithioacetal aldehyde shown was followed by a DMTSF mediated alkylation to give 17. Reductive desulfuration with Raney Ni and oxidation of the remaining alcohol under Albright/Goldman conditions (Swern-oxidation) furnished 18.

Scheme 5

Wittig reaction of the keto group then produced (+)-condylocarpine (and (-)-isocondylocarpine respectively) which was reduced in the presence of platinum oxide to give (+)-tubotaiwine.

Scheme 6

Nice… I had the schemes finished a few days ago but also had to write my last serious exam so … What do you think? Any comments?

Total Synthesis of (+)-Clavolonine, (-)-Deacetylfawcettiine and (+)-Acetylfawcettiine

Total Synthesis of (+)-Clavolonine, (-)-Deacetylfawcettiine and (+)-Acetylfawcettiine

Kai M. Laemmerhold and Bernhard Breit

DOI: http://dx.doi.org/10.1002/anie.200907248

It took some time but now it’s finished… I was very busy with studying, practicing and visiting the “Frontiers in Medicinal Chemistry”-symposium in Münster (Germany) the past weeks. Nice meeting with some cool posters and as the main act Prof. Fürstner himself. Really awesome stuff…

Ok, back to some chemistry: I found this paper on my desk almost a month after I printed it out during a period with only sporadically published total syntheses. It features a nice methodology and obviously that’s the one of the reasons why this work was done. Nevertheless the molecules we’re dealing with can easily be accomplished by making use of this (new) concept of synthesis the authors demonstrate here:

Overview

Syntheses of all 3 are rare and if published all except for one are racemic so an enantioselective approach might be useful for further biological evaluation.

Retro

The whole synthesis was built around this DPPB directing group which guides the rhodium catalyst and later the Lewis acid (BF3) during the synthesis. If you’re interested in this methodology in more detail you should have a look in this. Other steps include a neat Mannich- and aza-Wittig reaction. Furthermore an unusual addition of a cuprate on an alkyne

Scheme 1

The synthesis starts right off with a simple alcohol which was oxidized and elongated using Swern- and Wittig-chemistry. Saponification, acid chloride formation and alkynone formation was followed by CBS reduction of the ketone moiety. Next the TMS was cleaved with TBAF and the propargylic alcohol transformed into an allylic one by addition of a Normant cuprate (Cuprate addition of propargylic alcohols).

Scheme 2


Esterification with DPPBA was followed by the first of two hydroformylation reactions in this synthesis with very good regioselectively and yield. A stereoselective Prins-reaction then closes the first ring (or should it be named oxa-Alder-ene?). The alcohol was protected as the TIPS ether and the second alkene hydroformylated again with high yield and stereoselectivity giving this odd looking cyclohexane with 5 stereocenters already in place.

Scheme 3

Grignard reaction and protection of the resulting alcohol as the TIPS ether was followed by reductive cleavage of the DPPB group and acetylation of the free alcohol. Azide formation under Finkelstein conditions, global TIPS deprotection and DMP oxidation yields a cyclohexanone intermediate ready for some ring closing chemistry.

Scheme 4

The second ring was closed employing an aza-Wittig reaction resulting in an imine which was directly used to close the third ring by means of a Mannich reaction. The enol ether was opened with HBr in AcOH which results in cyclization and formation of (+)-clavolonine.

Nice stuff… This hydroformylation looks very promising to me especially in association with this Prins reaction.

Any comments?

Asymmetric Construction of Rings A-D of Daphnicyclidin-Type Alkaloids

Asymmetric Construction of Rings A-D of Daphnicyclidin-Type Alkaloids

Travis B. Dunn, J. Michael Ellis, Christiane C. Kofink, James R. Manning, and Larry E. Overman

DOI: http://dx.doi.org/10.1021/ol902373m

It’s finished… took me some weeks to complete this review but here it is: a sweet “towards”-total synthesis from the Overman group.

The compounds to be made are Daphnicyclidin A – D whose biological profile is poorly studied yet.

The crude extracts of the plant are used in Chinese folk medicine… so some biological effect could be expected.

Retro:

The paper skips the last 2 stages which I painted in grey so a full account could be expected in the near future.

I think the retro does not need some comment, questions should being answered in the following schemes… so let’ get started.

Blue fragment:

First a nice DA reaction developed by the MacMillan group formed the cyclohexene carbaldehyde was followed by a stereoselective methylation under conditions described by Woodward. TBDPS protection, Saegusa-like oxidation and TBS enol ether formation completes the first part in overall great yield and e.r..

Red fragment:

Hydroxybenzamide was oxidised with periodate to give in situ nitrosocarbonylbenzene which undergoes a hetero-DA in acceptable yield and diastereoselectivity. The crude mixture was used in the next step, a Mo(CO)6 induced cleavage of the N-O bond and deprotection. to yield the cyclohexenone shown. Conjugate addition of DMPS-lithium and epimerization of the benzoylamide was followed by de-benzoylation/reduction, alkylation and Swern oxidation to give the red fragment ready for the crucial aza-Cope/Mannich-reaction sequence developed by Overman some years ago.

Green fragment:

After some efforts to tune the reaction conditions for the introduction of the side chain, a premixed solution of the ketone with CeCl3 and LiCl was treated with the iodie and t-BuLi giving the alcohol in good yield. Some silver nitrate then induced the key transformation, the aza-Cope-Mannich reaction, forming 2 of the 6 rings required.

2 different approaches were employed to form the fused pyrrolidine rings which will be presented in 2 schemes:

Scheme 1

The first approach starts with TBDPS deprotection, mesylation of the delivered alcohol, which directly undergoes SN2 displacement, and double debenzylation. The free alcohol was tosylated, followed by Grignard addition of allylmagnesium chloride on the ketone. Treatment of the tosylate ester with the p-nitrophenyl selenide anion and subsequent oxidation with mCPBA yielded the required terminal bis-olefin. Grubbs II then did the job and closed the fourth ring ready for further transformations.

Scheme 2

A Grignard addition under Lewis acid conditions starts this sequence off. Grubbs II closed again the seven membered ring in excellent yield. Alcohol transposition with thionyl chloride and DMP oxidation (if I remember right CrO3 should do the same job in one step?) gave the α-β-unsatured ketone. TBDPS deprotection and mesylation/in situ ring closing yielded a structure similar to the one in the scheme before.

Yeah, I really appreciate the work from Overman’s groups. He’s really one of the best chemists alive. What do you think?