Total Synthesis of (-)-Dendrobine

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/SN2 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 SN2 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.

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?