Total Synthesis of the Galbulimima Alkaloid (-)-GB17

Total Synthesis of the Galbulimima Alkaloid ()-GB17

 Reed T. Larson, Michael D. Clift, and Regan J. Thomson

 DOI: http://dx.doi.org/10.1002/anie.201108227

GB 17 belongs to the family of Galbulimima alkaloids which can be found in the bark of a rainforest tree with himbacine as a promising lead structure for muscarinic receptor antagonists. Himbacine-like compounds were tested for the treatment of Alzheimer’s disease as thrombin agonists.

Other family members including himandrine, GB13, himgaline, and GB16 have been synthesized. To date no synthesis of GB17 is known so the Thomson group accepted this last challenge. The retrosynthetic analysis is shown below. Nothing real spectacular but a nice access to the tetracyclic carbon skeleton is presented.

 Scheme 1

 

The first building block is readily available by a methodology developed by Lhommet et al. Reaction of ester 1 with (S)-phenylglycinol yielded oxazolidine 2 which was hydrogenated to give piperidine 3. The yields are not reported but considering the original publication about 40 % yield can be achieved.

The linchpin was synthesized starting from monoprotected diol 4 which was converted to iodide 5 and dithiane 6 which alkylation with 7 to give acetal 8. 5 and 8 were coupled and the aldehyde and alcohol were freed with dilute HCl in acetone. [3]

 Scheme 2

Treatment of ester 3 with lithiated phosphonate and Boc protection of the naked amine gave ketone 10 [4]. HWE reaction with linchpin 9 under Masamune-Roush conditions and subsequent DMP oxidation furnished aldehyde 11. After some model studies the group found that a TMS-prolinol catalyst gave highest yields and enantiomeric excess on a multigram scale. In a one-pot procedure the aldehyde was converted to unsaturated ester 12. Base induced cyclization, amine deprotection, and lactamization yielded tetracycle 13 in moderate yield. Nevertheless it was found that the wrong isomer had been formed together with complete inversion of the stereocenter next to the amine.

Scheme 3

 

Obviously the (E)-configured ester gave the wrong stereochemistry in the Michael addition step, so the group proceeded from 11a through a Still-Gennari modified HWE to give again under Masamune-Roush conditions ester 14. Boc deprotection and this time sodium methanolate induced cyclization did the job. Under these conditions the lactamization occurred to give 15. The keto group in 15 was removed under standard conditions by formation of the vinyl triflate which was reductively removed in the presence of Pd and formic acid as the hydrogen source. Stereoselective alkylation of the lactam was followed by dithiane removal, reduction, and oxidative cleavage of the exo-methylene group to give GB17.

Scheme 4

 

To explain the stereochemistry in the organocatalytic step I would propose the following transition state. Enamine formation of the prolinol ether should lead to the transition state with the least steric interactions. [5] McMillan’s catalyst or proline gave much lower ee values.

 Scheme 5

 

The outcome of the cyclization step can be explained considering the transition states shown below. In structure 12 steric interactions between the large Boc group and the ester force the double bond into an axial position. Alternatively without the Boc group and with a (Z)-double bond the ester group is equtorial so steric interactions can be minimized in the conformer shown.

 Scheme 6

And as usual THX to Bobby for proofreading.

[1] DOI: http://dx.doi.org/10.1016/j.tet.2005.05.079

The specificity of the reduction step can be explained by looking at the particular bonds which are reduced. 1) The enamine bond is reduced stereoselectively by facial differentiation from the Re face. 2) The aminal opens up to an imine which is again reduced to the amine. 3) The auxiliary is cleaved off.

[2] DOI: http://dx.doi.org/10.1021/ja055740s

[3] Have a look at BRSM’s blog for a sweet discusison on linchpins…

[4] Interestingly the group protected the amine after the BuLi chemistry which results in the usage of > 2 eq of lithiated phosphonate. Maybe earlier Boc-protection gave racemisation through DoM-chemistry with some help from the Boc group. Racemization was later found to occur in the presence of tBuOK.

[5] The group stated that the dithiane protecting group was essential for the reactivity of the substrate. Without this group almost no transformation was observed. Considering the great Thorpe-Ingold effect of this protecting group it might be an explanation.

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2 Responses

  1. hi there,
    Nice post! Wanted to ask about your explanation of scheme 6. For compound 14 the desired isomer you state the “ester group is also axial”, are you sure? To get the opposite stereochem. at that ring junction, I would think it would be equatorial and it even looks equatorial in your pic.

  2. Oops, you are right… I will fix it. Otherwise the whole synthetic sequence would not make much sense…

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