A Novel Approach to Indoloditerpenes by Nazarov Photocyclization: Synthesis and Biological Investigations of Terpendole E Analogues
Fa´tima Churruca, Manolis Fousteris, Yuichi Ishikawa, Margarete von Wantoch Rekowski, Candide Hounsou, Thomas Surrey, and Athanassios Giannis
As the title suggests it’s time for some sunlight chemistry… Ok only one step but the rest of the synthesis is also worth reading. The Terpendoles are a family of indoleterpenes which show weak activity as acyl-CoA:cholesterol acyltransferase inhibitors. Recently it was discovered that the terpendoles inhibit the kinesin spindle protein (KSP).
In this paper the synthesis of one member of this class is described. The retro is rather short as the paper is, too. We’re starting with some FGI and cut the molecule into two halves by using the above mentioned Nazarov cyclization strategy. As can easily be seen, the molecule should be accessible directly from the known Wieland-Miescher-ketone.
So here we go:
The scheme starts with a selective protection of the unconjugated carbonyl. Phenylthiomethylation (search for Kirk-Petrow-reaction for further information) which was followed by a SET reduction under Birch conditions and subsequent trapping of the anion with allylbromide then yields the allylated/methylated ketone. LAH reduction of the remaining ketone, boronation of the terminal olefin and oxidation results in lactone formation. Oxidation of the ketone lactone to an α-β-unsatured one was achieved under more or less unconventional conditions. Epoxidation with H2O2, epoxide opening with phenylselenide and protection of the resulting alcohol as the MOM ether closes the first scheme.
Because the phenylthiomethylation looks a bit odd, here is the mechanism:
The first few steps should be clear. The Birch reduction step might involve an intermediate radical anion which is trapped by allylbromide and reprotonated under thermodynamic control.
Furthermore the dehydration dehydrogenation step with this to me unknown reagent:
This is only a proposal of what I think the mechanism might be… I’m open for better ideas or corrections.
With the blue intermediate in hand we can move on. Selective reduction of the lactone was achieved with DIBAL-H and the aldehyde olefinated. Epoxidation of the alkene with mCPBA was followed by Sc(III) mediated pyran formation, oxidation and epimerization of the isomeric ethers to give one single pyran ring. Grignard reaction with methylmagnesium chloride, PG interconversion and acetal cleavage sets the stage for the final few steps.
The first step involves an aldol condensation/hydrogenation to link both halves of the molecule together. Methylation of the ketone, benzylic oxidation with DDQ, dehydration of the tertiary alcohol with Burgess reagent to the exocyclic alkene and isomerization of the latter one to the endocyclic alkene prepares the key intermediate for the Nazarov cyclization. This [2+2] cyclization was mediated by UV light and closes the ring in a disrotatory manner.
Protecting group removal and complete reduction of the ketone then yields Terpendole E.
Overall a nice synthesis but I would have preferred a bit more details in the paper. The authors only give the used reagents without any more information like conditions or eq’s. Nevertheless nice chemistry but there’s a little mistake in the published paper. Maybe you find it too…