Total Syntheses of (-)-Acutumine and (-)-Dechloroacutumine
Sandra M. King, Nicholas A. Calandra, and Seth B. Herzon
Recently the Herzon group dislosed the neat syntheses of (-)-acutumine and (-)-dechloroacutumine. Driven by the interesting biological features (e.g. inhibition of human T-cell proliferation) and the densely functionalized structure the group devised a versatile approach towards both natural products. The common tetrahydroindolone core of the acutumines and the hasubanane alkaloids offered the opportunity to rely to some extend on earlier work on hasubanonine and related congeners. The main steps of the synthesis include the earlier employed lithium acetylide addition to an iminium ion, an intramolecular Hosomi-Sakurai reaction and a nice introduction of an unsaturated ketone.
The first two fragments are not featured in full detail in the paper so I present them separately. Fragment 5 can easily be accessed in five steps from glucose 1. Acetonide and acetal formation was followed by an Appel reaction and concomitant reductive ring opening to give aldehyde 3. Addition of vinyl Grignard, RCM in the presence of Grubbs-I and oxidation of the alcohol yielded known ketone 5 in good overall yield.
The second fragment was synthesized from trimethoxy acetophenone ketal 6 which underwent an interesting reductive ketal cleavage / hydroboration / oxidation procedure to give alcohol 7. Mesylation and SN2 replacement with sodium azide then furnished 8.
The following sequence of steps has been used in the synthesis of the hasubanane alkaloids. Oxidative dearomatization of 8 was followed by stereoselective Diels Alder reaction of the less hindered double bond. Finally trimethylphosphine mediated Aza-Wittig reaction produced key intermediate 11.
Elaboration of ketone 5 began with stereoselective Michael addition of (TMS)2 in the presence of catalytic Pd(OAc)2 and subsequent cleavage of the resultant TMS enol ether. Enol triflate formation and Stille coupling produced acetylide 14.
Next methylation of the imine and addition of the lithium acetylide of 14 furnished a single diastereomer of 15. The diastereoselectivity in this step is not straightforward to explain. Building a model does not help much because addition seems to occur from the concave site which should be less favored. The group offers an explanation in the paper: “The contrasteric diastereoselectivity in the addition step may be due to unfavorable torsional strain within the pyrrolidine ring in the alternate diastereomer”. For related addition products the group had access to X-ray structures which proved the relative stereochemistry.
Extrusion of TMS-pentadiene under thermal conditions was followed by regioselective hydrostannylation to give 17. TBAF mediated Hosomi-Sakurai reaction proceeded in moderate yield to close the remaining five-membered ring. Metal-halogen exchange with CuCl2 and deprotection of the diol then yielded 19.
Introduction of the remaining oxygen functionality proved to be fairly difficult. To the end the group had to rely on a rather steppy but successful approach. Oxidation of the diol to the vicinal diketone was followed by methyl sulfide addition and methylation to give 21. SN2’ replacement by formic acid and thermally induced Claisen rearrangement and subsequent aminolysis furnished hemiketal 24.
With fragment 24 only a few steps were left to complete the endeavor. Oxidation of the hemiketal and succeeding reduction with sodium borohydride gave 25 in good overall yield in excellent diastereoselectivity. In the presence of rhodium and high pressure hydrogen 25 was transformed into acutumine in low yield. In the presence of palladium on charcoal beside the double bond the chlorine could be removed to give dechloroacutumine in good yield.
Overall a really nice paper which is definitely worth a read.