Total Synthesis of Rameswaralide Utilizing a Pharmacophore-Directed Retrosynthetic Strategy

Abstract

A pharmacophore-directed retrosynthetic strategy was applied to the first total synthesis of the cembranoid rameswaralide in order to simultaneously achieve a total synthesis while also developing a structure-activity relationship profile throughout the synthetic effort. The synthesis utilized a Diels-Alder lactonization process, including a rare kinetic resolution to demonstrate the potential of this strategy for an enantioselective synthesis providing both the 5,5,6- and, through a ring expansion, 5,5,7-tricyclic ring systems present in several Sinularia soft coral cembranoids. A pivotal synthetic intermediate, a tricyclic epoxy α-bromo cycloheptenone, displayed high cytotoxicity with interesting selectivity toward the HCT-116 colon cancer cell line. This intermediate enabled the pursuit of three unique D-ring annulation strategies including a photocatalyzed intramolecular Giese-type radical cyclization and a diastereoselective, intramolecular enamine-mediated Michael addition, with the latter annulation constructing the final D-ring to deliver rameswaralide. The serendipitous discovery of an oxidation state transposition of the tricyclic epoxy cycloheptenone proceeding through a presumed doubly vinylogous, E1-type elimination enabled the facile introduction of the required α-methylene butyrolactone. Preliminary biological tests of rameswaralide and precursors demonstrated weak cytotoxicity; however, the comparable cytotoxicity of a simple 6,7-bicyclic β-keto ester, corresponding to the CD-ring system of rameswaralide, to that of the natural product itself suggests that such bicyclic β-ketoesters may constitute an interesting pharmacophore that warrants further exploration.

Figure 1.

Sinluaria cembranoid family members informing our proposed pharmacophore for rameswaralide.

Figure 2.

Proposed transition state arrangements for the enamine-mediated Michael D-ring annulation process leading to ketones 50a,b.

Figure 3.

Cytotoxicity of rameswaralide and synthetic intermediates toward the A549 lung cancer cell line.

Scheme 1.

Pharmacophore-Directed Retrosynthesis (PDR): (a) application to rameswaralide showing proposed ‘pharmacophore’ i and gradual, increasing structural complexity leading to general targets ii-iv that will inform SAR (b) generalized retrosynthetic strategy highlighting required disconnections for targeted derivatives (e.g. ii, iii) serving as waypoints in a projected total synthesis including a model study for D-ring annulation to also access the CD ring system.

Scheme 2.

Improved synthesis of the key, versatile epoxy α-bromo enone 20. Demonstration of a kinetic resolution leading to optically active enol ether (−)-15 and the transposed α-bromo enol ether (−)-16 (absolute stereochemistry determined through anomalous dispersion by X-ray analysis, inset).

Scheme 3.

D-Ring annulation strategy #1: (a) Retrosynthetic strategy for the intramolecular Diels-Alder (IMDA) cycloaddition. (b) Serendipitous C-ring oxidation state transposition discovered during attempted synthesis of the IMDA substrate.

Scheme 4.

D-Ring annulation strategy #2: Intramolecular Giese-type, 6-endo trig cyclization of an allylic radical generated from alcohol 29.

Scheme 5.

Synthesis of required coupling partners 31, 37 for generation of the allylic radical for proposed intramolecular Giese-type cyclization.

Scheme 6.

Model study of the intramolecular Giese-type, 6-endo trig cyclization leading ot the synthesis of the CD-ring system of rameswaralide.

Scheme 7.

Synthesis of the required Cs-oxalate 48 for the intramolecular Giese-type cyclization for D-ring annulation which failed to cyclize.

Scheme 8.

D-ring annulation strategy #2: (a) Retrosynthetic analysis of rameswaralide for D-ring annulation strategy employing an enamine-mediated Michael cyclization. (b) Model study of the enamine-Michael cyclization leading to synthesis of the CD-ring system of rameswaralide (inset: single crystal X-ray analysis of alkene 42a).

Scheme 9.

D-ring annulation strategy #3: Synthesis of enamine-Michael cyclization substrate 61 and cyclization, olefination, and deprotection leading to (±)-rameswaralide (1).