A Synthesis-Driven Structure Revision of Berkelic Acid Methyl Ester

A Concise Total Synthesis of (-)-Berkelic Acid

Reported here is a concise total synthesis of (-)-berkelic acid in eight linear steps. This synthesis features a Catellani reaction/oxa-Michael cascade for the construction of the isochroman scaffold, a one-pot deprotection/spiroacetalization operation for the formation of the tetracyclic core structure, and a late-stage Ni-catalyzed reductive coupling for the introduction of the lateral chain. Notably, four stereocenters are established from a single existing chiral center with excellent stereocontrol during the deprotection/spiroacetalization process. Stereocontrol of the intriguing deprotection/spiroacetalization process is supported by DFT calculations.

Total Synthesis, Stereochemical Revision, and Biological Reassessment of Mandelalide A: Chemical Mimicry of Intrafamily Relationships

Mandelalide A and three congeners had recently been isolated as the supposedly highly cytotoxic principles of an ascidian collected off the South African coastline. Since these compounds are hardly available from the natural source, a concise synthesis route was developed, targeting structure 1 as the purported representation of mandelalide A. The sequence involves an iridium-catalyzed two-directional Krische allylation and a cobalt-catalyzed carbonylative epoxide opening as entry points for the preparation of the major building blocks. The final stages feature the first implementation of terminal acetylene metathesis into natural product total synthesis, which is remarkable in that this class of substrates had been beyond the reach of alkyne metathesis for decades. Synthetic 1, however, proved not to be identical with the natural product. In an attempt to clarify this issue, NMR spectra were simulated for 20 conceivable diastereomers by using DFT followed by DP4 analysis; however, this did not provide a reliable assignment either. The puzzle was ultimately solved by the preparation of three diastereomers, of which compound 6 proved identical with mandelalide A in all analytical and spectroscopic regards. As the entire "northern sector" about the tetrahydrofuran ring in 6 shows the opposite configuration of what had originally been assigned, it is highly likely that the stereostructures of the sister compounds mandelalides B-D must be corrected analogously; we propose that these natural products are accurately represented by structures 68-70. In an attempt to prove this reassignment, an entry into mandelalides C and D was sought by subjecting an advanced intermediate of the synthesis of 6 to a largely unprecedented intramolecular Morita-Baylis-Hillman reaction, which furnished the γ-lactone derivative 74 as a mixture of diastereomers. Whereas (24R)-74 was amenable to a hydroxyl-directed dihydroxylation by using OsO4 /TMEDA as the reagent, the sister compound (24S)-74 did not follow a directed path but simply obeyed Kishi's rule; only this unexpected escape precluded the preparation of mandelalides C and D by this route. A combined spectroscopic and computational (DFT) study showed that the reasons for this strikingly different behavior of the two diastereomers of 74 are rooted in their conformational peculiarities. This aspect apart, our results show that the OsO4 /TMEDA complex reacts preferentially with electron deficient double bonds even if other alkenes are present that are more electron rich and less encumbered. Finally, in a brief biological survey authentic mandelalide A (6) was found to exhibit appreciable cytotoxicity only against one out of three tested human cancer cell lines and all synthetic congeners were hardly active. No significant fungicidal properties were observed.

A total synthesis of spirastrellolide A methyl ester

A concise total synthesis of spirastrellolide A methyl ester (1 a, R(1) =Me) as the parent compound of a series of highly cytotoxic marine macrolides is disclosed, which exploits and expands the flexibility of a synthesis plan previously developed by our group en route to the sister compound spirastrellolide F methyl ester (6 a, R(1) =Me). Key to success was the masking of the signature Δ(15,16) -bond of 1 a as a C16-carbonyl group until after the stereogenic center at C24 had been properly set by a highly selective hydrogenation of the C24 exo-methylene precursor 66. Conformational control over the macrocyclic frame allowed the proper stereochemical course to be dialed into this reduction process. The elaboration of the C16 ketone to the C15-C16 double bond was accomplished by a chemoselective alkenyl triflate formation followed by a palladium-catalyzed hydride delivery. The role of the ketone at C16 as a strategic design element is also evident up-stream of the key intermediate 66, the assembly of which hinged upon the addition of the polyfunctionalized dithiane 37 to the similarly elaborate aldehyde fragment 46. Other crucial steps of the total synthesis were an alkyl-Suzuki coupling and a Yamaguchi lactonization that allowed the Northern and the Southern sector of the target to be stitched together and the macrocyclic perimeter to be forged. The lateral chain comprising the remote C46 stereocenter was finally attached to the core region by a modified Julia-Kocienski olefination. The preparation of the individual building blocks led to some methodological spin-offs, amongst which the improved procedure for the N-O-bond cleavage of isoxazolines by zero-valent molybdenum and the ozonolysis of a double bond in the presence of other oxidation-prone functionality are most noteworthy. Preliminary biological data suggest that the entire carbon framework, that is the macrocyclic core plus the lateral chain, might be necessary for high cytotoxicity.

Synthesis of (-)-berkelic acid

An extremophilic challenge: Stereospecific condensation of a fully functionalized ketal aldehyde and a 2,6-dihydroxybenzoic acid is the key step in the synthesis of (-)-berkelic acid confirming Fürstner's reassignment of the stereochemistry at C18 and C19, establishing the absolute stereochemistry, and tentatively assigning the stereochemistry at C22.