Total synthesis of the presumed amphidinolide A

Synthetic studies toward amphidinolide B1: synthesis of the C9-C26 fragment

[reaction: see text] The synthesis of the C9-C26 portion of amphidinolide B1 is described. A Fleming allylation followed by elimination was employed for the construction of the C13-C15 diene portion. Sharpless asymmetric dihydroxylation was utilized for regioselective functionalization of a styrene-derived alkene, in the presence of the C13-C15 diene functionality. A highly diastereoselective aldol reaction was developed to establish the C18 stereochemistry.

Total Synthesis of (+)-Amphidinolide A. Assembly of the Fragments

The structure elucidation of (+)-amphidinolide A, a cytotoxic macrolide, has been accomplished by employing a combination of total synthesis and NMR spectroscopic analysis. Amphidinolide A possesses two skipped 1,4-diene subunits which are accessible by ruthenium-catalyzed alkene-alkyne couplings. Previous total syntheses had revealed that the reported structure was incorrect; therefore, to incorporate maximum flexibility into the synthesis, with the ultimate goal of determining the correct structure, a highly convergent approach was chosen. Furthermore, liberal use was made of catalytic asymmetric transformations to set individual stereocenters. Three different strategies were envisioned for the end game, and due to the highly convergent nature of the synthesis, all three routes disconnect to the same three key intermediates, 5, 6, and 7. Diastereomers of 6 and 7 were easily prepared by modification of the synthetic routes to allow access to multiple diastereomers of 1 for structural determination.

Total Synthesis of (+)-Amphidinolide A. Structure Elucidation and Completion of the Synthesis

The structure elucidation of (+)-amphidinolide A, a cytotoxic macrolide, has been accomplished by employing a combination of NMR chemical shift analysis and total synthesis. The 20-membered ring of amphidinolide A was formed by a ruthenium-catalyzed alkene-alkyne coupling to forge the C15-C16 bond. Using the reported structure 1 as a starting point, a number of diastereomers of amphidinolide A were prepared. Deviations of the chemical shift of key protons in each isomer relative to the natural material were used as a guide to determine the locations of the errors in the relative stereochemistry. The spectroscopic data for the synthetic and natural material are in excellent agreement.

Total syntheses of amphidinolides T1 and T4 via catalytic, stereoselective, reductive macrocyclizations

Described in this work are total syntheses of amphidinolides T1 and T4 using two nickel-catalyzed reductive coupling reactions of alkynes, with an epoxide in one case (intermolecular) and with an aldehyde in another (intramolecular). The latter was used to effect a macrocyclization, form a C-C bond, and install a stereogenic center with >10:1 selectivity in both natural product syntheses. Alternative approaches in which intermolecular alkyne-aldehyde reductive coupling reactions would serve to join key fragments were investigated and are also discussed; it was found that macrocyclization (i.e. intramolecular alkyne-aldehyde coupling) was superior in several respects (diastereoselectivity, yield, and length of syntheses). Alkyne-epoxide reductive couplings were instrumental in the construction of key fragments corresponding to approximately half of the molecule of both natural products. In one case (T4 series), the alkyne-epoxide coupling exhibited very high site selectivity in a coupling of a diyne. A model for the stereoselectivity observed in the macrocyclizations is also proposed.

A comparative analysis of the total syntheses of the amphidinolide T natural products

In this article we compare and contrast the strategies and tactics used in the syntheses of the amphidinolide T family of natural products that have been reported by Fürstner, Ghosh and ourselves. Similar approaches to the trisubstituted THF ring present in the targets are utilized in all of the syntheses, but each strategy showcases a different means of macrocyclization.

Synthesis of ent-haterumalide NA (ent-oocydin A) methyl ester

[reaction: see text] Stille coupling of allylic chloride 26 with vinylstannane 4a afforded 65% of 27 with the requisite skipped diene, vinyl chloride, and allylic oxygen functionality. Yamaguchi macrolactonization of 28 provided 65% of 29, which was elaborated to haterumalide NA methyl ester (32) by a Nozaki-Hiyama-Kishi coupling with 31.

Unified synthesis of C19-C26 subunits of amphidinolides B1, B2, and B3 by exploiting unexpected stereochemical differences in Crimmins' and Evans' aldol reactions

The efficient synthesis of the C(19)-C(26) subunit of amphidinolide B(1) and B(2) has been completed using a boron-mediated aldol reaction. The synthesis of the C(19)-C(26) subunit of amphidinolide B(3) has also been accomplished through an unexpected anti aldol reaction using a titanium-mediated process. In addition, the first reported examples of a stereochemical discrepancy between the Evans' boron-mediated oxazolidinone and the Crimmins' titanium-mediated oxazolidinethione aldol reactions are disclosed. A working hypothesis is put forth to explain the results.

Structure Elucidation of (+)-Amphidinolide A by Total Synthesis and NMR Chemical Shift Analysis

The structure elucidation of (+)-amphidinolide A, a cytotoxic macrolide, has been accomplished by employing a combination of NMR chemical shift analysis and total synthesis. Using the reported structure as a starting point, a number of diastereomers of amphidinolide A were prepared. The deviations of the chemical shifts of key protons in each isomer relative to the values reported for the isolated material were used to determine the locations of the errors in relative stereochemistry. The spectroscopic data for our proposed structure of (+)-amphidinolide A and the isolated material are in excellent agreement. The key step, a [Cp*Ru(MeCN)3]PF6-catalyzed alkene-alkyne coupling, was used to form the 20-membered ring in the final step of the synthesis.

Ruthenium-catalyzed alkene-alkyne coupling: synthesis of the proposed structure of amphidinolide A

The ruthenium-catalyzed alkene-alkyne coupling provides a powerful method for the synthesis of 1,4 dienes and a way to simplify synthetic strategy. The latter potential is explored in the context of a synthesis of the assigned structure of amphidinolide A, which also raises the question of the applicability of this reaction for macrocyclizations. Employing this reaction allows simplification of the target to three subunits corresponding to C-1 to C-6, C-7 to C-15, and C-16 to C-25. The C-7 to C-15 subunit involves introduction of chirality by an asymmetric dihydroxylation. The route to the C-16 to C-25 subunit introduces chirality by a Pd-catalyzed asymmetric allylic alkylation and an asymmetric epoxidation. Assembly of the three subunits employs the Ru-catalyzed addition inter- and intramolecularly. The synthesis culminated in the formation of the assigned structure and is identical to the synthetic samples prepared independently by two completely different routes. As noted by the other two groups, this structure appears to be a diastereomer of the natural product. Because this synthesis introduces all of the stereochemistry of the subunits by catalytic asymmetric processes, either enantiomer as well as diastereomers can be readily accessed to define the correct structure. Notably, the Ru-catalyzed macrocyclization to this macrolide proceeded in better yields than either a Pd-catalyzed cross-coupling or a Ru-catalyzed metathesis, macrocylization methods for the other two total synthesis.