Total Synthesis of Aflastatin A

The total syntheses of aflastatin A and its C3-C48 degradation fragment (6a, R = H) have been accomplished. The syntheses feature several complex diastereoselective fragment couplings, including a Felkin-selective trityl-catalyzed Mukaiyama aldol reaction, a chelate-controlled aldol reaction involving soft enolization with magnesium, and an anti-Felkin-selective boron-mediated oxygenated aldol reaction. Careful comparison of the spectroscopic data for the synthetic C3-C48 degradation fragment to that reported by the isolation group revealed a structural misassignment in the lactol region of the naturally derived degradation product. Ultimately, the data reported for the naturally derived aflastatin A C3-C48 degradation lactol (6a, R = H) were attributed to its derivative lactol trideuteriomethyl ether (6c, R = CD₃). Additionally, the revised absolute configurations of six stereogenic centers (C8, C9, and C28-C31) were confirmed.

Synthesis of central spiro-bis-THF fragments of symbiospirols A–C

Four diastereomeric spiro-bis-THF derivatives representing the central C22–C33 and C36–C47 fragments of symbiospirols A–C have been synthesized in an attempt to establish the relative stereochemistry of these fragments.

Synthesis of C3-C21 Segment of Aflastatin A Using Remote Asymmetric Induction Reactions

The C3-C21 segment of aflastatin A has been synthesized by converging three segments including the C3-C8 segment, the C9-C15 segment, and the C16-C21 segment. Each segment has been synthesized from a vinylketene silyl N,O-acetal possessing a chiral auxiliary by a wide-range stereocontrol strategy. The C3-C8 segment was constructed in seven steps including the stereoselective vinylogous Mukaiyama alkylation, while the C9-C15 segment and the C16-C21 segment were synthesized using the vinylogous Mukaiyama aldol reaction in seven and eight steps, respectively.

Determination of the Absolute Configuration of Gliomasolide D through Total Syntheses of the C-17 Epimers

The absolute configuration at C-17, the carbon bearing the distal hydroxy group of the 14-membered natural product gliomasolide D, was assigned as R by comparison of ¹³C NMR shifts and specific rotation values of the epimers at C-17. The first total synthesis of gliomasolide D along with its C-17 epimer, regioselective macrocyclization (18 membered vs 14 membered), and regioselective Wacker oxidation are highlights of the present work.