Macquarimicin B

A total synthesis of the macquarimicins A-C (1-3), novel natural products with intriguing tetra- or pentacyclic frameworks, has been achieved. The synthesis features an extensive investigation of the biosynthesis-based intramolecular Diels-Alder (IMDA) reactions of (E,Z,E)-1,6,8-nonatrienes. Considering possible biosynthetic sequences, four types of substrates were synthesized, and their IMDA reactions were examined. From one of the four substrates, the total synthesis was achieved via a transannular Diels-Alder reaction, which led to the stereoselective construction of the unique molecular framework. The convergent and efficient synthetic pathway afforded (+)-1 in 27 linear steps with 4.3% and 9.9% overall yields from readily available ethyl (2E,4S)-4,5-(isopropylidene)dioxy-2-pentenoate (22) and (R)-epichlorohydrin (30), respectively. Furthermore, efficient syntheses of 2, 3, and the 9-epi-cochleamycins A (57) and B (58) were accomplished. Additionally, the present work established the absolute stereochemistry of macquarimicins and revised the C(2)--C(3) geometry of 1.

A novel series of carbocyclic compounds has been isolated from two related Micromonospora cultures. The C-19 and C-22 macquarimicins represent different end products on a similar biosynthetic scheme. 1-D and 2-D homonuclear and heteronuclear NMR experiments allowed assignment of the basic structures of the macquarimicins. An X-ray structure of macquarimicin B suggested the stereochemistry for the series which was not discernible from spectroscopic data alone.

The highly stereoselective synthesis of the AB-ring system of macquarimicins, a novel class of microbial metabolites with inhibitory activity for neutral sphingomyelinase, has been achieved. The present synthesis features the highly stereocontrolled construction of the cis-tetrahydroindan structure via the intramolecular Diels-Alder reaction of an (E,Z,E)-1,6,8-nonatriene derived from D-glyceraldehyde acetonide. Reaction: see text.