Formamicin

The enantioselective total synthesis of the cytotoxic plecomacrolide natural product formamicin (1) is described. Key aspects of this synthesis include the efficient transacetalation reactions of MOM ethers 28 and 38 to form the seven-membered formyl acetals 29 and 39, a late-stage Suzuki cross-coupling reaction of the highly functionalized vinyl boronic acid 6 and vinyl iodide 7, a highly beta-selective glycosidation reaction of beta-hydroxy ketone 4 with 2,6-dideoxy-2-iodoglucopyranosyl fluoride 3, and the global desilylation of penultimate intermediate 77 mediated by in situ generated Et(3)N.2HF.

[structure: see text] The total synthesis of formamicinone (2), the aglycone of formamicin (1), has been accomplished via the late-stage Suzuki cross-coupling of fragments 5 and 6, the macrolactonization of seco ester 14, and the Mukaiyama aldol reaction of aldehyde 3 and methyl ketone 4. An efficient and highly stereoselective second generation synthesis of vinyl iodide 6 is also described.

The plecomacrolides are a large family of natural products typically featured with a 16- or 18-membered macrolactone possessing two conjugated diene units and a hemiacetal side chain. The macrocycle skeleton is connected with the side chain through a three-carbon linker, forming a biologically important intramolecular hydrogen bonding network among the lactone/linker/hemiacetal structural motif. Many members of the plecomacrolides act as selective inhibitors of vacuolar H+-ATPases (V-ATPases) and they are considered to have the potential for treatment of postmenopausal osteoporosis. Significant progress has been achieved in recent years in the area of plecomacrolide total synthesis. These include the total synthesis of bafilomycin A1 by Evans, Toshima, Hanessian, and Roush, of bafilomycin V1 by Marshall, of hygrolidin by Hashimoto and Yonemitsu, of concanamycin F by Paterson and Toshima, and of formamicin by Roush. The synthetic strategies and key transformations used in the total synthesis are discussed.