Journal of the American Chemical Society, Vol.125, No.45, 13784-13792, 2003
Application of conformation design in acyclic stereoselection: Total synthesis of borrelidin as the crystalline benzene solvate
The total synthesis of (-)-borrelidin (treponemycin), a structurally distinct 18-membered macrolide antibiotic, has been achieved. It was isolated as the crystalline benzene solvate, and its structure was confirmed by a single-crystal X-ray analysis. The deoxypropionate subunit consisting of four alternating C-methyl groups with a C-4-C-10 syn/syn/anti orientation was elaborated by a new method of iterative cuprate additions to acyclic a,p-unsaturated esters relying on two consequtive 1,3-inductions and starting with D-glyceraldehyde as the chiral progenitor. The unique Z/E cyanodiene unit was obtained as a single isomer by application of the Still-Gennari olefination protocol. The gamma-hydroxycyclopentane carboxylic acid subunit was prepared from L-malic acid utilizing a sequential introduction of C-vinyl and C-allyl groups, capitalizing on 1,2-induction in an acyclic alpha,beta-unsaturated ester and carbocyclization by a Grubbs ring closure metathesis reaction. The prevalence of 1,3-syn-disposed deoxypropionate triads in the cuprate additions is rationalized on the basis of minimized syn-pentane interactions in the transition states. A virtual diamond lattice was used as a visual tool to portray the low-energy conformations of the acyclic substrates, and corroborated by H-1 NMR homodecoupling studies.