Scheme 4.
Completion of the left-hand fragment. Reagents and conditions: a) hv, NMQPF6, O2, Na2S2O3, NaOAc, DCE, PhMe, 45% yield of isolated product (75% based on recovered starting material); b) mCPBA, Sc(OTf)3, CH2Cl2, 68%; c) IBX, DMSO, 93%. IBX = 2-iodoxybenzoic acid, mCPBA=meta-chloroperoxybenzoic acid, Tf =tri-fluoromethanesulfonyl.