Total syntheses of prenylflavonoids and polyketide-derived natural products
Concise syntheses of the natural products brosimones A and B have been achieved using sequential dehydrogenative Diels-Alder (DHDA) cycloadditions. The syntheses employ either Pt/C-cyclopentene or DDQ to effect dehydrogenation of prenylchalcone substrates in combination with silver nanoparticles (AgNP’s) to promote subsequent Diels-Alder cycloadditions. A concise, biomimetic approach to sorbiterrin A has been developed employing consecutive Michael additions of a 4-hydroxypyrone to a sorbicillinol derivative and silver nanoparticle-mediated bridged aldol/dehydration to construct the [3.3.1] ring system. The relative stereochemistry of sorbiterrin A was unambiguously confirmed by X-ray crystallographic analysis. Metal-catalyzed, double Claisen rearrangement of a bis-allyloxyflavone has been utilized to enable a concise synthesis of the hydrobenzofuro[3,2-b]chromenone core structure of the natural products sanggenon A and sanggenol F. In addition, catalytic, enantioselective [4+2] cycloadditions of 2’-hydroxychalcones have been accomplished using B(OPh)3/BINOL complexes. Asymmetric syntheses of the flavonoid Diels-Alder natural products sanggenons C and O have been achieved employing a stereodivergent reaction of a racemic mixture (stereodivergent RRM) involving [4+2] cycloaddition. Diaporine is a natural product containing a novel epoxyquinol dimer framework. An efficient annulation involving pyrone addition to a quinone has been developed for rapid assembly of the γ-naphthopyrone core structure. Dimerization was achieved through a Pd(II)-mediated dehydrogenative coupling. A natural product and precursor to diaporine, aurofusarin, was synthesized in excellent yield through an oxidation and demethylation sequence. In addition, diastereoselective epoxidation of aurofusarin was achieved using a phase transfer catalytic system.