Mechanistic investigation in the [1,4] and [1,2] Wittig rearrangement reactions: a DFT study

The mechanistic pathways for the [1,4] and [1,2] Wittig rearrangements of 2-silyl-6-aryl-5,6-dihydro-(2H)-pyrans have been studied at the M06-2X/6-31+G(d,p), 6-311++G(d,p) and cc-pVTZ level of theory. The crucial C-O bond cleavage step in the mechanism has been analysed initially, using two model reactions covering aliphatic as well as cyclic allylic ethers. The barriers for the one-step as well as two-step pathways have been calculated and the mechanisms for both the [1,4] and [1,2] Wittig rearrangement reactions are predicted to occur through a two-step mode. An energetic analysis of the reaction pathways reveals that the [1,4]-rearrangement has a lower barrier than the [1,2]-Wittig rearrangement. The C-O cleavage transition state was found to have the highest barrier and is thus the rate determining transition state for all of the studied molecules. This is in agreement with the previously published experimental studies. The role of the allylic trimethylsilane group in the stabilization of the intermediate anions of the Wittig reactants has also been investigated while comparing it with the phenyl and allylic t-butyl groups through Natural Bond Orbital (NBO) calculations.

Research Progress of Polycyclic Polyprenylated Acylphloroglucinols

Polycyclic polyprenylated acylphloroglucinols (PPAPs) are a class of hybrid natural products sharing the mevalonate/methylerythritol phosphate and polyketide biosynthetic pathways and showing considerable structure and bioactivity diversity. This review discusses the progress of research into the chemistry and biological activity of 421 natural PPAPs in the past 11 years as well as in-depth studies of biological activities and total synthesis of some PPAPs isolated before 2006. We created an online database of all PPAPs known to date at http://www.chem.uky.edu/research/grossman/PPAPs . Two subclasses of biosynthetically related metabolites, spirocyclic PPAPs with octahydrospiro[cyclohexan-1,5'-indene]-2,4,6-trione core and complicated PPAPs produced by intramolecular [4 + 2] cycloadditions of MPAPs, are brought into the PPAP family. Some PPAPs' relative or absolute configurations are reassigned or critically discussed, and the confusing trivial names in PPAPs investigations are clarified. Pharmacologic studies have revealed a new molecular mechanism whereby hyperforin and its derivatives regulate neurotransmitter levels by activating TRPC6 as well as the antitumor mechanism of garcinol and its analogues. The antineoplastic potential of some type B PPAPs such as oblongifolin C and guttiferone K has increased significantly. As a result of the recent appearances of innovative synthetic methods and strategies, the total syntheses of 22 natural PPAPs including hyperforin, garcinol, and plukenetione A have been accomplished.

Asymmetric Dearomatization/Cyclization Enables Access to Polycyclic Chemotypes

Enantioenriched, polycyclic compounds were obtained from a simple acylphloroglucinol scaffold. Highly enantioselective dearomatization was accomplished using a Trost ligand-palladium(0) complex. A computational DFT model was developed to rationalize observed enantioselectivities and revealed a key reactant-ligand hydrogen bonding interaction. Dearomatized products were used in visible light-mediated photocycloadditions and oxidative free radical cyclizations to obtain novel polycyclic chemotypes including tricyclo[4.3.1.0^1,4]decan-10-ones, bicyclo[3.2.1]octan-8-ones and highly-substituted cycloheptanones.