Silicon in organic synthesis. 16. A short synthesis of (+±)-α-vetispirene

Benefits of Unconventional Methods in the Total Synthesis of Natural Products

This article provides a survey of four "unconventional" methods employed in the synthesis of natural products in the Hudlicky group. The utility of flash vacuum pyrolysis is highlighted by examples of many natural products attained via vinylcyclopropane-cyclopentene rearrangement and its heterocyclic variants. Preparative organic electrochemistry was used in oxidations and reductions with levels of selectivity unattainable by conventional methods. Yeast reduction of ketoesters was featured in the total synthesis of pyrrolizidine alkaloids. Finally, the use of toluene dioxygenase-mediated dihydroxylations in enantioselective synthesis of natural products concludes this presentation. Recently, synthesized targets in the period 2010-2019 are listed in the accompanying table. The results of research from the Hudlicky group are placed in appropriate context with the work of others, and a detailed guide to the current literature is provided.

A computational triage approach to the synthesis of novel difluorocyclopentenes and fluorinated cycloheptadienes using thermal rearrangements

Electronic structure calculations have been used for the effective triage of substituent effects on difluorinated vinylcyclopropane precursors and their ability to undergo vinyl cyclopropane rearrangements (VCPR). Groups which effectively stabilised radicals, specifically heteroarenes, were found to result in the lowest energy barriers. Ten novel precursors were synthesised to test the accuracy of computational predictions; the most reactive species which contained heteroarenes underwent thermal rearrangements at room temperature to afford novel difluorocyclopentenes and fluorinated benzocycloheptadienes through competing VCPR and [3,3]-rearrangement pathways, respectively. More controlled rearrangement of ethyl 3-(1'(2'2'-difluoro-3'-benzo[d][1,3]dioxol-5-yl)cyclopropyl)propenoate (22) allowed these competing pathways to be monitored at the same time and activation energies for both reactions were determined; E_a(VCPR) = (23.4 ± 0.2) kcal mol^-1 and E_a([3,3]) = (24.9 ± 0.3) kcal mol^-1. Comparing our calculated activation energies with these parameters showed that no single method stood out as the most accurate for predicting barrier heights; (U)M05-2X/6-31+G* methodology remained the best for VCPR but M06-2X/6-31G* was better for the [3,3]-rearrangement. The consistency observed with (U)B3LYP/6-31G* calculations meant that it came closest to a universal method for dealing with these systems. The developed computational design model correctly predicted the observed selectivity of rearrangement pathways for both our system and literature compounds.

An entry to vinylcyclopropane through palladium-catalyzed intramolecular cyclopropanation of alkenes with unstabilized allylic tosylhydrazones

Intramolecular cyclopropanation of unstabilized allylic tosylhydrazones with different alkenes was developed in the presence of a Pd catalyst.