Applications of α-alkoxyorganocuprate reagents in the regiospecific synthesis of cyclic homoaldol products

Asymmetric [4 + 3] cycloadditions between vinylcarbenoids and dienes: application to the total synthesis of the natural product (-)-5-epi-vibsanin E

The total synthesis of (-)-5-epi-vibsanin E (2) has been achieved in 18 steps. The synthesis combines the rhodium-catalyzed [4 + 3] cycloaddition between a vinylcarbenoid and a diene to rapidly generate the tricyclic core with an effective end game strategy to introduce the remaining side-chains. The [4 + 3] cycloaddition occurs by a cyclopropanation to form a divinylcyclopropane followed by a Cope rearrangement to form a cycloheptadiene. The quaternary stereogenic center generated in the process can be obtained with high asymmetric induction when the reaction is catalyzed by the chiral dirhodium complex, Rh(2)(S-PTAD)(4).

Total Synthesis of (±)-Vibsanin E

Vibsanin E (1), a structurally rare complex diterpene, consisting of a compact 3-oxatricyclo[6.3.2.05,10] tridecane core and an unprecedented 3,3-dimethylacroyl enol ester functional group, formulate a considerable synthetic challenge. Williams and Davies failed to independently synthesize this nemesis, however, a ‘two heads are better than one’ approach delivered the first total synthesis of the molecule, since its diamond aniversary isolation.

Total Synthesis of Sordaricin

An enantioconvergent total synthesis of sordaricin (3), the diterpene aglycon of an important class of antifungal compounds, is described. Two approaches were explored, the first of which utilized a possible biogenetic intramolecular [4 + 2] cycloaddition to form the complete carbon skeleton of the target molecule as a single regioisomer 30. A second approach employed a tandem cycloreversion/intramolecular [4 + 2] cycloaddition process to afford not only the desired product 30 but also significant quantities of the undesired regioisomer iso-30. An investigation into the reasons for the difference in regioselectivity between these two reactions revealed the intervention of a cycloreversion/cycloaddition pathway at elevated temperatures leading to the formation of iso-30. Experimental evidence supports the hypothesis that iso-30 is the more thermodynamically stable of the two regioisomers.

Reactivity and Enantioselectivity in the Reactions of Scalemic Stereogenic α-(N-Carbamoyl)alkylcuprates

Stereogenic 2-(N-carbamoyl)pyrrolidinylcuprates prepared from scalemic (i.e., enantioenriched) N-Boc-2-lithiopyrrolidine and THF soluble CuCN.2LiCl react with vinyl iodides, vinyl triflates, beta-iodo-alpha,beta-enoates, propargyl mesylates, and allyl bromide to afford the substitution products with excellent enantioselectivity. Excellent enantiomeric ratios are obtained in the conjugate addition reactions with methyl vinyl ketone while low enantiomeric ratios can be achieved with acrylate esters using HMPA/TMSCl activation. Enantiomeric ratios vary with substrate substitution patterns and the observed enantioselectivities appear to be more a function of cuprate-electrophile reactivities than of the reaction type (e.g., substitution, conjugate addition). Low enantiomeric ratios are obtained with the alpha-(N-carbamoyl)benzylcuprates. The lithium-copper transmetalation and cuprate vinylation reactions proceed with retention of configuration.

Total synthesis of sordaricin

[structure: see text] The total synthesis of sordaricin, the diterpene aglycone of an important class of antifungal compounds, is described. Two approaches were explored, the first of which utilized a possible biogenetic intramolecular [4 + 2] cycloaddition to form the complete carbon skeleton of the target molecule. A second approach using a tandem cycloreversion/intramolecular [4 + 2] cycloaddition sequence is also detailed.