Enantiodivergent Synthesis of Allenes by Point‐to‐Axial Chirality Transfer

An enantiodivergent method for the synthesis of multiply substituted allenes is described. Highly enantioenriched, point‐chiral boronic esters were synthesized by homologation of α‐seleno alkenyl boronic esters with lithiated carbamates and eliminated to form axially chiral allene products. By employing either oxidative or alkylative conditions, both syn and anti elimination could be achieved with complete stereospecificity. The process enables the synthesis of either M or P allenes from a single isomer of a point‐chiral precursor and can be employed for the enantioselective assembly of di‐, tri‐, and tetrasubstituted allenes.     

Stereodivergent Hydroboration of Allenes

Full details of a stereodivergent hydroboration of allenes are reported. While hydroboration of an allene with 9‐BBN provided a thermodynamically stable (E)‐allylic alcohol after oxidative work‐up, the reaction of an identical allene with HB(Sia)₂ (disiamylborane) formed a (Z)‐allylic alcohol as the kinetic product. The developed conditions allowed for the synthesis of trisubstituted olefins in a highly stereoselective fashion, which is known to be challenging. The method was also applied to the stereodivergent synthesis of structural motifs such as skipped dienes and allylbenzenes, which are often embedded in biologically active natural products.     

1‐Phenyl‐1,2‐cyclohexadiene: Generation,Interception by Activated Olefins,Dimerisation and Trimerisation

Four possible precursors of 1‐phenyl‐1,2‐cyclohexadiene ( 2 ) were examined, namely, 6,6‐dibromo‐1‐phenylbicyclo[3.1.0]hexane, (1α,5α,6α)‐6‐bromo‐6‐fluoro‐1‐phenylbicyclo[3.1.0]hexane, 1‐bromo‐2‐phenylcyclohexene and 1‐bromo‐6‐phenylcyclohexene. All four compounds could be converted into 2 , as demonstrated by the products of the interception of 2 with activated olefins. Styrene, 1,1‐diphenylethene, indene, furan and 2,5‐dimethylfuran were employed as such. Whereas the first three gave [2+2] cycloadducts of 2 , the last two provided one [4+2] cycloadduct each. To create the [2+2] cycloadducts, the π bond of 2 that is more remote from the phenyl group reacted, whereas the π bond of 2 conjugated with the phenyl group exclusively produced the [4+2] cycloadducts. The generation of 2 in the absence of a trapping reagent brought about relatively good yields of a dimer or a trimer of 2 depending on the mode of the liberation of 2 . Being derivatives of triphenylene, the dimer as well as the trimer have unusual structures, thereby indicating that a phenyl group is participating in the formation of these compounds. The most surprising structure of the trimer was elucidated by X‐ray crystal diffraction. As to the mechanisms, diradical intermediates are proposed both for the cycloadditions and for the dimerisation. The initial steps of the latter seem to proceed also in the trimerisation.     

Facile synthesis of sulfur-substituted allenes by a three-component reaction

The three-component reaction of lithium alkylthiolate, 1-alkynylphosphine oxide and aldehyde in THF affords sulfur-substituted allenes in good to excellent yield.     

New synthesis of quinolines from lithiated allenes and isothiocyanates

A new route was proposed for the synthesis of quinolines based on the reaction of lithiated allenes with phenyl isothiocyanate.Translated frimIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 3024–3025, December     

Iron‐catalyzed Cross‐Coupling of Propargyl Carboxylates and Grignard Reagents: Synthesis of Substituted Allenes

Presented herein is a mild, facile, and efficient iron‐catalyzed synthesis of substituted allenes from propargyl carboxylates and Grignard reagents. Only 1–5 mol % of the inexpensive and environmentally benign [Fe(acac)₃] at ?20 °C was sufficient to afford a broad range of substituted allenes in excellent yields. The method tolerates a variety of functional groups.     

Synergistic Kinetic Resolution and Asymmetric Propargyl Claisen Rearrangement for the Synthesis of Chiral Allenes

The asymmetric propargyl Claisen rearrangement provides a convenient entry to chiral allene motifs. Herein, we describe the development of a kinetic resolution and asymmetric rearrangement of racemic propargyl vinyl ethers. This transformation afforded chiral allene products along with the enantiomerically enriched substrate in good yields with excellent diastereo‐ and enantioselectivity. The complete chirality transfer and facially selective rearrangement enabled the simultaneous construction of an axially chiral allenic unit and a quaternary carbon stereocenter.     

Stereodivergent and Protecting‐Group‐Free Synthesis of the Helicascolide Family: A Rhodium‐Catalyzed Atom‐Economical Lactonization Strategy

Natural products of polyketide origin, in particular small‐sized lactones often possess a very broad range of impressive biological activities. An efficient way to demonstrate the concise access to six‐membered lactones was emphasized as part of a stereodivergent and protecting‐group‐free synthesis of all three representatives of the helicascolide family. This strategy features an atom‐economical and highly diastereoselective rhodium‐catalyzed “head‐to‐tail” lactonization by an intramolecular addition of ω‐allenyl‐substituted carboxylic acids to terminal allenes, including the selective construction of a new stereocenter in the newly formed core structures. The excellent selectivities with which the helicascolide precursors were obtained are remarkable, thus resulting in an expeditious and highly efficient natural product synthesis.     

Synthesis of a Carbodicyclopropenylidene: A Carbodicarbene based Solely on Carbon

The first carbodicarbene stabilized by flanking cyclopropenylidenes is reported. Tetraphenylcarbodicyclopropenylidene ( 2 ) is accessed by deprotonation of the corresponding triafulvene cyclopropenium salt, and has been spectroscopically characterized in [D₈]THF solution at ?60 °C. Main‐group and transition‐metal complexes of 2 have been accessed, and have revealed the high sigma donating ability, and exclusive η¹ binding of this neutral all carbon ligand. Variable temperature NMR spectroscopy studies reveal varying degrees of free rotation in the flanking cyclopropenylidene groups of 2 in its coordination compounds.