The past couple of decades have witnessed a surge in the popularity of allenes as viable building blocks for a wide variety of synthetic applications.1 Their versatility allows them to participate in nucleophilic and electrophilic additions, cycloadditions and cyclizations, as well as various palladium- and gold-catalyzed reactions. In addition, the allene is present in many bioactive natural compounds and pharmaceutical reagents.
Bolte and Gagosz have recently accomplished the hydroalkylation of allenyl ethers to generate various fused and spiro tetrahydrofurans and tetrahydropyrans when catalyzed by a gold(I) phosphite complex or Brønsted acid (Scheme 1). The Brønsted acid catalyzed reaction proceeds under mild conditions and in a stereoselective manner, forming two adjacent asymmetric centers. Interestingly, when the gold(I) catalyst is employed, the product selectivity is reversed and the fused product dominates.2
Scheme 1.Hydroalkylation of allenyl-containing ethers
A recent report from Poonoth and Krause described the cycloisomerization of allenic hydroxyl ketones to form 3(2H)-furanones by aqueous NaOH at room temperature (Scheme 2).3 The reaction proceeds smoothly without any heating or cooling, nor does it require any metal catalysts.
Scheme 2.Cycloisomerization of allenes to form 3(2H)-furanones
The Krische group at the University of Texas at Austin has reported the ruthenium-catalyzed hydrohydroxyalkylation of 1,1-disubstituted allenes under various conditions to form quaternary centers with excellent levels of anti-diastereoselectivity (Scheme 3).4 Similarly, Lu and coworkers have devised an enantioselective [3+2] cycloaddition of allenes to acrylates to form cyclic scaffolds containing quaternary stereogenic centers (Scheme 4).5 Various amino acid based phosphines were found to catalyze the reaction with the O-TBDPS-D-Thr-L-tert-Leu-derived variant outperforming the others.
Scheme 3.Hydrohydroxyalkylation of 1,1-disubstituted allenes
Scheme 4.Enantioselective [3+2] cycloaddition of allenes to acrylates
Ryu and coworkers have reported a regioselective radical bromoallylation of allenes that generates 2-bromo-1,5-dienes when AIBN is used as the initiator (Scheme 5)6. Under similar conditions, Kwon and coworkers have demonstrated the triphenylphosphine-catalyzed b’-umpolung addition of nucleophiles to activated a-alkyl allenoates to produce functionalized alkenes with high levels of E/Z selectivity (Scheme 6).7
Scheme 5.Generation of 2-bromo-1,5-dienes via radical bromoallylation
Scheme 6.b’-Umpolung addition of nucleophiles to activated allenes
Fujii, Ohno, and coworkers have recently disclosed the enantioselective total synthesis of (+)-lysergic acid and related indole alkaloids wherein a key feature of the synthesis is a Pd(0)-catalyzed domino cyclization of an allene that contains amino and bromoindolyl moieties (Scheme 7).8 This cyclization forms the C and D rings of the alkaloid skeleton and proceeds with good diastereoselectivity (dr = 92:8).
Scheme 7.Pd(0)-catalyzed domino cyclization of allene in the synthesis of (+)-lysergic acid