Even after decades of use, allenes can still be classified as an underutilized functional group. However, great advances in the chemistry of allenes have been made since the 1970s when they were regarded as difficult to prepare and very reactive, and not commonly encountered. Focusing on a particular functional group at first glance may seem trivial and obvious, but the importance of these types of studies is underscored by the fact that numerous Nobel prizes have been awarded for the study “alkenes in synthesis”. More importantly, transformations involving allenes can provide new scaffolds that lead to an expansion of chemical space where currently nearly half of all existing organic compounds can be described by a mere 143 molecular frameworks. This data supports the notion that we tend towards compounds and functionalities with which we are familiar and understand, but this, nevertheless, creates a limitation to our knowledge of our field.
Graphical Abstract
Scheme 1: 1 gram scale reaction of allenol 1k.
Scheme 2: The oxidation of 1l and 1m under 1 atm of oxygen.
Graphical Abstract
Scheme 1: Rh(I)-catalyzed Pauson–Khand type reaction of 1 and 4.
Scheme 2: Formation of 4,4-bis(methoxycarbonyl)-6-methylbicyclo[4.3.0]non-1(9)-en-8-one (9g).
Graphical Abstract
Scheme 1: 1,3-Hydrogen shifts of allenes.
Scheme 2: Synthesizing amido-dienes from allenamides.
Scheme 3: Synthesis of 1-amido-dienes from allenamides.
Figure 1: X-ray Structure of 10b.
Figure 2: Proposed mechanistic models.
Scheme 4: A favored pro-E TS.
Scheme 5: Unexpected competing 1,7-hydrogen shifts.
Scheme 6: Applications in pericyclic ring-closure.
Scheme 7: Cyclic 2-amido-diene synthesis.
Graphical Abstract
Scheme 1: Synthesis of 5-aryl-11H-benzo[b]fluorenes via benzannulated enyne–allenes.
Scheme 2: Synthesis of 1,1'-binaphthyl-substituted 11H-benzo[b]fluorene 3c.
Scheme 3: Synthesis of 5-(2-methoxyphenyl)- and 5-[2-(methoxymethyl)phenyl]-11H-benzo[b]fluorene 13a and 13b.
Scheme 4: Synthesis of 5-(1-naphthyl)- and 5-(2-methoxy-1-naphthyl)-11H-benzo[b]fluorene 20a and 20b.
Scheme 5: Synthesis of 5-[2-(methoxymethyl)-1-naphthyl]-11H-benzo[b]fluorene 20c.
Scheme 6: Demethylation of 22b to form 5-(2-hydroxy-1-naphthyl)-11H-benzo[b]fluorene 24.
Graphical Abstract
Scheme 1: A new entry to substituted cross-conjugated trienes.
Scheme 2: A proposed reaction pathway.
Scheme 3: [4 + 2] cycloaddition reaction of 3a with PTAD and TCNE.
Graphical Abstract
Scheme 1: Gold-catalyzed intermolecular oxidation of alkynes and allenes.
Scheme 2: A side reaction from 1l.
Scheme 3: A proposed reaction mechanism.
Graphical Abstract
Scheme 1: Conversion of propargyl acetate 1 to spirooxindole 2 containing the core framework of welwitindolin...
Scheme 2: Preparation of enantiopure propargyl acetate 7 (R = Ac).
Figure 1: Chiral NMR shift analysis of propargyl acetate 7.
Figure 2: Chiral NMR shift analysis of allenyloxindole 8.
Scheme 3: Microwave irradiation of allenyloxindole 8.
Figure 3: Chiral NMR shift analysis of spirooxindole 9.
Figure 4: Thermally generated biradical intermediate 10.
Graphical Abstract
Scheme 1: One-pot hydrozirconation-reductive coupling of allene 2 and nitrile 7.
Scheme 2: Cyclization of allenylnitrile 18.
Figure 1: Coupling constant analysis of the Boc-protected aminopyran ring in 21.
Scheme 3: Proposed chelated transition state model.
Scheme 4: Conversion of homoallylic amines to β-amino acid derivatives.
Graphical Abstract
Figure 1: Known natural endocyclic allenes and related germacranes.
Scheme 1: C–C fragmentation strategy to yield endocyclic allenes.
Scheme 2: Endocyclic allene 17.
Scheme 3: Preparation of endocyclic allene 25.
Scheme 4: Bromo-olefination products from diketone aldehyde 19.