Studies on Thermal Reactivity of β-(1,2-Allenyl)butenolides and 2-Allyl-3-allenylcyclohex-2-enones

Allenes in Diels–Alder Cycloadditions

For a long time, allenes—and cumulenic systems in general—played a relatively minor role in Diels–Alder cycloadditions. This situation has changed, since allenes are more readily available and as their unique stereochemical features in [4+2]cycloadditions are more widely recognized. This review presents a comprehensive overview of allenes in Diels–Alder processes using selected examples. Allenes in dienes, dienophiles and cycloadducts are covered, inter- and intramolecular Diels–Alder cycloadditions are discussed, and stereochemical features of the addition process are described. Areas of emerging importance are also covered, including allenic components in dehydro-Diels–Alder processes, and dendralenic allenes in Diels–Alder sequences for the rapid generation of target-relevant molecular complexity. Preparatively useful methods for allenic precursor synthesis are also discussed.

1 Introduction

2 Allenic Dienes

2.1 Vinylallenes

2.2 Bisallenes

2.3 Cross-conjugated Allenes

3 Allenic Dienophiles

4 Intramolecular Diels–Alder Cycloadditions

5 Allenic Cycloadducts

6 Conclusions and Outlook

Tetravinylallene

The first chemical synthesis of tetravinylallene (3,5-divinylhepta-1,3,4,6-tetraene) is reported. The final, key step of the synthesis involves a palladium-catalyzed, Negishi-type cross-coupling involving 1,5-transposition of a penta-2-en-4-yn-1-ol methanesulfonate. The unprecedented fundamental hydrocarbon is sufficiently stable to be purified by flash chromatography. A similar synthetic pathway grants access to the first substituted tetravinylallenes, which provide insights into the influence of substitution upon stability and reactivity. Tetravinylallenes are shown to break new ground in swift structural complexity creation, with three novel sequences reported.

2,3-Allenoic acids via palladium-catalyzed carboxylation of propargylic alcohols

The first example of Pd-catalyzed carboxylation of propargylic alcohols in the presence of CO (1 atm) and water affording 2,3-allenoic acids has been developed.

Palladium-catalyzed cyclization reactions of allenes in the presence of unsaturated carbon-carbon bonds

Modern synthetic chemists have looked for rapid and efficient ways to construct complex molecules while minimizing synthetic manipulation and maximizing atom-economy. Over the last few decades, researchers have made considerable progress toward these goals by taking full advantage of transition metal catalysis and the diverse reactivities of allenes, functional groups which include two cumulative carbon-carbon double bonds. This Account describes our efforts toward the development of Pd-catalyzed cyclization reactions of allenes in the presence of compounds that contain unsaturated carbon-carbon bonds such as alkenyl halides, simple alkenes, allenes, electron-deficient alkynes, or propargylic carbonates. First, we discuss the coupling-cyclization reactions of allenes bearing a nucleophilic functionality in the presence of alkenyl halides, simple alkenes, functionalized and nonfunctionalized allenes, or electron-deficient alkynes. These processes generally involve a Pd(II)-catalyzed sequence: cyclic nucleopalladation, insertion or nucleopalladation, and β-elimination, reductive elimination, cyclic allylation or protonation. We then focus on Pd(0)-catalyzed cyclization reactions of allenes in the presence of propargylic carbonates. In these transformations, oxidative addition of propargylic carbonates with Pd(0) affords allenylpalladium(II) species, which then react with allenes via insertion or nucleopalladation. These transformations provide easy access to a variety of synthetically versatile monocyclic, dumbbell-type bicyclic, and fused multicyclic compounds. We have also prepared a series of highly enantioenriched products using an axial-to-central chirality transfer strategy. A range of allenes are now readily available, including optically active ones with central and/or axial chirality. Expansion of these reactions to include other types of functionalized allenes, such as allenyl thiols, allenyl hydroxyl amines, and other structures with differing steric and electronic character, could allow access to cyclic skeletons that previously were difficult to prepare. We anticipate that other studies will continue to explore this promising area of synthetic organic chemistry.

One-pot approach to installing eight-membered rings onto indoles

Ring fusion: The Pd(0)-catalyzed reaction of 2-allyl-3-iodo-1-tosyl-1H-indoles and propargylic bromides affords dihydrocycloocta[b]indoles (see scheme; M.S. = molecular sieves, TFP = tris(2-furyl)phosphine, Ts = 4-toluenemethanesulfonyl), and proceeds by carbon-carbon coupling, [1,5]-hydrogen migration, and electrocyclization. The newly established method was used to efficiently access iprindole.

An efficient and practical entry to 2-amido-dienes and 3-amido-trienes from allenamides through stereoselective 1,3-hydrogen shifts

Preparations of de novo acyclic 2-amido-dienes and 3-amido-trienes through 1,3-hydrogen shifts from allenamides are described. These 1,3-hydrogen shifts could be achieved thermally or they could be promoted by the use of Brønsted acids. Under either condition, these processes are highly regioselective in favour of the α-position, and highly stereoselective in favour of the E-configuration. In addition, 6π-electron electrocyclic ring-closure could be carried out with 3-amido-trienes to afford cyclic 2-amido-dienes, and such electrocyclic ring-closure could be rendered in tandem with the 1,3-hydrogen shift.

Torquoselective ring closures of chiral amido trienes derived from allenamides. A tandem allene isomerization-pericyclic ring-closure-intramolecular Diels-Alder cycloaddition

A new torquoselective ring-closure of chiral amide-substituted 1,3,5-hexatrienes and its application in tandem with [4 + 2] cycloaddition are described. The trienes were derived via either a 1,3-H or 1,3-H-1,7-H shift of alpha-substituted allenamides, and the entire sequence through the [4 + 2] cycloaddition could be in tandem from allenamides.

Electrophilic addition and cyclization reactions of allenes

Modern organic synthesis depends on the development of highly selective methods for the efficient construction of potentially useful target molecules. A primary goal in our laboratory is the discovery of new reactions that convert readily available starting materials to complex products with complete control of regio- and stereoselectivity. Allenes are one underused moiety in organic synthesis, because these groups are often thought to be highly reactive. However, many compounds containing the allene group, including natural products and pharmaceuticals, are fairly stable. The chemistry of allenes has been shown to have significant potential in organic synthesis. Electrophilic additions to allenes have often been considered to be synthetically less attractive due to the lack of efficient control of the regio- and stereoselectivity. However, this Account describes electrophilic reactions of allenes with defined regio- and stereoselectivity developed in our laboratory. Many substituted allenes are readily available from propargylic alcohols. Our work has involved an exploration of the reactions of these allenes with many different electrophiles: the E- or Z-halo- or seleno-hydroxylations of allenyl sulfoxides, sulfones, phosphine oxides, carboxylates, sulfides or selenides, butenolides, and arenes, and the halo- or selenolactonization reactions of allenoic acids and allenoates. These reactions have produced a host of new compounds such as stereodefined allylic alcohols, ethers, amides, thiiranes, and lactones. In all these reactions, water acts as a reactant and plays an important role in determining the reaction pathway and the stereoselectivity. The differing electronic properties of the two C=C bonds in these allenes determine the regioselectivity of these reactions. Through mechanistic studies of chirality transfer, isolation and reactivity of cyclic intermediates, (18)O-labeling, and substituent effects, we discovered that the E-stereoselectivity of some reactions results from the neighboring group participation of functional groups forming cyclic intermediates. We rationalize Z-stereoselectivity under other conditions by soft Lewis acid-base interactions and steric effects. These electrophilic reactions of allenes are efficient and useful methods for the synthesis of stereodefined alkenes and lactones, useful functionalities for synthesis.