Allenes in Diels–Alder Cycloadditions

Abstract

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 Introduction2 Allenic Dienes2.1 Vinylallenes2.2 Bisallenes2.3 Cross-conjugated Allenes3 Allenic Dienophiles4 Intramolecular Diels–Alder Cycloadditions5 Allenic Cycloadducts6 Conclusions and Outlook

Diastereoselective, Lewis acid-mediated Diels–Alder reactions of allenoic acid derivatives and 1,3-cyclopentadienes

“…The second issue is less straightforward to solve and has, to the best of our knowledge, not yet been sufficiently addressed in previous work on the Diels–Alder reactions of allenoic acid derivatives. 3,8,9 Yet, it would be desirable if one could control the position of the EWG either in the endo - or the exo -position at will.…”

Tricyclo[2.1.0.02,5]pent-3-ylidene: Stereoelectronic Control of Bridge-Flapping within a Nonclassical Nucleophilic Carbene

“…Tricyclo­[2.1.0.0 2,5 ]­pent-3-ylidene ( 1 ) − is an enigmatic carbene − accessible only, as yet, via computational modeling . The reactive intermediate has garnered much interest as a possible precursor to pyramidane ( 2 ), − ,, an elusive target featuring an inverted C atom. , Direct reaction of 1 to 2 has been posited to occur either by 1,3-C–H bond insertion via TS­( 1 / 2 ) a (Scheme , path a) or by C–C bond insertion via TS­( 1 / 2 ) b or TS­( 1 / 2 ) c (Scheme , paths b and c). , Pericyclic TS­( 1 / 2 ) b is unlikely (Scheme , path b), however, because a cheletropic reaction within 1 between its C3 atom and C1–C5 bond suffers from orbital misalignment. , In contrast, the elementary step 1 → 2 via zwitterionic TS­( 1 / 2 ) c (Scheme , path c), , which stems from C–C bond heterolysis, is the likely mechanism because TS­( 1 / 2 ) c is lower in energy than TS­( 1 / 2 ) a .…”

Total synthesis of lissodendoric acid A via stereospecific trapping of a strained cyclic allene

“…We questioned whether the azadecalin core and C8a quaternary stereocenter (for example, 7 ) could be introduced simultaneously by means of a [4+2] cycloaddition between cyclic allene 8 and 2-pyrone 9 (Fig. 1C) ( 31 ). The strain associated with cyclic allene 8 would be crucial to the success of the proposed reaction because such intermolecular cycloadditions of analogous cyclic alkene substrates are challenging ( 32 , 33 ).…”