[4+3] Cycloadducts from Lewis Acid Mediated Reactions of Acroleins with Cyclopentadiene

Abstract

More than 20 years ago Sasaki et al. [1] reported the Lewis acid promoted cycloaddition of 2-silyloxyacrolein to cyclopentadiene, which led to the isolation of the diastereomeric [4+3] cycloadducts 1a and 1b (Scheme 1). A variation of the silyl substituent and of the Lewis acid made little difference (Scheme 1), although starting materials and products were, of course, easier to handle. [2] Davies and Dai [3] have now looked more closely at a series of Lewis acid promoted cycloadditions of cyclopentadiene to acroleins and obtained a variety of [4+3] cycloadducts. They also showed that a 5-norbornene-2-carbaldehyde is frequently implicated as an intermediate en route to the final [4+3] cycloadduct. For example, by using a microwave-assisted procedure, they first prepared Diels-Alder adduct 2, which was then subjected to the original reaction conditions (10 % Sc(OTf) 3 , CH 2 Cl 2 , 0 8C) [2] to obtain the 1a adduct (Scheme 2).Tricyclic Diels-Alder adducts 4 a and 4 b were prepared from cyclopentene-1-carbaldehyde (3). Adduct 4 a has a sterically accessible formyl group on the exo face of the molecule: upon treatment with AlCl 3 ring expansion to 5 a occurred. In contrast, the diastereomeric adduct 4 b with a more shielded endo formyl group regenerated the substituted acrolein 3 in what appears to be the reverse of the forward reaction, that is, a retro-Diels-Alder reaction (Scheme 3).Despite the intensive work on DielsAlder additions of acroleins to cyclopentadiene, Davies and Dai showed that a number of very simple alkylated acroleins, including the parent 2-methacrolein, afford formal [4+3] rather than kinetic [4+2] cycloadducts under appropriate Lewis acid catalysis. For example, the cage-expanded adducts 6a and 7aa were formed from 2-methacrolein and 2-methyl-2-butenal, respectively (Scheme 4). Upon subsequent treatment of the respective products with HCl, the more-stable equatorial 6b was formed predominantly and the diequatorial 7ba epimer was formed exclusively. The cis-decalin system 8 is formed in lower yield (21 %) than the five-membered-ring annulated 5 a (92 %) (Scheme 3). 2-Butenal, which lacks the a-alkyl group, furnished conventional Scheme 1. Reagents and conditions: a) SnCl 4 , CH 2 Cl 2 , À78 8C, 72 %; b) Sc(OTf) 3 (10 mol %), CH 2 Cl 2 , 08C, 72 %. TMS = trimethylsilyl; TIPS = triisopropylsilyl. Scheme 2. Scandium triflate mediated rearrangement. LA = Lewis acid.Scheme 3. Reagents and conditions: a) AlCl 3 (1.1 equiv), CH 2 Cl 2 , À78!0 8C, 2 h, 92 % 5 a; 84% 3.

Lewis Acid-Catalyzed Tandem Diels−Alder Reaction/Retro-Claisen Rearrangement as an Equivalent of the Inverse Electron Demand Hetero Diels−Alder Reaction

“…Recently, we reported a formal [4 + 3] cycloaddition between cyclopentadiene and α,β-unsaturated aldehydes (eq 1) . The reaction was shown to proceed by a [4 + 2] cycloaddition followed by a Lewis acid-catalyzed ring expansion.…”

Sequential Diels–Alder Reaction/Rearrangement Sequence: Synthesis of Functionalized Bicyclo[2.2.1]heptane Derivatives and Revision of Their Relative Configuration

“…The combination of Diels–Alder (DA) cycloaddition reactions with other transformations without isolating the intermediates allows a very rapid increase in the complexity of products. − For example, Lewis acid promoted rearrangements of DA products have been reported recently. − Davies and Dai showed that DA addition of alkylated acroleins with cyclopentadiene affords the formal [4 + 3] rather than the kinetic [4 + 2] cycloadducts under appropriate Lewis acid catalysis. , The reaction sequence comprises a domino DA and two consecutive 1,2-shifts, delivering bicyclo[3.2.1]­oct-6-en-3-ones 4 (Scheme , eq A). The rearrangement proved to be stereoselective with retained enantioselectivity introduced during the DA step .…”

Versatile Reactivity of Alkynyl Fischer Carbene Complexes toward Pentafulvenes: Carbocyclization and C−H Insertion Reactions

“…From a mechanistic point of view it is not a simple matter to disclose a feasible reaction pathway for the transformations of the [4+2] cycloadducts 5 and 6 into either the [4+3] cycloadducts 4 or the formal insertion product 9 , particularly if the retro-Diels−Alder reaction is excluded (see above). On the basis of the above-described rearrangement, we tentatively propose at the present stage a common intermediate II to account for both rearrangements (Figure ). First, the [1,2]-alkyl shift would produce the zwitterion intermediate I , which in turn would evolve into the species II by a subsequent [1,2]-M(CO) 5 migration.…”

“…Concerning the ring expansion process of tungsten cycloadducts ( 5 → 4 ), it should be noted that related metal-free norbornene cycloadducts suffer rearrangement under appropriate reaction conditions. Thus, the Lewis acid rearrangement of the norbornene [4+2] cycloadducts of cyclopentadiene and acroleins into the [4+3] cycloadducts has been reported recently as a powerful methodology to access bicyclo[3.2.1]octen-2-ones …”