Homo Diels-Alder reaction intramolecular

Cycloaddition. The title complex is employed to catalyze intramolecular homo-Diels-Alder reaction/... 

Intramolecular homo Diels-Alder reaction of dienynes. Norbornadicnes bearing... 

Intramolecular homo Diels-Alder reaction of dienynes. Norbomadienes bearing a tethered alkyne group at C2 undergo an intramolecular homo Diels-Alder reaction when treated with 8 mol% of Co(acac)2, (C2H5)2A1CI (4 equiv.), and dppe at 25°. This reaction is particularly facile when a 5- or 6-membered ring is formed. No reaction is observed on thermal treatment of 1 at 140-170°. 

In the [4 + 2] cycloadditions discussed so far, the enol ether double bond of alkoxyallenes is exclusively attacked by the heterodienes, resulting in products bearing the alkoxy group at C-6of the heterocycles. This regioselective behavior is expected for [4+2] cycloadditions with inverse electron demand considering the HOMO coefficients of methoxyallene 145 [100]. In contrast, all known intramolecular Diels-Alder reactions of allenyl ether intermediates occur at the terminal C=C bond [101], most probably because of geometric restrictions. 

One sure way to make the hetero Diels-Alder reaction go well and with regio-and stereoselectivity is to make it intramolecular.14 The highly reactive diene 90 is formed by elimination with fluoride ion from 89. The cycloaddition is rapid because it restores aromaticity. The regiose-lectivity is as expected from HOMO of dienophile plus LUMO of diene but is probably controlled by the tether.15 The stereochemistry certainly is the azadiene is attached to the bottom face of the five-membered ring and is delivered to the bottom face of the dienophile to give the all-trans aza-oestrone 91. 

At higher temperatures (290°C), the 4.2.1 triene gives c/5 -8,9-dihydroindene by at least two pathways, the dominant of which (70%) results from an intramolecular Diels-Alder reaction to tetracyclo[4.2.1.0. 0 ]nonene followed by a homo-1,5-hydrogen shift to give tricyclo[4.3.0.0 ]nona-4,7-diene followed by another homo-1,5-hydrogen shift as revealed by deuterium and carbon-13 labeling in separate experiments summarized in Scheme 10.22. ... 

INTRAMOLECULAR HOMO-CYCLOADDITIONS (DIELS-ALDER REACTIONS)... 

Chiral enamine, dienamine, and trienamine intermediates have been shown to catalyse asymmetric Diels-Alder reactions via HOMO activation. These Diels-Alder reactions generally occur with high chemo-, regio-, and stereo-selectivities. ° The effect of the viscosity of pyridinium-based ionic liquids, with tetrafluoroborate and bis(trifluorosulfonimide) anions, on the kinetics of intra- and inter-molecular Diels-Alder reactions has been investigated. Results show that the intramolecular cycloaddition reaction is less susceptible to viscosity variations than the intermolecular reaction. 

In addition to cationic cyclizations, other conditions for the cyclization of polyenes and of ene-ynes to steroids have been investigated. Oxidative free-radical cyclizations of polyenes produce steroid nuclei with exquisite stereocontrol. For example, treatment of (259) and (260) with Mn(III) and Cu(II) afford the D-homo-5a-androstane-3-ones (261) and (262), respectively, in approximately 30% yield. In this cyclization, seven asymmetric centers are established in one chemical step (226,227). Another intramolecular cyclization reaction of iodo-ene poly-ynes was reported using a carbopaUadation cascade terminated by carbonylation. This carbometalation—carbonylation cascade using CO at 111 kPa (1.1 atm) at 70°C converted an acycHc iodo—tetra-yne (263) to a D-homo-steroid nucleus (264) [162878-44-6] in approximately 80% yield in one chemical step (228). Intramolecular aimulations between two alkynes and a chromium or tungsten carbene complex have been examined for the formation of a variety of different fiised-ring systems. A tandem Diels-Alder—two-alkyne annulation of a triynylcarbene complex demonstrated the feasibiHty of this strategy for the synthesis of steroid nuclei. Complex (265) was prepared in two steps from commercially available materials. Treatment of (265) with Danishefsky s diene in CH CN at room temperature under an atmosphere of carbon monoxide (101.3 kPa = 1 atm), followed by heating the reaction mixture to 110°C, provided (266) in 62% yield (TBS = tert — butyldimethylsilyl). In a second experiment, a sequential Diels-Alder—two-alkyne annulation of triynylcarbene complex (267) afforded a nonaromatic steroid nucleus (269) in approximately 50% overall yield from the acycHc precursors (229). 

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