Intramolecular [4+2] and [3+2] Cycloadditions in Organic Synthesis

Zahlreiche Beispiele intramolekularer Cycloadditionen von 1,3-Dienen, Nitronen und Azomethiniminen bezeugen den praparativen Wert dieser Varianten zur regio- und stereoselektiven Synthese anellierter sowie uberbruckter Ringsysteme. Die Gemeinsamkeiten, Unterschiede und Grenzen dieser Reaktionstypen werden systematisch umrissen.

Intramolekulare [4 + 2]‐ und [3 + 2]‐Cycloadditionen in der organischen Synthese

Complexation‐Initiated Intramolecular [4+2] Cycloaddition: Construction of Bridged‐Type Cycloadducts

A four step synthesis of Seychellene (1) is described. Intramolecular Diels-Alder reaction of dienon 2 furnished the tricyclic ketones 3 and 4 in good yield. Hydrogenation of 3 gave 5 and 6. The higher reactivity of 6 with methyllithium compared to 5 allowed the selective preparation of 8, which, on subsequent acid-catalyzed rearrangment afforded Seychellen in 57,5% yield. In an analogues manner epi-Seychellene was synthetized via the intermediates 5 and 7.

Synthese von (±) Seychellen†

Intramolecular Diels−Alder reactions of in situ generated masked o-benzoquinones are described. Oxidation of methyl vanillate (2) in the presence of allyl alcohol (1a), trans-crotyl alcohol (1b), cinnamyl alcohol (1c), and homoallyl alcohol (1d) resulted in the formation of masked o-benzoquinones 8a−d that underwent intramolecular Diels−Alder reactions under reaction conditions to furnish adducts 14a−d in 53−75% yields. This tandem oxidative acetalization−intramolecular Diels−Alder process was extended to other 2-methoxyphenols such as 2-methoxy-4-methylphenol (3), guaicol (4), methyl isovanillate (5), methyl syringate (6), and 2,6-dimethoxy-4-methylphenol (7) to obtain adducts 15a−d, 16a−c, 17a−d, 18a−d, and 19a−d, respectively. While intramolecular Diels−Alder reactions of the masked o-benzoquinones 10a, 12d, and 13d were found to be less efficient, the masked o-benzoquinones 9a−d, 10b, 10c, 11a−d, 12a−c, and 13a−c furnished the desired products in 38−80% yields. Masked o-benzoquinones 21a−d generated f...

Tandem Oxidative Acetalization−Intramolecular Diels−Alder Reactions of 2-Methoxyphenols. Simple Synthesis of Bicyclo[2.2.2]octenone Derivatives

Thermal rearrangement of trans-penta-2,4-dienyl mesityl ether (trans-1) gave, besides trans, cis-2 and trans, trans-2, the homotwistanone derivative 1,3,10-trimethyl-tricyclo[5.4.0.03,9]undeca-5,10-dien-2-one (3) as well as its isomer 1,3,9-trimethyl-tricyclo[5.3.1.03,8]undeca-5, 9-dien-2-one (4). The tricyclic ketones 3 und 4 were shown to be products of intramolecular Diels-Alder addition in the intermediate 2,4,6-trimethyl-6-(penta-2,4-dienyl)cyclohexa-2,4-dien-1-one (5). Preparation of 2,6-methyl-6-(3-methyl-penta-2,4-dienyl)-cyclohexa2,4-dien-1-one (6) and subsequent heating of 6 in benzene gave the homotwistanone derivative 1,3-6-trimethyl-tricyclo5.4.0.03,9]undeca-5,10-dien-2-one (7) together with its isomer 1,3,6-trimethyl-tricyclo[5.3.1.03,8]undeca-5,9-dien-2-one (8) in good yield.

Gy. Fráter

Papers

Intramolekulare Diels‐Alder‐Additionen in 6‐Methyl‐6‐(penta‐2,4‐dienyl)‐cyclohexa‐2,4‐dien‐1‐on‐Systemen. Vorläufige Mitteilung