Synthese angularer Tetraquinane durch Photo‐Thermo‐Metathese diastereomerenreiner Benzochinon–Tetrahydropentalen‐Addukte

Abstract

Synthesis of Angularly Annulated Tetraquinanes by Photo-Thermo Metathesis of Diastereomerically Pure Adducts Between Benzoquinone and Tetrahydropentalenes A retrosynthesis is presented for an angularly fused tetraquinane, the central step of which is the photo-thermo metathesis of a tetrahydropentalene - benzoquinone adduct. A sequence of Michael addition/condensation reaction gives the 3-methyl- l-phenyl-1,2-dihydropentalene (2) in 70% yield from benzalacetone and cyclopentadiene. Reaction with lithium aluminum hydride lead to a 1.0: 1.0:0.8 mixture of the 1,2,3,4-tetra-hydropentalenes 4a and 4b and the 1,2,3,5tetrahydropentalene 5. This reaction is highly diastereoselective (d.s. > 96%) and provides the cis compounds exclusively. Diels-Alder cycloadditon of 4a, b and 5 to benzoquinone also proceeds highly stereoselectively. The adducts 7a, b and 8 may be separated by fractional crystallization. Their relative configuration can be determined by means of 2D-COSY and NOE experiments. The endo arrangement is demonstrated by the photoreactivity of the cycloadducts. Gas-phase thermolysis (700°C/0.05 Torr) of the cage molecules 9a and 9b (from irradiation in benzene) gives the angularly annulated tetraquinanes 10a and 10b in 95 and 88% yield, respectively.