Fused cyclopentenone

A Nazarov cyclisation of l-(3,4-dihydro-2//-pyran-6-yl)-3-phenylpropenones affords cyclopenta[Z)]pyranones (Scheme 9) <03OL4931> and the conjugated ethoxytriene 7 derived from tetrahydropyran-2-one also yields a fused cyclopentenone <03JOC9728>. 

During the 1940s and 1950s Nazarov and his coworkers extensively studied the formation of allyl vinyl ketones by hydration of dienynes and their subsequent acid-catalyzed closure to 2-cyclopentenones (Scheme 1). The reaction was notable as one of the first general methods for the selective construction of isolated and, more importantly, ring-fused cyclopentenones. In many instances the dienynes could be transformed directly to cyclopentenones without isolation of intermediates. 

The efficient conversion of a ketone to a fused cyclopentenone of the general type outlined in Scheme 5 has been demonstrated by Paquette and coworkers.The overall strategy is illustrated in Scheme 6 and begins with Freidel-Crafts acylation of the cyclic vinylsilane (8) to give the enone (9), which undergoes... 

Rh-mediated intramolecular C-H insertion is effected by Rh2(OAc)4 to afford novel fused cyclic ketones.70 The ring fused cyclopentenone derivative (235) is developed based on Rh2(NHCOMe)4-catalyzed diazo decomposition reaction of 233 and 234.84... 

Intramolecular Pauson-Khand reaction. Fused cyclopentenones are formed. The reaction exhibits modest asymmetric induction when a chiral BINAP is present. 

Cycloadditions. Pyrolysis of 0-alkynyl cyclopropylcarbene complexes generates fused cyclopentenones. This transformation has been exploited in a synthesis of a vitamin Dj synthon. ... 

When the biscobalthexacarbonyl complex 17, prepared from 2-acetoxy-3,4,6-tri-O-acetyl-D-glucal, was treated with NMNO, a Pauson-Khand reaction occurred to give the fused cyclopentenone 18 in virtually quantitative yield. Treatment of 17 with triflic add caused epimerization at C-1, and the stereoisomer also underwent Pauson-Khand cyclization eflfidently. ... 

Branched-chain Sugars. - The oxazolidinone 35, and the novel fused cyclopentenone 36 (see Chapter 24 for synthesis). ... 

The spiro-fused cyclopentenone 31 has been prepared by 3-aza-Cope rearrangement, intramolecular Mannich reaction, p-elimination reaction sequence from 30. A mixture of enamine stereoisomers in the initial condensation leads to a mixture of spirocyclic epimers in 31 (Scheme 5). ... 

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