New methods for the construction of condensed five-membered ring systems continue to be developed at an accelerated pace. The challenges underlying this tremendous current upsurge of interest arise from several directions. One stems from the desire to elucidate and resolve those special problems associated with the incorporation of added strain not present when six-membered rings are mutually fused. The many structurally interesting polyquinane natural products isolated and characterized in recent years have provided a particularly delightful forum for application of various new synthetic protocols, many of which must equally well accommodate the particular stereochemical demands of each indi­ vidual target. Synthetic elaboration of a marvellous array of new unnatural molecules also holds continued fascination. In the past, we have attempted to keep others abreast of developments in this rapidly burgeoning area by authoring a pair of comprehensive reviews in Topics in Current Chemistry that appeared in 1979 [1] and 1984 [2]. During this period, others have also surveyed the developments in cyclopentannulation [3] and the cyclopentanoid field in general [4]. In the last couple of years, the pace at which new synthetic facets have been reported has become more frenetic than ever before. Accordingly, a suitable updating of the exciting newer findings was deemed appropriate and the present overview, which extends approximately to mid - 1986, was written. Once again, our hope is that compilations of this type will serve to stimulate imaginative new scientific ventures that will propel the field forward to still greater maturity.

Inhalt

I Introduction.- II New Synthetic Developments.- A Annulation Reactions.- 1 Acid-and Base-Promoted Cyclizations.- 2 Metal-Promoted Ring Closures.- 3 Free Radical Processes.- 4 [3 +2] Cycloaddition Methodology.- 5 Weiss-Cook Condensations.- 6 Intramolecular Diels-Alder Reactions.- 7 Cyclization of Diazo Compounds.- 8 Miscellaneous.- B Ring Expansion, Contraction, and Cleavage Processes.- C Pauson-Khand Reaction.- D Photochemical Approaches.- 1 Oxa-di-?-methane Entry.- 2 Meta Olefin Addition to Arenes.- E Rearrangement Routes to Polyquinanes.- 1 Thermochemical Pathways.- 2 Cleavage of Cage Molecules.- 3 Carbocation-Based Approaches.- 4 Photochemical Processes.- 5 Base-Promoted Isomerizations.- 6 Carbenic and Transition Metal-Catalyzed Schemes.- F Trapping of 1,3-Diyls.- G Transannular Cyclizations.- III Functional Group Manipulation Within Polyquinanes.- A Reactions Involving Ketonic Substrates.- B Carbocationic Processes.- C Reactions Involving Olefinic Centers.- D Miscellaneous.- IV Physical Data for, and Theoretical Analysis of, Polyquinanes.- A Crystal Structure Data.- B Photoelectron Spectra.- C Molecular Orbital Calculations.- D Miscellaneous.- V Molecules of Theoretical Interest.- A Pentalene.- B Semibullvalenes.- C Syn/anti-Sesquinorbornenes.- D Triquinacenes.- E Tricyclic [10]Annulenes.- F Fenestranes and Related Molecules.- G (D3)-Trishomocubanes and Congeners.- H Peristylanes.- I Chemistry Surrounding Dodecahedrane.- VI Natural Products Chemistry.- A Isolation and Physical Properties.- B Biosynthesis and Chemical Transformations.- VII Synthesis of Nonpolycyclopentanoid Natural Products by Way of Diquinane Intermediates.- A Allamcin.- B Boonein.- C BrefeldinA.- D Dendrobine.- E Forsythide Aglycone Dimethyl Ester.- F Ikarugamycin.- G Iridodial.- H Isoiridomyrmecin.- I Loganin and Analogues.- J Oplopanone.- K Plumericin.- L Trichodiene.- M Udoteatrial.- N Xylomollin.- VIII Synthesis of Diquinane Natural Products.- A Cedranoids.- B The Pentalenolactone Group.- C Quadrone and Terrecyclic Acid.- D Ptychanolide.- E Albene.- F Carbaprostacyclins.- G Ryanodine and the Ryanoid Insecticides.- IX Synthesis of Triquinane Natural Products.- A Linear Triquinanes.- 1 Hirsutene.- 2 The Capnellene Group.- 3 Coriolin.- 4 Hirsutic Acid.- B Angular Triquinanes.- 1 Isocomene Sesquiterpenes.- 2 Silphinene.- 3 Pentalenene and Pentalenic Acid.- 4 Senoxydene.- 5 Silphiperfolene and Congeners.- 6 Retigeranic Acid.- C Propellane Structures.- 1 Modhephene.- X References.