Cyclopent-2-enol ring

Haiogenobenzene from cyclopent-2-enol ring Metacyclophanes... 

The study of the Diels-Alder adducts of levopimaric acid has continued with an examination of the structure and stereochemistry of the adducts with cyclopentenone and cyclopent-l-ene-3,5-dione. The major product with cyclo-pentenone is the endo,cis adduct (83). The enedione gives a mixture of enolic endo,cis adducts whose stereochemistry was determined by photocyclisation to give compounds such as (84). A correlation was also achieved with the benzo-quinone adduct through a Favorskii-type ring-contraction of the epoxide (85) to (86). The 13(14) double bond of these adducts is hindered to oxidation, excepting the adduct with acetylenedicarboxylic acid. Condensation of 12-hydroxymethylabiet-7(8)-en-18-oic acid with formaldehyde gives the cyclic ether (87). 

The capnellane (109) has been synthesized, using two dififerent annulation procedures (Scheme 11). The a/b ring junction was formed through consecutive Michael and ene-type reactions by addition of a lithium cuprate to 3-methyl-cyclopent-2-enone and trapping of the enolate anion that is thus formed as the enol acetate (110). Lewis-acid-induced cyclization then formed the ketone (111). 

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