Cyclopentadiene oxyallyl cation

A one-step Lewis acid-catalysed intermolecular 4- -3-cycloaddition of aromatic a,)3-unsaturated aldehyde and ketones (105) with epoxides (106) formed seven-membered oxacycles (107) under mild conditions (Scheme 34).The effect of oxygen-, sulfur-, and halogen-substituents on the reactivity of nitrogen-stabilized oxyallyl cations in 4- -3-cycloaddition reactions has been extensively investigated. Aza-oxyallyl cationic intermediates react with cyclopentadiene and furan via an aza-4 -I- 3-cycloaddition reaction to form bicyclic cycloadducts in moderate yields. The intramolecular formal 4- -4-cycloaddition of conjugated enynes with an e-deflcient cyclobutene (108) yielded a strained six-membered cyclic allene (109) that isomerized to a 1,3-cyclohexadiene (110). This intermediate underwent a thermal or acid-promoted six-electron electrocyclic ring opening to yield a 2,4,6-cyclooctatrienone (111) (Scheme 35). ... 

Cycloadditions and Rearrangements. The addition of 2-oxyallyl cations to furan provides a route to oxabicyclo[3,2,l]octanes complementary to the cycloaddition of cyclopropanones to furan. Careful experimental studies have led to yields of preparative importance both with furan and with cyclopentadiene. Following the route to azabicyclo-octanes, dipolar addition to the pyrylium betaine (99) affords oxa-analogues (Scheme 23). Also reported are the addition of fiiran to 1-cyanonaphthalene, the formation of various cycloadducts of tropone and tropolone (Scheme 24), and the phototransformations of (100) (Scheme 25) and (101) (Scheme 26). Thermal addition gives (102) from (103) and similarly other 8-oxabicyclo-octanes are prepared from acyclic precursors. ... 

The reactions of the family of oxyallyl cations 5 with cyclopentadiene as diene can also be compared to study the... 

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