Bicyclic acetals acid catalysis

The presence of the catalyst can also favor multiple Diels-Alder reactions of cycloalkenones. Two typical examples are reported in Schemes 3.6 and 3.7. When (E)-l-methoxy-1,3-butadiene (14) interacted with 2-cyclohexenone in the presence of Yb(fod)3 catalyst, a multiple Diels-Alder reaction occurred [21] and afforded a 1 1.5 mixture of the two tricyclic ketones 15 and 16 (Scheme 3.6). The sequence of events leading to the products includes the elimination of methanol from the primary cycloadduct to afford a bicyclic dienone that underwent a second cycloaddition. Similarly, 4-acetoxy-2-cyclopenten-l-one (17) (Scheme 3.7) has been shown to behave as a conjunctive reagent for a one-pot multiple Diels-Alder reaction with a variety of dienes under AICI3 catalysis, providing a mild and convenient methodology to synthesize hydrofluorenones [22]. The role of the Lewis acid is crucial to facilitate the elimination of acetic acid from the cycloadducts. The results of the reaction of 17 with diene... 

An alternative and useful method for intramolecular conjugate addition when the Michael donor is a ketone is the formation of an enamine and its reaction with a Michael acceptor. This can be advantageous as enamine formation occurs under reversible conditions to allow the formation of the product of greatest thermodynamic stability. Treatment of the ketone 40 with pyrrolidine and acetic acid leads to the bicyclic product 41, formed by reaction of only one of the two possible regio-isomeric enamines (1.51). Such reactions can be carried out with less than one equivalent of the secondary amine and have recently been termed organo-catalysis (as opposed to Lewis acid catalysis with a metal salt). The use of chiral secondary amines can promote asymmetric induction (see Section 1.1.4). 

The isomeric epoxy triflates 153 and 157 undergo triflate displacement-epoxide opening with the dianion of methyl propanoyl acetate. Reaction of 153 generates the epimeric bicyclic tetrahydrofurans 154 and 155, subsequent treatment with triflic acid leading to isomerization about the alkene bond to a mixture of 154-156. Isomeric 157 under similar conditions gives 158 and 159, with triflic acid catalysis leading to some of the alkene isomers 160 along with 158 (Scheme 32). ... 

Reactions of nitriles with nucleophilic reagents normally require catalysis by acids or bases. Heating iminodiethanols with aliphatic nitriles in the presence of catalytic amounts of sodium alkoxide leads to oxazolo[2,3-6Joxazoles ( bicyclic amide acetals ), in a single process via oxazolidine intermediates (equation 18) (73AG(E)1055>. 

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