Domino formal cycloadditions

Scheme 4.17 Domino formal cycloaddition of CO2 with propylene oxide.

The platinum-catalysed intramolecular domino annulation reaction of o-alkynylben-zaldehydes has been described as a versatile approach to naphthalenes with annulated carbocycles or heterocycles of various sizes (Scheme 32).94 A plausible mechanism for the platinum(II)-catalysed annulation reaction shows that the double annulation process most probably proceeds through the benzopyrylium cation (117), which results from the nucleophilic attack of the carbonyl oxygen at the alkyne, activated by the Lewis-acidic platinum salt. A subsequent intramolecular Huisgen-type 3 + 2-cycloaddition of the second alkyne is assumed to generate intermediate (118). Rearrangement to (119) and the formal 4 + 2-cycloaddition product (118) leads to the aromatized final (116), liberating the active catalyst. In the case of FeCl3 as the Lewis acid, we assume that intermediate (118) is oxidatively transformed to (121). 

The same group [50] developed a remarkable enantioselective synthesis of chiral indenylamines 179 via domino Rh(I)-catalyzed C-H activation/cyclization of aryl ketimines 177 and internal alkynes 170 (a formal [3+2] cycloaddition) (Scheme 3.46). A major challenge of this transformation was the requirement of a catalyst that could accommodate the many different steps in the reaction sequence and show selectivity for instance, imine-directed ortho C-H activation/... 

Lee and coworkers presented a facile synthesis of fused pyrans 116 in 65-90% yields in water by reacting a variety of cyclic 1,3-dicarbonyls 33 with a,fi-unsaturated aldehydes 115 (Scheme 12.45). The key step is a formal [3+3] cycloaddition though a domino Knoevenagel/6 [i-electrocyclization. This methodology has been applied to the synthesis of biologically interesting pyranocoumarin, pyranoquinolinone, and pyranonaphthoquinone derivatives [69]. 

Cooperative catalysis using cinchona alkaloid derivatives in combination with metals such as silver have also been widely developed. On the basis of this concept, Escolano et al. have disclosed an enantioselective domino Michael-cyclisation reaction. This formal [3 + 2] cycloaddition occurred between isocyanoacetates and enones in the presence of a combination of a chiral hifunctional cinchona alkaloid, such as cupreine, and AgNOs to provide the corresponding chiral 2,3-dihydropyrroles in low to high yields and... 

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