A new access to 2′-amino-substituted vinylindoles as donor-activated heterocyclic dienes and their first diels-alder reactions

Multicomponet Synthesis of Pyrrolo [3,4-a] Carbazole-1,3-Diones

“…Notably, the Knoevenagel condensation of cyclic 1,3dicarbonyls (12) and aldehydes (13) can readily generate α,β-unsaturated carbonyl compounds (14). Cycloaddition of these resulting heterodienes (14) with isocyanides (5) is known to afford 2aminofuranes (15) [20,21], and subsequent Diels-Alder reaction with dienophiles (6) present in the reaction medium could purportedly afford tricyclic isoindoles (16) in the same pot (Scheme 4). Thus, equimolar amounts of dimedone (12), p-nitrobenzaldehyde (13), cyclohexyl isocyanide (5b) and N-phenylmaleimide (6) were stirred in refluxing acetonitrile for 22 h, affording the expected.…”

“…Surprisingly, little attention has been paid to the synthesis of pyrrolo [3,4-a] carbazoles, in spite of the many reports on the synthesis of some of their isomers, such as 3,6-dihydropyrrolo [2,3-c] carbazoles [15]. Most of the methods to obtain [3,4-a] carbazoles are based on the Diels-Alder cycloaddition of 3-vinylindoles [13,14,16,17], which often requires harsh reaction conditions and nonreadily available starting materials [12,13,15,16]. Thus, it is vital to develop new efficient and flexible syntheses of these important heterocyclic structures.…”

Diastereoselective synthesis of functionalised carbazoles via a sequential Diels–Alder/ene reaction strategy

“…The vinyl-indole synthesis of carbazoles, originally developed by Nolan, Pindur, Porter and others, involves the D-A cycloaddition of a 2-or 3-vinyl-1H-indole (typically either unprotected or containing an electron donating N-protecting group) with a dienophile. [10][11][12][13][14][15][16][17][18][19][20][21][22] The resulting D-A cycloadducts are oen unstable and are therefore typically oxidised or undergo an in situ 1,3-H shi to rearomatise the indole. We postulated that if the intermediate D-A cycloadduct could instead be intercepted via an alternative intermolecular reaction this would provide a new multi-component route to functionalised carbazoles.…”

“…PAPER N-protecting group as we postulated that this would stabilise the desired D-A cycloadducts sufficiently to allow either isolation or further in situ chemistry. 19,20 Despite the extensive body of work that has been published on the vinyl-indole synthesis of carbazoles, [11][12][13][14][15][16][17][18] the incorporation of electron withdrawing Nprotecting groups has been less well studied with the phenylsulfonyl group being the most common. 25,[30][31][32][33] We therefore decide to focus our initial investigation on tosyl protected systems and embarked on the synthesis of two Ntosyl protected 3-alkenyl-indoles through reaction of 1Hindole-3-carbaldehyde with tosyl chloride, followed by a Wittig reaction with methylenetriphenyl-l 5 -phosphane or ethyl 4-(triphenyl-l 5 -phosphanylidene)butanoate to give 1a and 1b respectively (Scheme 1).…”