Evidence for Atropisomerism in Polycyclic γ-Butenolides: Synthesis, Scope, and Spectroscopic Studies

An approach to functionalized carbazoles from Z-enoate propargylic alcohols. A unified total synthesis of N-Me-carazostatin, N-Me-carbazoquinocin C and N-Me-lipocarbazole A4

Development of an acid catalyzed, intramolecular benzannulation of indoles for the synthesis of functionalized carbazoles has been reported. The indole appended Z-enoate propargylic alcohols have been employed. The N-EDG-indoles involve the 5-exo-dig cyclization followed by 1,2-migration to give the carbazole-butenoates, whereas the N-EWG-indoles undergo the Z-enoate assisted Meyer-Schuster rearrangement to give the dihydrocarbazole-4-oxo-butanoates. Utilizing one of the 2-methyl-carbazole-butyraldehyde (obtained from the corresponding carbazole-butanoate) as the key intermediate, we have developed a simple approach for an efficient synthesis of N-Me-carazostatin, N-Me-carbazoquinocin C and N-Me-lipocarbazole A4.

TfOH catalysed domino-double annulation of arenes with propargylic alcohols: a unified approach to indene polycyclic systems

The design and development of a TfOH catalysed domino strategy for the double annulation of arenes with propargylic alcohols for the rapid generation of indene based polycyclic systems is reported. The dehydration, intramolecular 6-endo-dig hydroarylation, and cationic cyclization were consecutively promoted by TfOH. The key features of this strategy are the formation of two C-C bonds, unified access to indene polycyclic systems, excellent yields (up to 95%), high atom economy (>90%), an operationally simple procedure, and water being the only byproduct. By extending this strategy, a two-step synthesis of the pentacyclic systems of hypoxylonol A (43% overall yield from α-tetralone), daldinone A (63% overall yield from β-tetralone) and spiro-tetracyclic framework of incarviatone A has also been achieved.

An Approach for the Generation of γ-Propenylidene-γ-butenolides and Application to the Total Synthesis of Rubrolides

Design and synthesis of a new class of γ-butenolides, viz. β-aryl-γ-propenylidene-γ-butenolides, have been reported from β-aryl-Z-enoate propargylic alcohols in the presence of acid. Isolation of β-aryl-γ-propenylidene-γ-butenolides and their O¹⁸-isomer confirmed the intermediacy of the allenyl-lactonium ion as well as the cyclic-hemiacetal during the proposed mechanism. By utilizing the β-aryl-γ-methylenecyclohexenylidene-γ-butenolides as starting materials, a highly stereoselective and efficient approach has been developed for the syntheses of frameworks of rubrolide natural products. This strategy was further extended for the total synthesis of rubrolide E.