Chapter 5 - [4 + 2] Annulation With Nonactivated Phthalides (Sammes Annulation)

Add time:08/23/2019    Source:sciencedirect.com

Anionic annulation of nonactivated phthalides (phthalides without a nucleofuge at C3), also referred to as Sammes annulation is a unique reaction for regiospecific synthesis of α-naphthols. It typically involves the reaction between a phthalide without a C3 nucleofuge and a α,β-unsaturated ester or lactone in the presence of a strong base to produce 4-hydroxytetralone-2-carboxylates. Dehydration of the tetralones by acidic catalysts leads to 1-hydroxy-2-carboxylates. With α-alkyl/aryl acrylates, the annulations (Mal annulation) with the phthalides divert to give 2-alkyl/aryl-α-naphthols in one pot. Like the Hauser annulation, the regiochemical integrity is the most attractive feature of the annulations. The annulation between a phthalide and an aryne precursor in the presence of LDA or LTMP directly produces an anthraquinone. This reaction has been useful for the synthesis of a wide variety of natural products and biosynthetic intermediates which include tetracyclines, olivin, versicolorins, enediynes, carbazoles, arylnaphthalene lignans, neotanshinlactones, clausevatine D, alnumycins and calothrixin, etc. A new class of reactions is observed when α-azidoacrylates are reacted with phthalides in the presence of LHMDS. Unlike the Mal annulation, it gives 5-hydroxy-2-benzazepin-1-ones.

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