Synthesis of Erythrina and Related Alkaloids. XXX. Photochemical Approach. (1). Synthesis of Key Intermediates to Erythrina Alkaloids by Intermolecular [2+2] Photocycloaddition Followed by 1, 3-Shift

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A novel synthetic route to Erythrina alkaloids consisting in [2+2] intermolecular photocycloaddition of the isoquinolinodioxopyrroline 4 to 2-trimethylsilyloxybutadienes and the subsequent ring enlargement reaction of the trimethylsilyloxyvinylcyclobutane 5 by thermal or tetrabutylammonium flouride(TBAF)-induced 1, 3-shift was developed. The TBAF method was particularly useful, offering a good yield under mild reaction conditions. Thus, photoannulation of 4 with 1-methoxy-3-trimethylsilyloxybutadiene, followed by hydride reduction, TBAF-induced 1, 3-shift, and hydrogenation gave the 7α-hydroxy-2, 8-dioxoerythrinan 19 in overall 45% yield, and this product was converted, in 70% yield, into the 1, 7-cycloerythrinan 24, a key intermediate for the total synthesis of Erythrina alkaloids.