Synthesis of a tetracyclic core of Erythrina alkaloids and analogs of crispine A

Both enantiomers of C10b methyl analogs of crispine A and a tetracyclic core of Erythrina alkaloids were synthesized starting from l-aspartic acid via a common chiral N-arylethyl succinimide intermediate. The pivotal C10a-C10b bond formation and construction of the C10b stereogenic center were achieved via diastereoselective N-acyliminium ion cyclization. The dibenzylamino group gave stereochemical control where 1,2- and 1,3-stereoinductions led to the cyclized products with opposite configurations at the newly generated stereogenic center. Cope elimination of the dibenzylamino group gave a cyclic enamide which underwent Michael addition and ring closing metathesis of the resulting bis-allyl intermediate in the synthesis of a tetracyclic core of Erythrina alkaloids.

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