A Biomimetic Total Synthesis of (−)-Spirotryprostatin B and Related Studies

The prenylated natural products spirotryprostatin A and B derived from the Trp-Pro diketopiperazine also feature an oxidative rearrangement of the indole to form a spirooxindole. Synthetically, formation of the oxindole ring was stereoselectively accomplished by reaction of the appropriate indole precursor with N-bromosuccinimide. For optimum results, the oxidation should be carried out prior to diketopiperazine cyclization. In this manner, we have synthesized the tetrahydro- and dihydro- analogues of spirotryprostatin B in four steps from l-tryptophan methyl ester. The dihydro derivative was then converted to spirotryprostatin B by unsaturation of the pyrrolidine ring to a pyrroline, thus unambiguously confirming the structure of the natural product.