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Page 36 of 54 Yang et al. Chem Synth 2023;3:7 https://dx.doi.org/10.20517/cs.2022.38
Scheme 48. Spirolactonization reaction of 3-hydroxyoxindoles 69 with enals 95. This figure is used with permission from the Royal
[121]
Society of Chemistry .
Mechanistic investigation of this reaction revealed that the annulation was initiated with (1) the in situ
generation of the chiral α, β-unsaturated acyl azolium intermediate or via (2) a formal [3 + 2] pathway to
give the corresponding spirocyclic product 97 [Scheme 48, bottom].
Maremycin A, a diketopiperazine alkaloid, was isolated from the culture broth of marine Streptomyces
[122]
species B 9173 . The application of a new synthetic strategy for the total synthesis of complex natural
products is generally considered an important validation of its synthetic potential and usefulness.
[123]
In 2012, Bergonzini et al. established a chiral secondary amine C8-catalyzed enantioselective [3 + 2]
annulation of α, β-unsaturated aldehydes 98 with 3-hydroxy oxindoles 69, generating spiro oxindole-
butyrolactones 99 and 100 in high enantioselectivities and low diastereoselectivities [Scheme 49A].
Although the diastereoselectivity is not as good as expected, the diastereomerically pure compounds 99 and
