To gain a deeper understanding of the deprotona on of compound 2, we conducted an experiment where D2O was added instead of iodine a er deprotona on with an excess of base. In the case of the 2 endo,endo isomer, the reac on yielded compound 8 as expected. However, in the lithia on-deutera on of the exo,exo and exo,endo isomers, the iden cal mixture of deuterated diastereomers 2(D) was obtained (Scheme 4) based on the 2 H NMR spectra (see SI). Due to the epimeriza on process, the accurate assignment of signals in the obtained 2 H NMR spectra was challenging. Further structural informa on was obtained by analyzing the 1 H NMR spectra of the deuterated products. A comparison of the 1 H NMR spectra of the deutera on products from both isomers revealed their similarity (Figure ). Moreover, careful integra on of both spectra indicated the incorpora on of approximately one deuterium atom per molecule (the integral intensity of all MeO groups was used as the comparison standard). This result suggests that while double deprotona on with dianion forma on occurs, it is an insignificant process. However, we have conducted a more detailed study of the possibility of double deprotona on. Overlapping the 2D 1 H COSY spectra of each diastereomer and the deutera on product (Figure ) revealed that the mixture contained all three (not just two) diastereomers. Addi onally, a comparison of linear combina ons of 1 H NMR spectra of pure diastereomers with the spectra of the mixture indicated an approximate exo,endo:endo,endo:exo,exo ra o of 2:1:2. Superposi on of all the data obtained in this study revealed valuable insights into the forma on of different diastereomers and the mechanis c aspects of the synthesis of stellane 1,5-dicarboxylic acid. The forma on of exo,endo and exo,exo isomers was a ributed to the protona of the mono-deprotonated intermediate I3 from different sides of the "enolate plane" (see Scheme 4). The presence of the endo,endo diastereomer could be explained by the fast double protona on of dianion I4 from less hindered sites. Interes ngly, this fast protona on, which occurs more rapidly than the intramolecular Claisen cycliza on, provides a reasonable explana on for the forma on of the endo,endo diastereomer. Another pathway leads to the forma on of enolate I2, which affords compound 8 in quan ta ve yield under given condi ons. These experiments demonstrated that during the treatment of exo,exo and exo,endo diastereoisomers of diester 2 with an excess of a strong base, both monoand double deprotona on is possible, with an expected ra o of I3/I4 of approximately 4:1. However, the precise mechanism of the target stellane 1 forma on from I3 or I4 remains unclear.