The target size of the MCNB was determined using DFT calculations. We found that MCNBs have a higher strain energy than CNBs with the same size (for details, see Fig. in the Supplementary Information (SI)), and that the strain of the MCNBs is mainly induced during the final bond-formation step. Fig. shows the hypothetical homodesmotic reactions using (n,n)MCNBs, (n,n)CNBs, and their corresponding precursors (pre(n,n)MCNBs and pre(n,n)CNBs), based on which the strain induced in the final bond-formation step (DHFBF/kcal•mol -1 ) was estimated. Cisstilbene (sti) and phenanthrene (phe) were used as reference molecules. For belts with a similar size, the DHFBF of the MCNB is much higher than that of the CNB (e.g., (6,6)CNB: DHFBF = 40.2 kcal•mol -1 ; (7,7)MCNB: 121.1 kcal•mol -1 ). As (6,6)CNB has been successfully synthesized using a nickel-mediated homocoupling reaction, the strain energy allowed by this synthetic method was estimated to be approximately 40 kcal•mol -1 . Based on these considerations, the synthetic pathway and the symmetry of the product, (15,15) Our synthetic route to MCNBs is shown in Fig. . In order to improve the solubility of the intermediates and products, n-butoxy groups were introduced to the starting material 2. Thus, (25,25)MCNB with 20 butoxy groups (1) was targeted and synthesized from simple precursors 2 and 5 over 14 steps. First, the unsymmetric functionalization of phenanthrene 2 was investigated to ensure a Z-selective Wittig reaction sequence. During the screening of the Lewis-acid-catalyzed formylation of 2, we found that monoformylated 3 was obtained selectively using TiCl4 and MeOCHCl2 in high yield (75%), and that a subsequent chloromethylation with ZrCl4 and MeOCH2Cl smoothly afforded bifunctional phenanthrene 4a with formyl and chloromethyl groups in 84% yield. The formyl and chloromethyl groups of 4a were then converted into acetal and phosphonium groups, respectively, to yield 4b. The sequential Wittig reaction of 5 with 4a followed by 4b produced key intermediate 7c.