## 4 Conclusions  
In this work, we studied DMM as a green alternative to traditional solvents used in SPPS. This ether has shown good ability to solubilize amino acids and coupling agents, as well as good capability to swell some commercially available resins and to remove the Fluorenylmethyloxycarbonyl protecting group. It was tested first during a small peptide synthesis and then on a larger one, obtaining excellent results. DMM also showed full applicability in microwave-assisted peptide synthesis, demonstrating that the green solvent could be a valid alternative to hazardous DMF in all processes of SPPS. Based on these findings, we intend to apply the developed protocol to our studies in order to evaluate its potential for the large-scale production of therapeutic peptides. Certainly, the cost of this solvent is high, so recovery and recycling are considered and studied to reduce the PMI and make SPPS green. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Entry** & **Solvent** & **Decapeptide (\%)** & **Des-Aib (\%)** & **Other (\%)** \\ \hline
14 & DMF & 53.73 & - & 46.27 \\
15 & DMM & 53.69 & 9.24 & 37.07 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: HPLC purities of Aib-ACP decapeptide assembled on Rink amide CM. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Entry** & **Solvent** & **PMI** & **Recovery (Yield \%)** & **PMI after Recovery** \\ \hline
1 & DMF & 3896.28 & - & - \\
2 & DMM & 3750.20 & 80 & 1370.68 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Process mass intensity (PMI) of synthesis and recovery \({}^{\mathrm{a}}\). **Supplementary Materials:** The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/pharmaceuticals15061773/s1](https://www.mdpi.com/article/10.3390/pharmaceuticals15061773/s1)]([https://www.mdpi.com/article/10.3390/pharmaceuticals15061773/s1](https://www.mdpi.com/article/10.3390/pharmaceuticals15061773/s1)), Table S1: Solubilization efficacy of Fmoc-AA(PG)-OH amino acids and coupling agents in DMF and DMM; Figures S1 and S2: Solubilization efficacy of Fmoc-AA(PG)-OH amino acids and coupling agents in DMM; Table S2: Physical parameters of resins evaluated for swelling tests; Figure S3: Swelling of several resins in DMM and DMF; Table S3: Calculations of standard deviations for the values of the swelling tests in triplicate; Figure S4-S11: Chromatogram of Fmoc-Val-OH deprotection in DMM at t = 0, 2, 4, 6, 8, 10, 15 and 30 min; Figure S12-S16: Chromatogram of Fmoc-Val-OH deprotection in DMF at t = 0, 2, 4, 6 and 8 min; Figure S17: 1H NMR spectrum (400 MHz, DCM-6) after deprotection with 20% piperidine in DMM of Fmoc-Val-OH; Figure S18-S26: Chromatogram of NH\({}_{2}\)-Tyr-Ala-OH dipeptide; Figure S27: ESI-MS of NH\({}_{2}\)-Tyr-Ala-OH dipeptide ion [M + H]+ (253.0832 Da); Figure S28-S31: Chromatogram of Fmoc-Phg-Pro-NH\({}_{2}\) dipeptide ion [M + H]+ (470.2083 Da); Figure S33-S36: Chromatogram of Aib-enkephalin; Figure S37: ESI-MS of Aib-enkephalin ion [M + H]+ (611.3532 Da); Figures S38 and S39: Chromatogram of Aib-ACP; Figure S40: ESI-MS of Aib-ACP ion [M + H]+ (1090.5859 Da); Tables S4-S7: Overview of starting materials used for decapeptide (Aib-ACP), their total mass and PMI before and after recycling; Figure S41: (a) \({}^{1}\)H NMR spectrum (CD\({}_{2}\)Cl\({}_{2}\), 400 MHz, 298 K) of DMM recovered from distillation processes; (b) \({}^{1}\)H NMR spectrum of DMM. **Author Contributions:** M.S., M.C.S. and P.C. : Supervision. M.S., M.M. and P.C. : Conceptualization and project administration. G.V. and M.M. : Investigation. G.V., M.S. and M.C.S. : Data curation and writing--original draft preparation. M.S., M.C.S. and P.M.: Formal analysis. G.V., M.S. and M.C.S. : Resources, methodology, and synthesis. P.C. : Funding acquisition and editing. All authors have read and agreed to the published version of the manuscript. **Funding:** Research was supported by the project \"PROGEMA\" (PROcessi Green per l'Estrazione di principi attiv e la deporazione di MAtrici di scarto e non), ARS01_00432-CUP B26G18000700005.