Abstract Tubulysins are natural products isolated from myxobacterial species and are mitotic poisons which exceed the cell growth inhibition of any clinically relevant traditional chemotherapeutic. Structurally, tubulysins are linear tetrapeptides comprised of N-methyl pipecolic acid (Mep), isoleucine (Ile), the novel amino acid tubuvaline (Tuv), and the novel tyrosine analogue tubutyrosine (Tut). All isolated tubulysins possess an acid- and base-sensitive N-acyloxymethyl substituent not previously found in nature. This N,O-acetal of formaldehyde is attached as a side chain to the amide N-atom of the Tuv fragment. The isolation of natural tubulysins from culture extracts provides only limited quantities. Recently, we reported a large scale total synthesis of natural tubulysin B. Among the multiple synthetic and stereochemical challenges, the most striking were: a) the incorporation of the labile N,O-diacyl N,O-acetal and b) the regioselective hydrolysis of the C-terminal methyl ester (OMe) in the tripeptide Mep-Ile-Tuv-OMe. Herein, we present a convergent total synthesis of tubulysin analogues incorporating an alkoxymethyl side chain. Such molecular architecture allows for compounds that are more base and esterase inert, thus providing additional metabolic/catabolic stabilization. The key step in our synthetic strategy relies on use of a novel reagent for the efficient conversion of the N-acyloxymethyl substituent to novel side chain groups with concomitant facilitated hydrolysis of the C-terminal methyl ester. Following LiOH-based hydrolysis of the C-terminal ester, the Tut fragment is added resulting in novel tubulysin analogues. This novel process opens the door to tubulysin analogs with improved potency for treating cancer. Citation Format: Iontcho Vlahov, Longwu Qi, Fei You, Spencer Hahn, Hari K. Santhapuram, Kevin Wang, Paul Kleindl, Jeremy Vaughn, Marilynn Vetzel, Joseph Reddy, Christopher Leamon. Total synthesis of tubulysins: A new chemical reaction leads to analogues with enhanced cytotoxicity. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4499. doi:10.1158/1538-7445.AM2013-4499