Batch Versus Flow Lithiation–Substitution of 1,3,4‐Oxadiazoles: Exploitation of Unstable Intermediates Using Flow Chemistry

• Published in: Chemistry : a European journal Vol. 25; no. 53; pp. 12439 - 12445
• Main Authors: Wong, Jeff Y. F., Tobin, John M., Vilela, Filipe, Barker, Graeme
• Format: Journal Article
• Language: English
• Published: WEINHEIM Wiley 20.09.2019; Wiley Subscription Services, Inc
• Subjects: alkylation; Chemistry; Chemistry, Multidisciplinary; Continuous flow; Exploitation; flow chemistry; Fragmentation; heterocycles; Interception; Intermediates; lithiation; Organic chemistry; Oxadiazoles; Physical Sciences; Science & Technology; Substitutes; synthetic methods; Temperature effects; Trapping; MICROREACTOR SYNTHESIS; COMPLEXES; lithiation; PHOTOPHYSICAL PROPERTIES; C-H FUNCTIONALIZATION; OXADIAZOLES; DISCOVERY; LITHIUM ENOLATE; STEREODYNAMICS; heterocycles; ESTERS; REPLACEMENTS; alkylation; synthetic methods; flow chemistry
• DOI: 10.1002/chem.201902917
• ISSN: 0947-6539

1,3,4‐Oxadiazoles are a common motif in pharmaceutical chemistry, but few convenient methods for their modification exist. A fast, convenient, high yielding and general α‐substitution of 1,3,4‐oxadiazoles has been developed using a metalation‐electrophilic trapping protocol both in batch and under continuous flow conditions in contradiction to previous reports which suggest that α‐metalation of this ring system results in ring fragmentation. In batch, lithiation is accomplished at an industrially convenient temperature, −30 °C, with subsequent trapping giving isolated yields of up to 91 %. Under continuous flow conditions, metalation is carried out at room temperature, and subsequent in flow electrophilic trapping gave up to quantitative isolated yields. Notably, lithiation in batch at room temperature results only in ring fragmentation and we propose that the superior mixing in flow allows interception and exploitation of an unstable intermediate before decomposition can occur. A convenient new substitution of alkyl‐1,3,4‐oxadiazoles, a key pharmaceutical motif, is described. The reaction tolerates a wide scope of substrates and can be carried out in either batch or continuous flow conditions. In batch cooling to −30 °C is required to avoid decomposition of an unstable intermediate while in flow this may be quickly intercepted and synthesis carried out at room temperature.