Crystallographic evidence for unintended benzisothiazolinone 1-oxide formation from benzothiazinones through oxidation

Abstract

1,3-Benzothiazin-4-ones (BTZs) are a promising new class of drugs with activity against Mycobacterium tuberculosis, which have already reached clinical trials. A product obtained in low yield upon treatment of 8-nitro-2-(piperidin-1-yl)-6-(trifluoromethyl)-4H-benzothiazin-4-one with 3-chloroperbenzoic acid, in analogy to a literature report describing the formation of sulfoxide and sulfone derived from BTZ043 [Tiwari et al. (2015). ACS Med. Chem. Lett. 6, 128–133], is a ring-contracted benzisothiazolinone (BIT) 1-oxide, namely, 7-nitro-2-(piperidine-1-carbonyl)-5-(trifluoromethyl)benzo[d]isothiazol-3(2H)-one 1-oxide, C14H12F3N3O5S, as revealed by X-ray crystallography. Single-crystal X-ray analysis of the oxidation product originally assigned as BTZ043 sulfone provides clear evidence that the structure of the purported BTZ043 sulfone is likewise the corresponding BIT 1-oxide, namely, 2-[(S)-2-methyl-1,4-dioxa-8-azaspiro[4.5]decane-8-carbonyl]-7-nitro-5-(trifluoromethyl)benzo[d]isothiazol-3(2H)-one 1-oxide, C17H16F3N3O7S. A possible mechanism for the ring contraction affording the BIT 1-oxides instead of the anticipated constitutionally isomeric BTZ sulfones and antimycobacterial activities thereof are discussed.

New Insight into Dearomatization and Decarbonylation of Antitubercular 4H‐Benzo[e][1,3]thiazinones: Stable 5H‐ and 7H‐Benzo[e][1,3]thiazines

“…The crystal of 3 b is thus a co‐crystal of diastereomers, indicating that the diastereomers of 3 b are also not easily if at all separable by crystallization. A similar situation was previously encountered in the crystal structure of the corresponding benzisothiazolinone 1‐oxide derived from 1 b , where oxidation led to a second centre of chirality at the sulfur atom [6] . Structural parameters of the 5 H ‐benzo[ e ]‐1,3‐thiazine heterocyclic system in 3 b are similar to those in 3 a .…”

“…A similar situation was previously encountered in the crystal structure of the corresponding benzisothiazolinone 1‐oxide derived from 1 b , where oxidation led to a second centre of chirality at the sulfur atom. [6] Structural parameters of the 5 H ‐benzo[ e ]‐1,3‐thiazine heterocyclic system in 3 b are similar to those in 3 a . The piperidine ring exhibits a chair conformation in all four distinct molecules.…”

“…Recently, we showed by X‐ray crystallography that treatment of 1 a and 1 b with moist 3‐chloroperbenzoic acid unexpectedly led to the corresponding benzo[ d ]isothiazol‐3(2 H )‐ones and their 1‐oxides, [6] instead of the anticipated corresponding BTZ sulfones and sulfoxides, [7] presumably involving nucleophilic addition of water at C2. In their search for the structure of a human plasma metabolite of 1 b , Kloss et al .…”

“…[4] We did, however, not observe nucleophilic attack at C7 when incubating BTZs with DprE1. [5] Recently, we showed by X-ray crystallography that treatment of 1 a and 1 b with moist 3-chloroperbenzoic acid unexpectedly led to the corresponding benzo[d]isothiazol-3(2H)-ones and their 1-oxides, [6] instead of the anticipated corresponding BTZ sulfones and sulfoxides, [7] presumably involving nucleophilic addition of water at C2. In their search for the structure of a human plasma metabolite of 1 b, Kloss et al found that treatment of the compound with the reducing agent NaBH 4 yielded Meisenheimer complexes, which were also encountered in ex vivo studies.…”

BTZ-Derived Benzisothiazolinones with In Vitro Activity against Mycobacterium tuberculosis

“…Unexpected upfield shifts of the two aromatic 1 H NMR signals, as compared with the parent BTZ043, were attributed to the influence of the lone pair at the sulfur atom and loss of aromaticity due to puckering of the thiazinone ring, as observed in AM1 computed molecular structures of the anticipated “BTZ-SO” and “BTZ-SO 2 ”. Recently, X-ray crystallography revealed that the supposed BTZ sulfones resulting from treatment of BTZs 1a and 1b with m -chloroperoxybenzoic acid are in fact the corresponding benzisothiazolinone (BIT) 1-oxides (Scheme ), which represent constitutional isomers . Crystallographic evidence for the unintended BIT 1-oxide formation, retrosynthesis of a BIT and revisiting the 1 H NMR data eventually led to the revision of the chemical structures as depicted in Scheme by Eckhardt et al in 2020.…”