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A microwave-assisted, sequential, one-pot protocol has been developed for the synthesis of a variety of benzothiadiazin-3-one-1,1-dioxides. This protocol utilizes a copper-catalyzed N-arylation of α-bromo-benzenesulfonamides with a number of amines to generate the corresponding 2-aminobenzenesulfonamides, which undergo cyclization to the desired sultams using carbonyl diimidazole (CDI). A range of conditions was evaluated for the key C–N bond formation step with tolerance toward functionalized amines.
The development of protocols for the synthesis of skeletally diverse heterocyclic scaffolds is a critical step in the drug discovery process. The growing demand for libraries of small molecules as potential small molecule therapeutic agents for high-throughput screening presents challenging opportunities in this field. One-pot strategies are highly efficient pathways to rapidly synthesize complex heterocyclic molecules from simple substrates.1 When coupled with transition metal-catalyzed processes, one-pot processes enable the generation of complex heterocyclic scaffolds from simple building blocks. In this regard, α-haloarylsulfonamides represent an attractive building block for the production of benzofused sultams.2,3
Sultams and their sulfonamide precursors possess a number of advantageous chemical properties making them ideal building blocks for the titled process, the most prominent of these include: (i) click coupling between starting α-halobenzenesulfonyl chlorides and amines under mild conditions, (ii) the α-halo group can be utilized in transition metal-catalyzed cross coupling (iii) the α-halo group enhances the acidity of the aryl sulfonamide N–H enabling Mitsunobu and conventional alkylation reactions to occur under mild conditions, and (iv) the commercial availability of a variety of substituted α-halo benzenesulfonyl chlorides. Taken collectively, these attributes have guided our efforts to develop a microwave-assisted, sequential one-pot protocol for the synthesis of benzothiazdiazin-3-one-1,1-dioxides based on a pivotal copper-catalyzed N-arylation strategy.
Traditionally, sultams have been synthesized using a number of classical cyclization protocols such as Friedel–Crafts, [3+2] cycloadditions, Diels–Alder reactions, and recently the application of oxa- and aza-Michael reactions.4 Notably, there have been a number of transition metal-catalyzed protocols reported for the generation of diverse sultams.3,5
In addition to their inherent chemical properties, sultams have emerged as important targets for drug discovery due to their potent biological activities. In particular, benzothiadiazin-3-one-1,1-dioxides and their derivatives have shown promising activity, including hypoglycemic,6 anti-HIV,7 RSV inhibitory activity,8 as well asand serving as selective antagonists of CXR2 (Fig. 1).9
Since the observation of copper-catalyzed coupling of a arylbromide with an acetanilide by Goldberg in 1907,10 copper-catalyzed N-arylation represents an effective reaction for the formation of C–N and C–O bonds.11 Early reports classically required harsh reaction conditions and stoichiometric quantities of copper. Seminal work by Buchwald, Hartwig and Ley reported notable advances in both ligands and reduced reaction temperatures for copper-catalyzed couplings.12
Traditionally, benzothiadiazin-3-one-1,1-dioxides have been synthesized in a number of linear protocols.6–9,13 Envisioning a copper-catalyzed approach to benzothiadiazin-3-one-1,1-dioxides, a variety of conditions were evaluated to probe and subsequently optimize the N-arylation of allyl amine with N-allyl-2-bromo-4-fluorobenzenesulfonamide 1 to yield N-allyl-2-(allylamino)-4-fluorobenzenesulfonamide 2 (Scheme 1, Table 1).14 An array of copper sources (Table 1, entries 1–3) and ligands (Table 1, entries 4–7) were initially evaluated followed by a survey of reaction solvent. Under conventional heating, the desired sulfonamide 2, could be isolated in 92% yield after 6 h.
Further optimization was achieved using microwave irradiation, which reduced reaction times to 11 min at 150 °C with comparable yields (Table 1, entry 8 vs 11).15 With these results in hand, a number of 2-aminobenzenesulfonamide derivatives were synthesized to demonstrate the versatility of the protocol with a variety of amines, amides, and sulfonamide starting materials (Scheme 2, Table 2).
With an array of 2-aminobenzenesulonfamides in hand, cyclization to the corresponding benzothiadiazin-3-one-1,1-dioxides with carbonyl diimidazole (CDI) was achieved in excellent yields under thermal conditions (Scheme 3, Table 3).13b,17
Finally with both protocols in hand, a sequential, two-step, one-pot approach was achieved whereby microwave irradiation afforded the desired benzothiadiazin-3-one-1,1-dioxides in good yield (Scheme 4).18 To achieve this, the CDI cyclization was conducted under microwave irradiation following the initial copper-catalyzed step in the same microwave vial. This required a change of solvent to DMF which was the optimum compatible solvent for both the N-arylation and CDI cyclization steps while maintaining good yields.
In conclusion, we have developed a microwave-assisted, copper-catalyzed, sequential, one-pot synthesis of benzothiadiazin-3-one-1,1-dioxides. A variety of derivatives of benzothiadiazin-3-one-1,1-dioxides can be rapidly accessed by combining a copper-mediated N-arylation followed by cyclization with CDI. Further efforts toward employment of this method in library production will be published in due course.
This publication was made possible by the Pilot-Scale Libraries Program (P41 GM076302), the National Institutes of General Medical Sciences (KU Chemical Methodologies and Library Development Center of Excellence P50 GM069663) and by Grant Number P20 RR015563 from the National Center for Research Resources, a component of the National Institutes of Health, and the State of Kansas. Its contents are solely the responsibility of the authors and do not necessarily represent the official view of the NCRR or NIH.