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Toward a selective and facile method for the synthesis of CF 3 -containing pyrazolo[1,5- c ]quinazolines, we developed a [3 + 2] cycloaddition/1,3-H shift/rearrangement/dehydrogenation cascade involving in situ generated CF 3 CHN 2 and 3-ylideneoxindoles with DBU as a base. The reaction is distinguished by its mild conditions, metal-free process, operational simplicity, and broad functional group tolerance, thus presenting a convenient protocol for the construction of pyrazolo[1,5- c ]quinazolines that are of interest in medicinal chemistry., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

An inexpensive copper-catalyzed sequential reaction process, proceeding via a nucleophilic attack of amine to Cu-carbene generated in situ from heterocyclic N -tosylhydrazone precursors followed by a 1,2-H shift/oxidative cyclization cascade of N -ylides, has been described, smoothly generating the corresponding structurally various spiro-dihydropyrrolo[1,2- a ]quinoxaline derivatives. Furthermore, the significance of this protocol can be also highlighted by its diverse conversions of the synthetic compounds to the potentially bioactive molecules such as the 2-substituted pyrrolo[1,2- a ]quinoxalins.

We present herein a visible-light-induced [3 + 2] cycloaddition of a hypervalent iodine(III) reagent with α-ketoacids for the construction of 5-CF 3 -1,3,4-oxadiazoles that are of importance in medicinal chemistry. The reaction proceeds smoothly without a photocatalyst, metal, or additive under mild conditions. Different from the well-established trifluorodiazoethane (CF 3 CHN 2 ), the diazotrifluoroethyl radical [CF 3 C(·)N 2 ], a trifluoroethylcarbyne (CF 3 CĊ:) equivalent and an unusual CF 3 -containing building block, is involved in the present reaction system.

A new method for the ex situ generation of difluorodiazoethane (CF 2 HCHN 2 ) and a procedure for its efficient use in [3 + 2] cycloaddition with nitroolefins by the AcOH/O 2 catalyst system were developed by using a simple two-chamber system. The method provides a facile and straightforward access to a series of 4-substituted 5-difluoromethyl-3-nitro-1 H -pyrazoles that are of interest in medicinal chemistry. Interestingly, the key factor for the success of this method is the efficient preparation of CF 2 HCHN 2 by an ex situ process.

Herein is disclosed a selective and facile approach for the construction of CF 2 H-containing pyrazolo[1,5- c]quinazolines from easily accessible 3-ylideneoxindoles and in situ generated CF 2 HCHN 2 . The reaction involving a [3 + 2] cycloaddition/1,3-H shift/rearrangement/dehydrogenation cascade proceeded smoothly at room temperature in the absence of catalyst and additive. Moreover, this metal-free process along with mild conditions is desirable and valuable for the pharmaceutical industry. Importantly, this reaction features a broad substrate scope, good functional group tolerance, and gram-scale synthesis.

An efficient [3 + 2] cycloaddition of 3-ylideneoxindoles with in situ generated CF 2 HCHN 2 for the syntheses of spirooxindoles has been developed. This methodology gives access to a range of relatively complex spirooxindoles featuring a CF 2 H group and three contiguous stereogenic centers in up to 84% yield and 99 : 1 trans/cis.