1,3,5-Trisubstituted-hexahydro-1,3,5-triazines (referred to as triazinanes in this text) are stable precursors of formaldimines and have been utilized as effective aminomethylation reagents by Krische and as Mannich-type reagents by Feng and Kang. Moreover, application of triazinanes have been further expanded as 1,n-synthons (n = 2-4) in various cycloadditions to construct a wide range of N-heterocycles. Of special interest to the current study, Sun and co-workers first discovered that triazinanes could be used as 1,4-dipole precursors in the gold-catalyzed (4+1)/(4+3) cycloadditions with diazo compounds to produce five-/sevenmembered N-heterocycles. Besides, Werts and co-workers further developed a (4+3) cycloaddition/ring-opening strategy of donor-acceptor cyclopropanes (DACs) with triazinanes to form 1,4-diamines via (4+3) 1,3-diazepane cycloadducts. Inspired by these two pioneering reports and our recent findings on highorder cycloaddition of BCBs, we are intrigued by the potential of developing a (4+3) cycloaddition/ring-opening reaction of BCBs with triazinanes to produce structurally intriguing cyclobutyl diamines 4 via (4+3) aza-BCO cycloadducts 3 (Scheme 1B). This strategy would not only incorporate 1-amino and 3-methylamino functional groups into cyclobutane ring simultaneously to enhance the molecular complexity, but also result in the aminalring pre-fixed cis-configuration. Mechanistically, the complexation of Lewis acid with BCB may produce intermediate I or enolate species II. So, either an SN1 or SN2-like pathway is conceivable for the nucleophilc attack of triazinane 1 on BCB 2 to form intermediate III, which then loses one molecule of formaldimine to generate a shorter chain iminium species IV. Subsequent cyclization forms the desired (4+3) cycloadduct 3 (Scheme 1B, path a). However, under certain conditions, triazinane 1 could decompose into formaldimines. Thus, a competitive Leitch's process leading to (3+2) aza-BCH or cyclobutenyl methanamine poses a big challenge for the target (4+3) cycloaddition (Scheme 1B, path b). Alternatively, intermediate IV might release another molecule of formaldimine and cyclize to afford (3+2) aza-BCHs (Scheme 1B, path c). In this study, we successfully realized a stepwise (2+2+3) rather than (4+3) cycloaddition of BCB ketones with triazinanes under B(C6F5)3 catalysis, leading to aza-BCOs 3. A direct acidic treatment of the obtained aza-BCOs resulted in efficient cleavage of the aminal moiety, producing the designed cis-cyclobutyl diamines 4. Interestingly, when BCB esters were used instead of BCB ketones to react with triazinanes under In(OTf)3 catalysis, butterfly-shaped biscyclobutenyl amines 5 were unexpectedly obtained (Scheme 1C). Herein, we report these preliminary results.