As a recent expansion, the annulation of BCBs with three-atom partners has been used to obtain bicyclo[3. 1.1]heptanes (BCHeps) using the same three activation modes (Scheme 1.B: Molander, Li, Waser, Deng ). Cycloadditions of BCB affording larger saturated bicycloalkanes have however remained unexplored so far, and only one example exists, in which this kind of transformation is employed to form unsaturated thiabicyclo[5. 1.1]nonanes (Scheme 1.C; Glorius). Medium sized carbocycles and their bridged variants are abundant among both natural and pharmacologically relevant compounds. One example is bicyclo[3. 2.1]octane ([3.2.1]-BCO), which represents a conformationally rigid analog of cycloheptane. This scaffold can be found in thousands of bioactive terpenoid derivatives, and extensive research has focused on its synthesis (Scheme 1D). In comparison, bicyclo [4.1.1]octane (BCO) is rarer in nature, it has been much less studied, and the very few preparative methods that have been established so far are limited in scope and lack convergence. In a recent study, the group of Grygorenko showcased the improved lipophilicity of this unique motif and its potential function as an isosteric replacement for both aromatic and saturated monocyclic carbocycles. Furher investigations on BCO ring systems would be of great benefit in the perspective of their applications in medicinal chemistry. Nonetheless, progressing in this direction is hampered by the lack of efficient synthetic methods granting an expedient access to these scaffolds. The annulation of BCBs with four-carbon partners such as dienes appears as an attractive convergent strategy to access BCOs. However, dienes can also act as twocarbon partners, leading to the competitive formation of BCHexs. This is especially true when a radical-based mechanism in involved. In previous reports, using weakly or non-polarized dienes under photochemical conditions had resulted in the formation of the (2π+2σ) BCHex products. On the other side, the only reported transformation giving access to medium-sized bicyclic scaffolds relied on a photo-induced dearomative expansion of thiophenes. Therefore, we wondered if Lewis acid catalysis might constitute a more viable alternative. Herein, we describe the synthesis of BCO through the formal (4π+2σ) cycloaddition of BCB ketones with dienol silyl ethers under Lewis acid catalysis through the successful implementation of this strategy (Scheme 1.E). To the best of our knowledge, this is the first application of BCBs to synthesize medium-sized bridged all-carbon carbocycles, as well as the first example of a formal Diels-Alder cyclization involving a 2σ dienophile. At the start of our studies, more stable naphthoyl BCB 1a was selected as our model substrate and treated with an excess (2.2 mmol) of tert-butyl diphenylsilyl (TBDPS) dienol ether 2a in DCM and in the presence of TMS-OTf (20 mol%, the catalyst reported by Studer for BCB activation ) at room temperature. A check of the reaction after 16 hours showed the full conversion of 1a and the formation of a less polar compound (later identified as silyl enol ether 3). After methanol was added and the resulting mixture was stirred for 2 hours, we observed that 3 had been completely transformed into BCO ketone product 4aa, which could be isolated in 71% yield. Because the purification of the intermediate silyl enol ether was challenging, we focused directly on optimizing the formation of ketone 4aa. As the complete conversion of 3 to 4aa through the sole addition of MeOH was difficult to achieve, an excess of TMS-OTf was used. A screening of silyl protecting groups on the dienol ether using 20 mol% of TMS-OTf as catalyst showed that, compared to TBDPS (Table , entry 1) the smaller and less stable TBS (entry 2) and TIPS (entry 3) provided 4aa in lower yield. Ga(OTf)3the catalyst of choice in the annulation of BCB ketones with imines published by the group of Leitch led to an increased yield of over 80% (entry 4). Other Lewis acids furnished inferior results (see ESI, for details). Reducing the catalytic loading to 10 mol% did not affect the efficiency of the reaction (entry 5). On the contrary, a smaller amount of the dienol ether (1.2 instead of 2.2 equivalents) afforded a significantly diminished yield (entry 6). Al(OTf)3 was next investigated as a more sustainable alternative to Ga(OTf)3. No diminution of yield occurred when the reaction was performed using 10 mol% Al(OTf)3 (entry 7). Testing other solvents confirmed the superiority of DCM to other chlorinated (entry 8) and non-chlorinated ones (entry 9). In addition, further lowering the catalytic loading of Al(OTf)3 to 5 mol% provided the product in even higher 90% yield (entry 10); this was not the case with Ga(OTf)3 (see ESI). Finally, the influence of the silyl-deprotecting work-up after the formal cycloaddition step was investigated. For ensuring reproducibility, the scale of the process was doubled to 0.30 mmol ketone 1a. Treatment with TBAF upon solvent-switch to THF gave inferior results (entry 11) compared to the protocol involving the addition of TMS-OTf and MeOH (entry 12), which was therefore adopted as our optimal procedure. Changing TMS-OTf to methanolic HCl provided 4aa in a comparable yield (entry 13), and can be thus considered as a more costeffective alternative.