The heteroatom is stabilizing radical intermediates generated from a hydrogen atom transfer (HAT) process. As a result, the Norrish type II C-C bond cleavage, which involves a HAT process, became more favorable than the Norrish type I direct fragmentation. After C-C cleavage, an aldol-type reaction led then to formation of the ring-contracted product I. In addition, the competitive Norrish-Yang pathway to give strained ring II is also less favorable than in the case of acyl-substituted carbocycles. To achieve selective C-C bond cleavage on carbocycles, we envisioned the use of strained rings to lower the bond dissociation energy of the C-C  bond (Scheme 1B). In the case of cyclic substrates, a ring expansion to give products III or IV could be achieved from the direct excitation of carbonyl compounds, complementing the ring-contraction strategy of Suarez and Sarpong. When considering that bicyclic structures containing carbonyl cyclopropanes are usually synthesized from the corresponding olefins, a sequential cyclopropanation/selective ring-opening reaction would provide a direct strategy for CH2 homologation of , unsaturated carbonyl compounds (Scheme 1C). The insertion would happen on the , bond of the carbonyl group, in contrast to most existing homologation reactions proceeding via insertion into the carbonyl -carbon bond. The key for success of this approach would rely on selective cleavage of the endocyclic C-C bond. Interestingly, most methods for reductive cyclopropane ring-opening are based on transition metals and favor cleavage of the exocyclic C-C bond. We envisioned however that selective endocyclic C-C bond cleavage could be achieved through formation of the most stable 1,3 biradical intermediate. In this work, we report first our mechanistic studies on the direct photoexcitation of cyclopropyl carbonyl compounds by experiments and quantum chemical computation. Our investigations resulted in the discovery of efficient photoisomerization processes in case of polycyclic substrates, as well as a cascade photoisomerization-photo Nazarov reaction for bicyclic compounds (Scheme 1D). By adding diphenyl disulfide as a hydrogen atom transfer (HAT) reagent, we were then able to achieve a unified ring opening reaction applicable to both linear and cyclic biradicals, paving the way for the development of a novel homologation methodology for , unsaturated ketones In our previous work, we reported preliminary results on the [3+2] annulation of cyclopropanes 1a and 1b with phenylacetylene to give products 2a and 2b (57% and 40% yield, Scheme 2A) in the absence of a photocatalyst. The reaction was successful for geminally disubstituted carbonyl cyclopropanes under irradiation at 390 nm with a UV Kessil lamp. A lower yield was obtained at 352 nm and no conversion was observed at 440 nm. In addition, Brown and coworkers reported the formation of biradicals from bicyclo[1,1,0] butane (BCB) and housane derivatives, which are more strained ring systems. We wondered if it would be possible to extend the annulation reaction to larger bicyclic ring systems such as bicyclo[3,1,0]hexane 3a or norbornene derivative 5a (Scheme 2B). Under our conditions, we however observed rearrangements leading to products 4a and 6a instead of the expected [3+2] annulations to give 4a1 and 6a1. While the mechanism of the [3+2] annulation has been investigated by our group 8 and others, the behavior of 1,3 biradicals in transformations beyond annulation has not been described thoroughly in the literature. 15e Therefore, we decided to initiate more systematic studies in the absence of alkynes as biradical trap (Scheme 2C). We first performed the irradiation of trans-cyclopropane 1c. After 48 hours of irradiation, epimerization was observed and 1c was recovered in 93% yield as a 4:1 mixture of trans and cis isomers, suggesting that homolysis of the C-C bond was occurring and was reversible, as had been observed previously. Performing the same control experiment with 1a resulted in a mixture of isomerization products 2a1 and 2a2 with 40% of 1a recovered. These products might arise from a Norrish type II reaction. The different reaction outcome between 1a and 1c suggested that there may be a competition between Norrish type II reaction and reversible C-C bond cleavage depending on the substrate. We then conducted the same experiments on more rigid polycyclic structures 3a and 5a. To our delight, products 4a and 6a were obtained in good yields (86% and 67% respectively). A trace amount of photoisomerization product 4a2 was also observed. This could indicate that both reactions proceeded via formation of the ring-opened alkene products, but in the case of 4a2 a fast photo-Nazarov reaction led to 4a. HAT process. However, this pathway is unlikely as the computed activation energy was 35.3 Kcal/mol. We hypothesize that 1a can undergo Norrish type I fragmentation, leading to generation of the allyl radical. It was reported that the radical-radical coupling can happen at a least steric hindered center between allyl radical and acyl radical, leading to olefin 7a2 (see SI for detail mechanism). We then performed similar computations on bicyclic cyclopropane 3a. In this case, the Norrish type II pathway A is energetically unfavorable for both 1,4 and 1,5 HAT with energy barriers of more than 20 Kcal/mol, while only 6.1 Kcal/mol is required for C-C bond cleavage. The reason might be due to the rigidity of structure 3a, making the HAT process geometrically less favored. Intermediate 4a1 could be obtained via 1,4 HAT from IB1-3aT to IB2-3aT then ISC and relaxation the singlet ground state IB2-3as. However, transition state TSB2-3aT was around 33 Kcal/mol higher in energy than IB1-3aT, which is challenging to reach under our conditions. When looking at the singlet state energy surface of intermediate IB1-3aT, we realized that after ISC, IB1-3aT would spontaneously transform to 4a2 via a 1,2 hydrogen shifts. In fact, it was demonstrated that the generation of 1,3 biradical often result in 1,2 hydrogen shifts in competition with radical-radical coupling to give cyclopropanes. 18b-f As described in literature, the photo-Nazarov reaction only happens if sufficient twisting of the double bond is possible (see SI). 17 This process is well established with cyclohexenyl phenyl ketones such as 4a2, furnishing product 4a under UV irradiation. In contrast, more rigid structures such as present in 5a disfavor double bond twisting, hence the intermediate olefin 6a can be isolated. We then explored synthetic applications of the photo-rearrangement of polycyclic cyclopropanes 3a-k and 5a-c (Scheme 4A). Starting from bicyclohexanes 3a-d, hydrofluorenones 4a-d bearing alkyl and fluoro substituents were formed in 71 -86% yield via the photo-isomerization-Nazarov cascade. Tetrahydropyran and thiophene derivatives 4e and 4f were also successfully obtained. Bicyclo[3,2,1]octene products 6a-c were obtained in 54 -67% yield via the photo-induced isomerization starting from norbornene derived cyclopropanes 5a-c.