The chemical industry has to shift from fossil fuels to renewable carbon feedstocks to mitigate its impact on the climate. Lignocellulosic biomass is a promising candidate as it is ubiquitous on land and formed by carbon sequestered from the atmosphere. However, its chemical composition differs from fossil hydrocarbons, presenting challenges and opportunities. Specifically, the abundance of oxygenated groups increases biomass reactivity, which promotes degradation during chemical treatments, therefore requiring active stabilization to prevent condensation. Such stabilization allows the production of biobased platforms with O-functionalities that can be used in efficient conversions or the design of safer alternatives to petrochemicals. To that end, we studied the potential of acetalization to prevent biomass condensation and preserve O-functionalities in various applications. First, we expanded aldehyde-assisted fractionation (AAF) to endocarp, reporting mono-aromatic yields up to 30% higher than with wood due to high lignin content. AAF facilitated lignin valorization when using industrially relevant particle sizes (3-5 mm), yielding 80% to 170% more monomers than RCF on similarly sized particles. This difference correlates with the feedstock porosity, demonstrating the importance of acidic pretreatment for substrates with low accessibility. We then demonstrated efficient vanillin and syringaldehyde production from AAF-lignin via aryl-a-diketones. These previously poorly studied monomers were hydrogenated to their corresponding glycols with Pt/C (80°C, 20 bars H2) which were then cleaved oxidatively (VO(acac)2, 80°C, under air). Both steps gave quantitative yields. When applied to birch AAF-lignin, 4 wt.% of dry biomass was converted to aromatic aldehydes. After the production of drop-in chemicals from a-diketones, we explored their conversion to high value plant pigments anthocyanidins and associated novel synthetic analogues. Anthocyanidins were produced by converting a-diketones into their enol-acetate derivatives, before condensing them with phloroglucinol (33-69% yield), while synthetic analogues were produced by direct condensation with phloroglucinol (46-87% yield). Importantly, the analogues showed enhanced stability compared to the natural structures. Overall, this work illustrates the potential of reactive oxygenated lignin monomers in synthesizing valuable natural products. Finally, we expanded aldehyde functionalization to glycerol to demonstrate the potential of exploiting O-functionalities on one of the cheapest biobased molecules available today. We demonstrated the scalable production of hydroxy-acids by acetalization with glyoxylic acid at high yields (86%) that were converted to polyesters with tunable properties (crystallinity, Tg = 32-64°C) and to bicyclic lactones. The latter achieved polymers with high MW (280 kDa) through ring-opening polymerization, and that can be recycled at low temperature (170°C). In summary, we demonstrated the versatility of AAF in the production of biobased molecules that can be exploited by largely conserving their native structure, including their O-functionalities. Their reactivity can be leveraged to produce drop-in chemicals and explore new chemistry to access natural products and their analogs or even new polymer precursors. Overall, this thesis demonstrates the opportunities offered by the preservation of natural structures and especially O-functionalities in biobased molecules.