Upgrading furan and small oxygenates obtained from the decomposition of cellulosic materials via formation of carbon-carbon bonds is critical to effective conversion of biomass to liquid transportation fuels. Simulation-driven molecular level understanding of carbon-carbon bond formation is required to design efficient catalysts and processes. Accurate quantum chemical methods are utilized here to predict the reaction energetics for conversion of furan (C4H4O) to C-5-C-8 ethers and the transformation of furfural (C5H6O2) to C-13-C-26 alkanes. Furan can be coupled with various C-1 to C-4 low molecular weight carbohydrates obtained from the pyrolysis via Diels-Alder type reactions in the gas phase to produce C-5-C-8 cyclic ethers. The computed reaction barriers for these reactions (similar to 25 kcal/mol) are lower than the cellulose activation or decomposition reactions (similar to 50 kcal/mol). Cycloaddition of C-5-C-8 cyclo ethers with furans can also occur in the gas phase, and the computed activation energy is similar to that of the first Diets Alder reaction. Furfural, obtained from biomass, can be coupled with aldehydes or ketones with alpha-hydrogen atoms to form longer chain aldol products, and these aldol products can undergo vapor phase hydrocycloaddition (activation barrier of similar to 20 kcal/mol) to form the precursors of C-26 cyclic hydrocarbons. These thermochemical studies provide the basis for further vapor phase catalytic studies required for upgrading of furans/furfurals to longer chain hydrocarbons.