Trans-carbamate structures can be prepared in a diastereoselective approach by a judicious one-pot combination of in situ prepared organic carbonates and suitable amine reagents under appropriate reaction conditions. This unprecedented approach allows for stereo-divergence from a single oxirane substrate with easy access to both cis and trans carbamate isomers with high stereo- selectivity (dr > 19:1). Key to control the diastereo-selective nature of the conversions leading to the trans carbamates is the in situ formation of trans-configured oligo/polycarbonates through Al- catalysis providing the targeted products after aminolysis. The present results demonstrate the valorization of a renewable carbon-based reagent (CO2) into new valuable scaffolds and an unusual stereo- control exerted through carbonate intermediates. A series of control experiments support the proposed mechanistic rationale towards the trans-carbamate products which is based on the trapping of an in situ formed trans-configured oligo/polycarbonate.