A New Approach to 1,4-Oxazines and 1,4-Oxazepines via Base-Promoted Exo Mode Cyclization of Alkynyl Alcohols: Mechanism and DFT Studies

A new approach was developed to synthesize 1,4-oxazine and 1,4-oxazepine derivatives without solvent and metal. Regioselective cyclization occurred to afford exclusively the exo-clig product, and stereochemistry was studied by circular dichroism and specific optical rotation techniques. The Grignard reaction is a key synthetic step to produce high diastereomeric compounds via Cram's rule and was well supported by DFT calculations. A hydroalkoxylation mechanism was proposed and supported by DFT calculations.