Synthesis of optically active homotryptophan and its oxygen and sulfur analogues

d-Homotryptophan and its sulfur analogue have been synthesized by Sonogashira coupling between 3-iodoheteroarenes and ethynyloxazolidine followed by reduction of triple bond and oxidation of alcohol to acid. l-Homotryptophan and its oxygen analogue have been synthesized from silylated internal alkyne using Larock's heteroannulation as the key reaction. The alkynyloxazolidines were synthesized from l-serine and l-glutamic acid, respectively.