Synthesis of Tyrosine‐Derived Tetrahydroisoquinolines by Lewis Acid Catalyzed Cyclization of N ‐(Phenylsulfonyl)alkyloxazolidinones

N ‐Boc‐protected tyrosine esters 5a , b were converted into tetrahydroisoquinolines 13 and 14 in four steps by reduction and ring closure to oxazolidinones 9 and 10 , addition of benzenesulfinic acid and aldehydes to sulfones 11 and 12 and subsequent Lewis acid catalyzed cyclization. In the case of m ‐tyrosine derivative 5a , selective protection with bromine prevented the formation of undesired regioisomers. Debromination of target compounds 13 was readily achieved under radical reduction conditions by using Bu 3 SnH/AIBN. Tetrahydroisoquinolines 13 and 14 were isolated as single diastereomers whose trans configuration was confirmed by X‐ray crystal structure analysis. Partial epimerization of trans ‐ 13i and trans ‐ 21 to the corresponding cis diastereomers was achieved under basic conditions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)