LEWIS ACID-CATALYZED VINYL ACETAL REARRANGEMENT OF 4,5-DIHYDRO-1,3-DIOXEPINES: STEREOSELECTIVE SYNTHESIS OF CIS- AND TRANS-2,3-DISUBSTITUTED TETRAHYDROFURANS AND STRUCTURE-BASED DESIGN AND SYNTHESIS OF NOVEL INHIBITORS OF HIV-1 PROTEASE AND SARS-3CLPRO PROTEASE