ON THE TI-TADDOLATE-CATALYZED DIELS-ALDER ADDITION OF 3-BUTENOYL-1,3-OXAZOLIDIN-2-ONE TO CYCLOPENTADIENE - GENERAL FEATURES OF TI-BINOLATE-MEDIATED AND TI-TADDOLATE-MEDIATED REACTIONS

A systematic investigation of the enantioselective Diels-Alder additio n of 3-butenoyl-1,3-oxazolidin-2-one to cyclopentadiene under the infl uence of catalytic amounts of dichloro-Ti complexes of ha',alpha'-tetr aaryl-1,3-dioxolane-4,5-dimethanols (TADDOLs) is described. The influe nce of the mode of catalyst preparation, amount of catalyst, presence of molecular sieves, concentration of the reactands temperature, solve nt, and TADDOL structure on this reaction is studied. Best results (en antiomer ratio er 94:6) are obtained with the TADDOL-bearing Arl = C6H 5 and C6H5/CH3 substituents in the dioxolane 2-position (47) and with the C-2-symmetrical TADDOL with Arl = 2-naphthyl and two CH3 groups on the dioxolane 2-position (16). A surprising reversal of the absolute topicity of the reaction is observed with TADDOLs (15, 28, 32, 38) bea ring four 1-naphthyl groups: the 2(R)- instead of the 2(S)-bicyclo[2.2 .1]hept-5-en-2-carboxylic acid derivatives are formed with enantiosele ctivities of up to 86:14. The crystal structures of several TADDOLs (1 6, 28, 47) and of the tetracyclohexylanalog 50 are described and compa red with previously determined structures. A superposition of 29 struc tures reveals that the cyclic array of atoms of the TADDOLate moiety a lways has two axial and two equatorial aryl groups in a lambda-type co nformation when derived from (RP)-tartrate and in a delta-type conform ation when derived from(S,S)-tartrate. The binaphthols (BINOLs) show s imilar structural features (lambda in (P) or (S) and delta in (M) or ( R) enantiomers). A mnemonic rule is disclosed which applies to the ste ric course of Ti-BINOLate- and Ti-TADDOLate-mediated reactions involvi ng monodentate and bidentate electrophiles. The possible structure of the reactive complex involved in enantioselective reactions mediated b y Ti-BINOLates and -TADDOLates, i.e., inter- and intramolecular [2 + 2 ] and [4 + 2] cycloadditions and ene reactions, is discussed.