Studies on π-Face Selective Additions with 3, 4-Di-tert-butylthiophene 1-Oxide and 1-Imide

Abstract

3, 4-Di-tert-butylthiophene 1-oxide (1) and 1-imide (2), which possess anti- and syn-π-faces with respect to the S=X bond (X= O or NTs), provide an excellent model system to investigate π-face selectivity in a variety of addition reactions. Diels-Alder reactions of 1, including inverse electron-demand and hetero Diels-Alder reactions, all take place selectively at the syn-π-face of 1 with respect to the S=O bond in high yields under mild conditions. The observed π-face selectivity can be well explained by the difference in conformational changes that are brought about at the transition states for anti- and syn-π-face additions. Even 1, 3-dipolar cycloadditions of 1 take place predominantly at the syn-π-face to the S=O bond. Diels-Alder reactions of 2 also occur at the syn-π-face with respect to the S=NTs bond. The Diels-Alder reaction of 2 with PTAD provides the first synthesis of an S-unoxidized 1, 2-thiazetidine derivative. The addition of bromine to 1, though not π-face selective, occurs in a 1, 4-cis mode exclusively. Only Michael additions to 1 take place at the anti-π-face probably as a result controlled by nonequivalent orbital extension of π-LUMO.