[4.2]- and [4.3]-Metacyclophanes

Abstract

[4.2]Metacyclophanes may conveniently be prepared via carbene insertion into appropriately protected [2.2]-and/or [3.2]-metacyclophanediones. Thus, starting from [2.2]-metacyclophane-1,10-dione propylene thioacetal (1)[4.2]metacyclophane (6) was obtained (28%, three steps). For the synthesis of [4.3]metacyclophanes, [4.2] metacyclophane-2,12-dione (5) served as a key compound. Regioselective carbene insertion into (5) yielded [4.3]metacyclophane-2,13-dione (7) which was transformed into [4.3]metacyclophane (8)[7% starting from (1)]. The ring inversion barriers of (6)(60 kJ mol^–1, 308 K) and (8)(<38 kJ mol^–1,138 K) were estimated by ¹H n.m.r. spectroscopy.