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Synthesis and X-ray structural analysis of anti-[3,3]metacyclophane-2,ll-dione quinhydrone dimethyl ether (3) are described. The formation of anti-3 starting from syn-9 may be ascribed to the syn-mnti isomerization of the syn-quinhydrone 17 according to a benzoquinone ring inversion process. This is in contrast to a reverse antihsyn isomerization (4 + 5a) of anti-[3.3]metacyclophane quinhydrone dimethyl ether 4. Results of a conformational analysis of the parent [3.3]metacyclophanes (1 and 2) and the calculated relative stability of syn and anti isomers (3, 17 and 4, 5a) suggest that the major driving force of these syn-anti isomerizations originates from the relative thermodynamical stability of the syn and anti isomers rather than from the donor-acceptor interaction between the rings. The anti geometry of 3 was confirmed by an X-ray structural analysis.Charge transfer (CT) cyclophanes are suitable model compounds for studying the orientation and distance dependence of CT interactions because these cyclophanes adopt a well-defined conformation and do not dissociate into the interacting components. CT cyclophanes of [3.3] systems are considered to be more suitable models for studying these CT interactions than the corresponding [2.2] systems since the donor and acceptor units of the former are almost in parallel planes, and the distance between them is closer than in the intermolecular CT complexes. Various CT cyclophanes have previously been synthesized, and the orientation and distance dependence of CT interaction have been investigated"].A conformational study of the parent [3.3]metacyclophane 1 revealed that the most stable conformation has the syn geometry with a chair-chair arrangement of the trimethylene chains [syn(chair-chair) and syn(boat-boat) l c (1.31 kcal/mol). In contrast, the anti isomers Id were predicted to be much less stable than syn isomers by more than 7 k~al/rnol[~~l. MM3 calculations predicted that the major factor which determines the stability of syn-la and anti-ld is the tortional strain of the trimethylene bridges; the tortional strain of Id is higher than that of l a by ca. 5 kcal/mol due to eclipsed interaction in each CH2CH2CH2 bridge in anti-ld, whereas none is present in syn-la [2e,4b]. The observed energy barrier (1 1.6 kcallmol) for the chair-boat flipping of the trimethylene bridges of 1 is much higher than that for the benzene ring inversion

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