Photochemical control of a selfassembled supramolccular I : 1 pseudorotaxane (formed between a tetracationic cyclophane, namely the tetrachloride salt of cyclobis(paraquat-p-phenylene) , and 1 .S-bis[2-(2-(2-hydroxy)ethoxy)ethoxy]naphthalene) has been achieved in aqueous solution. The photochemical one-electron reduction of the cyclophane to the radical trication weakens the noncovalent bonding interactions between the cyclophane and the naphthalene guest-n-n interactions between the n-electron-rich and nelectron-poor aromatic systems, and hydrogen-bonding interactions between the acidic r-bipyridinium hydrogen atoms of the cyclophane and the polyether oxygen atoms of the naphthalene derivative-sufficiently to allow thc gucst to dcthread from the cavity; the process can be monitored by the appearance of naphthalene fluorescence. The radical tricationic cyclophane can be oxidized back to the tetracation in the dark by allowing oxygen gas into the system. This reversible process is marked by the disappearance of naphthalene fluorescence as the molecule is recomplexed by the tetracationic cy-

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[2]Pse~dorotaxanes [~~ and [2]rotaxane~[~~ can be assembled from cyclobis(paraquat-p-phenylene) as rings and 1,4-dioxysubstituted arenes as threads bearing macrocyclic polyethers at their termini. The [2]pseudorotaxanes can be readily disassembled by the addition of alkali metal cations. Here, we demonstrate how these [2]pseudorotaxanes can be used to produce an optical response on metal binding. The [l Slcrown-6 derivative (R,S)-5, containing a 1,5-dioxynaphthaIene residue, can act (see Scheme 2) both as a host toward alkali metal cations and as a guest toward the tetracationic cyclophane S4+. The binding of (R,S)-5 by S4+ can be reduced significantly by the addition of alkali metal cations, such as K' ions, which form complexes with [18]crown-6 derivatives."'] In this manner, we can achieve the unthreading of the

The reductive fission of cyclic trithiocarbonates provides a useful method for the synthesis of dithiols, and is applicable to the preparation of vicinal disecondary thiols which may not be readily accessible by other routes. The method offers particular advantages in the carbohydrate field, and has been used to synthesise three dithiols and one tetrathiol related to the hexitols.Although cyclopentene oxide does not give a trithiocarbonate with xanthate, cyclopentene sulphide readily gives tmns-1,2-(thiocarbonyldithio)cyclopentane (XXX), a compound containing two trans-fused five-membered rings. Reasons for this difference in behaviour are discussed.ALKALINE hydrolysis of cyclic trithiocarbonates is reported l p 2 to give only poor yields of dithiols. It is known, h~w e v e r ,~ that thiols can be prepared very satisfactorily by the reductive fission of xanthates with lithium aluminium hydride, and this procedure should evidently be applicable to trithiocarbonates, though the only report is of the small-scale reduction of the unsaturated cyclic trithiocarbonate (I) to give ethane-l,2-dithiol in unspecified yield.In some trial experiments, ethane-1,2-, propane-1,3-, propane-1,2-, and cyclohexanetrans-lJ2-dithiol were obtained by reduction of the corresponding trithiocarbonates with lithium aluminium hydride in yields of 86, 37, 75, and goyo, respectively; for the last two compounds the reported The configuration of the cyclohexane-trans-l,2-dithiol (IV) was not specified by the earlier workers, but since the reductive fission does not affect any asymmetric centre it must be the same as that of the trithiocarbonate (111) from which it is derived; the latter is trans, since it is formed through the intermediate episulphide (II).? The dithiol obtained by fission of the episulphide (11) with potassium hydrogen sulphide must also be trans. Although cyclohexane-trans-l,2-diol does not condense with aldehydes or ketones under normal condition~,~ the dithiol (IV) readily gave the crystalline isopropylidene derivative (V) on treatment with acetone and a trace of sulphuric acid at room temperature; this is no doubt due to the great stability of the resulting dithiolan system. The corresponding benzylidene and piperonylidene derivatives have already been described,2 but without comment.Conventional methods for the synthesis of simple thiols often fail when attempts are made to prepare vicinal dithiols in which both groups are secondary. Thus many vicinal disecondary halides undergo dehalogenation to the olefin, rather than substitution, on yields by alkaline hydrolysis are 26 and 14%.* Part XX, Fitt and Owen, J., 1957, 2250. t The mechanism of the formation of cyclic trithiocarbonates is outlined in the preceding paper.5

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