It has been shown that the presence of a methylenedioxy substituent in a planar aromatic molecule tends to favour its crystallisation in highly overlapped structures. Thus, in a series of methylenedioxycinnamic acid derivatives, there is a preference for the P-structure over the U -and y-forms. It is suggested that this substituent is a good steering device towards the p-form since, in this form, the oxygen atoms of the substituent can participate in stabilising non-bonded interactions involving p electrons and further, the compactness of the substituent allows the molecules to crystallise with a 4 A repeat. The crystal-structure determination of 3,4-methylenedjoxycinnamic acid, (1 ), illustrates these concepts. The crystals of (1 ) are triclinic, the space group is PI, Z = 2, a = 3.804(3), b = 10.502(5), c = 11.1 12(4) A, u = 77.84(4),

Abstract. M,=306.5, P2Jc, a=6.098(2), b= 14.685(3), c=15.143(2) A, fl=95.89(2) ° , V= 1348.9 (6) A,3, Z = 4, D m = 1.49 Mg m -3, D x--1-51Mgm -3, 2(CuKct)=l.54178/k, p(CuKct)= 39.9 mm -l, R = 0.066, R w = 0.099 with 1737 independent non-zero reflections. The S atom is tetrahedral and the phenyl ring makes an angle of 55 o to the plane of the heterocyclic ring. There is a close intermolecular approach of a C1 atom to an adjacent fused-ring system.

Die Kristallchemie von Zimtsäurederivaten wird in Hinsicht auf den Einfluß untersucht, den die Beteiligung von Alkoxygruppen an Wechselwirkungen ohne Bindungsbildung ausübt.