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toluene into the stable well-crystalline product of an intramolecular Diels-Alder reaction. Due to the complicated 'H-NMR spectrum it could not be decided unequivocally if the five-membered or the four-membered ring derivative, (3) or ( 4 ) , had been formed"]. Although postulated as intermediates of ally1 phenyl ether rearrangements, intramolecular [4 + 23 cycloadducts have not yet been isolated [']; such isolation has only been accomplished in the thermal rearrangement of propargyl phenyl ethersc3]. Thermolysis of 7-allyloxycycloheptatriene affor.ded a mixture of tricyclic reaction products of various ring X-Ray structure analysis of the stable rearrangement product (3) or ( 4 ) has now permitted a decision in favor of the structural alternative ( 4 ) , and also confirmed that both rearrangement steps (1 ) + 4 2 ) proceed suprafacially because the 0x0 group attached to phosphorus is located on the same side of the ring plane as the carbocyclic bridge newly formed from (2) by cycloaddition.Compound ( 4 ) crystallizes in the t$clinic space group Pi with a=9.940, b=12.164, c=10.660A; cr=82.11, p=115.25, y=110.25"; Z = 2 ; dC,,,=1.21 g~m -~. A single crystal (ca. 0.5 x 0.5 x 0.2 mm) was mounted on an automated Nonius four-circle diffractometer (Mo,, radiation) and 2931 independent reflections registered, 2555 with detectable intensity. Fo > 6 (Fo). The structure was solved by direct methods (MUL- Received: December 29, 1975 [Z 396 IE] German version: Angew. Chem. 88, 261 (1976) CAS Registry numbers: ( l j . 51-80-9: (2). 30354-18- 8: ( 3 a ) . 611-70-1: ( 3 b ) . 564-04-5:

The monoterpene glycoside secologanin is a key intermediate in the biosynthesis of most indole, cinchona, ipecacuanha, and pyrroloquinoline alkaloids, as well as of simple monoterpene alkaloids. More than a thousand alkaloids are formed from secologanin in vivo; this represents almost a quarter of this large group of natural products. It is also the parent compound of the secoiridoids. Many of the compounds derived from secologanin display a high degree of biological activity and are employed as pharmaceuticals, e.g., the dimeric indole alkaloid leurocristine (vincristine) which is used very successfully in the treatment of acute leukemia. A knowledge of the biosynthesis and biological reactions of secologanin provides a sound basis for the biosynthesis‐orientated classification of numerous natural products and the taxonomy of many plants. Secologanin and structurally related substances can be synthesized in a few steps by stereocontrolled photochemical and thermal cycloadditions. Its biomimetic reaction with amines and amino acids yields other natural products and compounds of pharmacological interest.

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