Tetrahedron Lmtem. zyxwvutsrqponmlkjihgfedcbaZYX Vol. 35, No. 30. pp. W l-5464, 1994 Elsevk !scialcc ud Printed in Great Britain w40-4039194 $7.00+0.00 zyxwvutsrqpo 0040-4039(94)01124-9 Acid-catalyzed Rearrangement of Pentacyclic Cage Compound zyxwvutsrqponmlkjihgfedc via a Strange By-path Ken-ichi Hirao,*a Hiroki Takahashi,a and Kimiko Kobayashib abomoayofor88nic~ isby.GtaduaRscboolofEnvirmmcntalEanhsdencc. HokMdo univasity, sappolo 060. Japan bIhe Instituted fiysicsl and ChemicalResearch (RIKEN). Hirosawa, Wake-shi. Saitama 35101. Japan Absuuct: The strange reaction pathway via an ally1catian 15 for the acidatalyzul Itereangemcnt of mcsybxy kclonc 2 TV diketones18 and 19 is rcvealai by an X-ray crystallogrephi study of mC zyxwvutsrqponmlkjihg inta m c d ii eamc 14. Some years ago. we found an efftcient method for the synthesis of the bisnordiamantane skeleton 111 by the completely legioqecitic leaWangement of the pentacyclic cage compound 12 using a BrBnsted acid (CF3CO2H apTsOH) as a catalyst. The rcgiospccificity of the &on (l+ 4; preferable rearrangement of bond a) was attributed to the favorable formation of the cadonic intermediate 4 ([PATH A]) over the formation of 5 ([PATH B]) on the basis of inspection of molecular mode1s.l The carbocation 4 then readily isomer&s to the OH-stabilized intermediate N-I+. which is then depmtonated to 8. Another set of analogous two-fold rearrangements involving bond a’ occurs to give the bisnotdiamantane 11. Later, this proposal was supported by the convincing explanation by using empirical fotce field calculations.3 This acid-catalyzed IWIIZ~~UI was intended for application to the preparation of other cage systems.4 When the pentacyclic mesyloxy ketone 2 was tteated with BF3cther in benzene. no trace amounts of expected 17 (having the skeleton ccnnqonding to 11) were detected, but two uuexpected diketones 18 (corrcspouding to 12.47%) and 19 (cortesponding to l&10%) were obtained, The loss of high regiospecificity observed during the teatrangement of 1 and the alteration of the mute fiom [PATH A] to [PATH Bl, which includes intetmcdiaries 10 and 16. have been ascribed to the steric hindrance of the mesylate groups to the bulky BF3 catalyst_4 However, it is necessary to modify this assumption in the following way. In order to look mote closely at the reaction of the mesyloxy ketone 2 with BF3. the re~tion ptuducts other than lg and 19 have been studied Among the pmducts. a set of isomeric enones (in 13 and 0.4% yields) has been obtained. The analysis of the spectral data of these isomeric enones reveals that the structutes should be 14 and 16.5 If the reaction of 2 with BF3 pmceeds by the presumed mechanism previously described (i.e. through [Path B]). the major enone (obtained in 13% yield) and the minor enone (obtained in 0.4% yield) should be 16 and 14, respectively. Because the distinction between the two cannot be unambiguously deduced from the spectral data, the confirmation of the s~ ~ uctunz. of the major enone has been made by X-ray analysis. In contrast to our anticipation, the structure, shown in Fig. 16 is indeed that of 14. When 2 and 14 wen mated with BF3 5461