Regioselective transannular cyclization of Deslongchamps's diketone. New entries to polycyclic cage structures: reductive opening of a cyclobutyl ketone with lithium in liquid ammonia and its photochemical rearrangement to a bridged cyclopentyl ether

Abstract

Regioselective bromination of Deslongchamps's diketone (6), followed by base-induced transannular cyclization, leads to f e t r a c y c l 0 [ 5 . 3 . 0 .~~~. 0~*~] decane-5,8-dione ( 8 ) , together with minor amounts of the isomeric tetracyclo[5.2.1 .02,6.03~8]decane-4,9-dione (9). The chemistry and photochemistry of the monoacetals of (8) have been studied, and structural assignments confirmed either by X-ray diffraction analysis or 2 D n.m.r. spectroscopy.Cage-structure compounds and their derivatives are of great interest in connection with a number of topics, such as the testing of the scope and limitations of synthetic methods, the determination of structure-reactivity relationships, and the theoretical and experimental estimation of strain energies. More recently, the lipophilic character of spherical hydrocarbon molecules has led to additional interest in the pharmacological properties of these compounds.2 b 3 a 4( 2 ) 10 general strategy are found in the synthesis of b~llvalene,~ t w i~t a n e ,~ cubane,' and dodecahedrane.6 In this context, properly functionalized derivatives having the skeleton of endo-tricycl0[5.2.l.O~*~]decane (1) are of interest for the synthesis of tetra-and penta-cyclo C,, systems (Scheme 1). Whereas photochemical intramolecular[2 + 2lcycloaddi-tions of derivatives of compound (1) have been an important synthetic starting point for bis-homocubane (2) derivative^,^ + E d 3 * cy; 4 4 0 ( 4 ) Scheme 1.Since compounds with cage structures have usually a substantial amount of strain energy, a general strategy for the synthesis of these compounds relies on the preparation of key intermediates which have adequate geometry and functionalization in order to create, through some irreversible process, the bond(s) which mostly contribute to the total strain of the molecule by means of proximity effects. Examples of such a nothing is known concerning the selective bond formation between C(3)-C(9), C(4)-C(9), and C(5)-C(9) which would lead to the tetracyclic skeletons (3), (4), and (5), respectively. In fact, derivatives of (3) and (4) have been obtained by selective reduction of one of the bonds of bishomocubane (2).8 On the other hand, derivatives of (5), and even the hydrocarbon itself have been synthesized starting from an intermediate with the