Abstract:The first synthesis of 4´-X-phenylverbenene (X = F and Cl) via reaction of myrtenal with 4 -X phenylmagnesium bromide and Bi(NO 3 ) 3 is described. Verbenene is formed due to the electron-withdrawing effect of the 4 -electronegative group, either fluorine or a Cl-Bi complex.Keywords: Myrtenal, verbenene, bismuth nitrate, pinane, lewis acid.In recent years, bismuth compounds have attracted great interest in organic synthesis. Although their chemistry is not well known, bismuth salts have been widely used as Lewis acids to catalyze numerous reactions, mainly functional group deprotections, oxidations, reductions, rearrangements and C-C bond formation [1]. The last of these reactions can be achieved by Friedel-Crafts acylation, aldolic and Knoevenagel condensations, Diels-Alder cycloaddition, and Michael addition, to mention just a few possibilities [2]. Recently, bismuth has also been used in the chemistry of natural products [3]. Studies on the chemistry of natural products of pinene derivatives have increased in recent years because of the wide biological activity that this group of compounds presents. One of these derivatives is verbenene 2 (Fig. 1), which is found as a secondary metabolite in many medicinal plants and has been investigated as a beetle attractor [4,5]. Verbenene has also been used in the study of cycloadditions on account of its restricted stereochemistry and can also be viewed as a precursor in the formal synthesis of verbenone, which can provide a possible starting point in the synthesis of the A ring of taxol and other interesting compounds [6][7][8][9][10]. Verbenene has been synthesized through an electron transfer from -or -pinene to 2,3,5,6-tetrachlorobenzoquinone on sensitized irradiation where a radical cation rapidly deprotonates to give verbenene with retention of chirality [11]. To study the mechanism of bismuth-catalyzed additions to double bond carbonyl-activated systems, in the present work we examined the addition of Grignard reagents to myrtenal 1 catalyzed by Bi(NO 3 ) 3 . We found no difference in diastereoselectivity, but surprisingly observed the production of phenylverbenenes 3c and 3d in good yield when 4 -Cl-or 4 -F-phenylmagnesium bromide was added at 0°C (Scheme 1). Under the same conditions but in the presence of methyl iodide, alcohols 4 and 5 (c and d) give 3 (c and d respectively) in 98% yield (Scheme 1).
RESULTS AND DISCUSSIONWe used (1R)-(-)-myrtenal as a model system because some of its derivatives are good chiral auxiliaries that induce high enantiomeric excess because of the conformational restriction that is present in the bicyclic system [12][13][14][15][16]. Results of the addition of 4 -MeO-, 4 -Cl-or 4 -F-PhMgBr to myrtenal in the presence or absence of bismuth(III) nitrate at three temperatures (0°C, 25°C and reflux) are listed in Table 1.The results show no significant difference in diastereoselectivity among the various reactions, and the bismuth salt does not have any effect on the reaction. Although no significant diastereoisomeric excess w...