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FULL PAPER Mild Protocols for Generating Molecular Complexity FULL PAPER Mild Protocols for Generating Molecular Complexity: A Comparative Study of Hetero-Domino Reactions Based on the Oxidant and the Substitution Pattern José I. Candela Lena,[a] Elena M. Sánchez Fernández,[a] Alwar Ramani,[a] Nicolas Birlirakis,[a] Alejandro F. Barrero,[b] and Siméon Arseniyadis*[a] Keywords: Domino reactions / Glycol fission reagents / Ring expansion / Synthetic methods A bicyclic unsaturated 1,2-diol framework was found to un- process. The substrates were chosen to test the limits of reac- dergo an interesting set of domino reactions on treatment tivity of various unsaturated 1,2-diol derivatives and to pro- with oxidants. One-pot syntheses of seven-membered-ring- vide guidelines for their use in constructing complex syn- containing frameworks were achieved by a Pb(OAc)4-medi- thetic targets. In this article we provide details of the reaction ated domino process, while with other oxidants investigated, conditions, preparation of the starting materials and charac- such as Mn(OAc)3, Dess–Martin periodinane, PhI(OAc)2, terization of the products, along with new, unpublished re- Ph3Bi(CO)3, NaIO4, and NaBiO3, the domino process was in- sults. Information is assembled to facilitate comparison be- terrupted halfway through. Finally, tetrapropylammonium tween the various oxidants, solvents and substitution pat- perruthenate (TPAP-NMO), Pd(OAc)2, pyridinium dichro- terns. mate (PDC), and pyridinium chlorochromate (PCC) gave only allylic oxidation and further diketone formation without any (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, of the bond cleavage that is necessary to initiate the domino Germany, 2005) Introduction ?with a series of ene-diols can lead to a cascade of reactions that provides an entry into a variety of molecular architec- Domino reactions continue to be a topic of substantial tures. The direct fragmentation of octaline diols provides [1] interest and intensive exploration. Their usefulness in syn- access to a range of functionalized cycloheptanes,[6] as well thesis stems from their ability to maximize molecular com- as bicyclo[3.2.2]nonanes. The domino sequence can be con- plexity while minimizing waste. This has significantly trolled by changing the stoichiometry: using only one equiv- broadened their applicability for multi-step syntheses of alent of the reagent generates the isolable, and stable, cyclic natural compounds. The reaction of lead tetraacetate with ene-acetal III, the so-called “half-cascade” intermediate, olefins, as well as the glycol fission reaction, were originally while two equivalents lead to the ring-expanded product II [2] investigated by Criegee and subsequently studied by sev- (Scheme 1). These rearrangements are of interest in view of eral other chemists. There is considerable precedent in the the synthesis of a number of bis-angularly substituted bicy- literature for cleaving glycols, such as the Malaprade reac- clic lactones V, as well as heavily substituted bridged and tion (periodic acid cleavage) or the Rigby oxidation (glycol fused seven-membered-ring-containing systems such as IV [3] fission with sodium bismuthate), and others. The presence and VI. Tricyclic lactones of type VII are the only bypro- of an adjacent olefin considerably increases the range of ducts obtained so far. The octalone-derived unsaturated [4] products accessible. Unsaturated 1,2-diols undergo glycol diol system I was chosen to initiate this study because it fission upon treatment with Pb(OAc)4, which initiates a presents optimum opportunities to examine the processes domino process under very mild conditions. In our efforts as the “half-cascade” intermediate III is isolable and stable to generate high molecular complexity in a one-pot reaction in this series. we examined the unsaturated bicyclic diol system I The mechanistic pathways involved in both the oxidative- (Scheme 1), which is a versatile template for the domino cleavage-induced domino transformations and in the base- transformations during lead tetraacetate mediated oxidative induced ring-system interchange reactions have been dis- cleavage in the synthesis of highly elaborated cyclic systems cussed in previous papers.[7] An overview of the mechanism [5] containing six- or seven-membered rings. We have already deduced from earlier studies is portrayed in Scheme 1 to ?shown in several publications that the reaction of Pb(OAc)4 assist the reader’s understanding. The domino process is shown to proceed by a cleavage/[4+2] cycloaddition/oxy- [a] Institut de Chimie des Substances Naturelles, CNRS, 91198 Gif-sur-Yvette, France plumbation/deplumbation/ring-expansion sequence, all the E-mail: [email protected] steps of which are initiated by a multi-task reagent: lead [b] Universidad de Granada, Avda. Fuentenuevas/n, 18071 Granada, Spain tetraacetate. Eur. J. Org. Chem. 2005, 683–700 DOI: 10.1002/ejoc.200400549 © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 683 FULL PAPER S. Arseniyadis et al. Scheme 1. Reagents and conditions: (a) 2.4 equiv. Pb(OAc)4, MeCN, 0 °C to room temp.; (b) 1.2 equiv. Pb(OAc)4, MeCN, 0 °C to room temp.; (c) K2CO3/MeOH/H2O, room temp.; (d) O3,CH2Cl2 at –78 °C, then K2CO3/MeOH/H2O, room temp.; (e) LiAlH4, THF, reflux then H+-acetone room temp. To extend the scope of the process, it was essential to acetoxy-enone 1 was converted into the unsaturated diol 2 this study that some other oxidants, less toxic than lead by a simple low temperature treatment with methyllithium tetraacetate, were studied in various solvents. To this aim, (THF, –78 °C); similarly, 4 gave the unsaturated diol 5. The the effect of the oxidant was examined using a number of next target, the unsaturated bicyclic diol 7, was synthesized known reagents employed in glycol cleavage. This paper de- in a three-step sequence from 6, which, in turn, was pre- scribes the synthesis of several variously substituted 2,3-di- pared from commercially available dimedone on a large hydroxy-4-ene bicyclic frameworks, and the rearrangements scale using the Heathcock procedure.[10] that these systems undergo when treated with an oxidant. Previously described methods were used to prepare oc- taline-diol 3, its free-ketone counterpart 8 and hence the exo-methylene derivative 9 from the well known Wieland– Preparation of Substrates Miescher ketone.[11] The four-step process from 3 was ef- We thus embarked on the synthesis of a set of substrates fected in 78% overall yield (Scheme 2). The allylically sub- for investigating the effect of stoichiometry, oxidant, substi- stituted substrates 11, 12, and 26 (Table 1) were generated from the corresponding acetoxy-enones by way of the inter- tution pattern, solvent, temperature, and reaction time. Di- [12] ols of type 2, 3, 5, 7, 8, 9, 11, 12 (Scheme 2), and 18 mediate dienol acetate species, using known procedures. (Scheme 3) were studied in order to optimize the condi- The dienol acetates, obtained by heating of the acetoxy- tions. On the whole, for the preparation of the bicyclic un- enones in pyridine in the presence of acetic anhydride and saturated 1,2-diols − the required domino substrates − the 4-(dimethylamino)pyridine (DMAP), were then epoxidized [8] with methyltrioxorhenium (MTO) as catalyst and aqueous well-known Robinson annulation procedure was used, [13] starting from the appropriate cyclic ketone and methyl vinyl hydrogen peroxide as terminal oxidant. ketone. The required diols were prepared, in a straightfor- As the effectiveness of the process could be further im- ward manner, from the bicyclic enones thus obtained fol- proved if the resulting complex heterocycle is poised to un- lowing our previously published procedure.[9] To prepare dergo spirocyclization, we came to favor octaline-diol 18, obtained by an asymmetric Robinson annulation,[14] as a acetoxy-enones (such as 1 Scheme 2), Pb(OAc)4 and the corresponding enones were heated in benzene at reflux, un- subgoal of the synthesis of spirocycles. This intermediate der Dean–Stark conditions for water removal. Acetoxylated would provide a platform structure for evaluating possibil- compounds were isolated in good yields (70–90% yield) ities for the construction of the spirocyclic frameworks along with unreacted starting material. The diols were then found in many natural products. The elaboration of the tar- get octaline diol 18 began with the known bicyclic enone obtained by reduction with lithium aluminum hydride in [15] diethyl ether at 0 °C. Exploratory studies focused on the alcohol 13a, which was converted into the bis-OTBS pro- synthesis of diols 2, 3, 5, 7, 8, 9, 11, and 12. The unsaturated tected acetoxy-enone 17 in four steps by the sequence diols 2 and 5, which possess a tertiary-secondary diol sys- shown in Scheme 3. TBS protection of the secondary hy- tem, were obtained from the acetoxy-enone 1 and acyloin droxyl group (TBSCl, DMF/imidazole, 0 °C, 24 h) followed 4, respectively (intermediates in our earlier studies).[9] Thus, by hydroboration (BH3·Me2S, H2O2/NaOH, EtOH, 8 h), 684 © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjoc.org Eur. J. Org. Chem. 2005, 683–700 Mild Protocols for Generating Molecular Complexity FULL PAPER Scheme 2. Reagents and conditions: (a) MeLi, THF, –78 °C (86–90%); (b) LiAlH4/Et2O, 0 °C (99%); (c) 10% HCl/THF, 1:1, 4 °C, 12 h (99%); (d) TBSCl, DMF/imidazole, 0 °C, 2 h (87%); (e) PPh3MeBr, tBuOK, PhMe, room temp., 4 h (94%); (f) TBAF/THF, 60 °C, 4 h (96%); (g) ethylene glycol, p-TosH, room temp. (95%); (h) 4 equiv. Pb(OAc)4, benzene, 90 °C, 4 d (85%); (i) Ac2O, Py, 4-DMAP, 80– 100 °C (90%). (j) MTO, 30% H2O2,Py,CH2Cl2, room temp.(70%); (k) TMSOTf, collidine, PhMe, –40 °C (95%) Scheme 3. Reagents and conditions: (a) TBSCl, DMF/imidazole, 0 °C, 24 h (89%). (b) BH3 ·Me2S, H2O2/NaOH, EtOH, 8 h (78%).
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