Silicon in organic synthesis. 25. Thermolysis and desiliconation-alkylation of [1-(trimethylsilyl)cyclopropyl]ethylenes as a route to spirocyclic sesquiterpenes

Abstract

Three routes to [ [ 1-(trimethylsilyl)cyclopropyl]methylene]cyclohexanes and -2-cyclohexenes have been developed. The condensations of [ 1-(trimethyKiyl)cyclopropyl]lithium with cycloalkyl carboxaldehydes and of a-lithio selenides with 1-(trimethylsily1)cyclopropanecarboxaldehyde (10) are useful and complementary methods for gaining access to the methylenecycloalkanes. For more highly unsaturated systems, the methodology involving titanium-(0)-promoted coupling of 10 to cyclic a,@-unsaturated ketones is preferred. Silylcyclopropane 26 has been transformed into the racemic forms of a-vetispirene, hinesol, and @-vertivone. Ita broad utility resides in the ease with which desiliconation-alkylation of this substrate can be effected and the high regioselectivity of new carbon-carbon bond formation. Additionally, thermal rearrangement of the trimethylsilyl, (methoxydimethyl)carbinyl, and cyano derivatives invariably proceeds with preferential recombination of the respective biradical intermediate from that surface which is less sterically congested. As a result, the product mixtures are dominated by those stereoisomers suited for elaboration into the spirovetivane sesquiterpenes.Interest in the development of new syntheses of spirovetivane sesquiterpenes2 continues unabated. Because stereoselective construction of the spiro[4.5]decane carbon framework and regioselective introduction of at least one double bond comprise the essential elements of any successful approach, these requirements serve as an effective testing ground of new methodology. T h e feasibility of deploying preparative techniques involving nucleophilic cyclization,3 electrophilic cyclization,4 photochemical cy-(8) For example: (a) Conia, J. M. Bull. SOC. Chim. Fr. 1968,3057. (b) Conia, J. M.; Drouet, J. P.; Gorge, J.

β-Silyl Arenes in the Arene-Olefin Photocyclization Reaction

“…It may be replaced by a proton, alkyl groups, or oxygen. 15 When treated with a source of fluoride, it may create a nucleophile that can react in high yield with proton sources, 16 alkyl iodides and triflates, 17 aldehydes, 18 and ketones, 19 as well as acylating groups such as acid chlorides 20 and carbon dioxide. 21 The unstable photoadducts were elaborated to more stable compounds in order to properly characterize them and, in particular, to verify the cis-syn-cis-relationship of the rings that should arise as a result of the endo selectivity (Scheme 4).…”

The McMurry Coupling and Related Reactions

“…25) 142 or α,β-unsaturated ketones (Eq. 26) 143 with monoaryl ketones or their corresponding aliphatic counterparts. The success of these reactions is also attributable to the sufficiently different redox potentials of two coupling components.…”

Low-Valent Titanium-Mediated Reductive Coupling of Carbonyl Compounds

“…Renard et al [98] recently reported on the scope and limitations of intermolecular McMurry cross-coupling reaction between a benzylic ester and a hindered ketone. Using TiCl 3 (10 equiv)-LiAlH 4 (5 equiv)-Et 3 N (6 equiv) in THF at 70˚C and slow addition of both carbonyl compounds, a simple 1.2 excess of 2-adamantanone over tert-butyl- [73,99]. In this regard, stereoselective cross coupling of fluorinated-with nonfluorinated carbonyl compounds represents a potentially versatile approach to new chiral fluorine-containing diols.…”