ChemInform Abstract: Unusual Cleavage of the Enolsilane C-O Bond: Transformation of 2-Silyloxy-1,3-dienes into 1,3-Dienyl-2-zirconium Compounds and Their Cross-Coupling Reactions (original) (raw)
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Tetrahedron Letters, 2004
A series of diazoketones tethered to tricarbonyliron coordinated dienes at the terminal position have been synthesized, and their decomposition reactions as carbene precursors studied. Intermolecular reactions with nucleophilic olefins were observed with Cu(acac) 2 as catalyst, the tricarbonyliron playing the role of an efficient protecting group of the diene fragment. With rhodium II acetate, intramolecular reactions predominate, with formation of five-membered rings, in the side chain, or less commonly, as a cyclopentenone joined side by side to a rearranged diene ligand. Unusually, due to cationic stabilization by the iron, six-membered rings in the side chain are also formed here. A still different reaction was observed during the thermal decomposition of a diazoketone tethered to a coordinated diene at position 2: the formation of a stable complex of a cyclohexa-2-4-dienone (iron assisted carbene reaction and multiple hydrogen migrations).
The Journal of Organic Chemistry, 1984
protons7 (Table I) on the linked carbons (C-8,9,12 and 13 in Figure 1). No couplings was observed between H-11 and H-12 as the dihedral angle is close to 90°. The NOE between the proton at 1-H and 3-Me(16-H) and one of the gem-dimethyls (19-H) indicated the locations of three methyls on the transoid diene system. 2D-NOE8 of 1 (3-Me/l-H, 19-H(Me)/lH, 2-H/13-H, 13'-H/20-H(Me), 13'-H/7-Me) revealed the gross conformation which is similar to that of its oxidation product 2.9 The location of the sec-methyl was suggested from the NOE between the 20-Me and 11-H of 2. 13C NMR (25 MHz) data (C&) also indicated a bicyclic structure with three trisubstituted double bonds (Table I); the assignments are based on the (1NEPT)'O method, selective decoupling, and the comparison with the data of 2. Compound 2, C2oH3zO (m/z 288.2452, calcd 288.2498) was isolated from the stored (6 years a t-20 OC) hexane extract of C. ugandensis soldier heads by column chromatography using 1O:l hexane/EtOAc over Si02. The tricyclic nature of compound 2 was disclosed from the 13C NMR data, which showed the presence of one trisubstituted double bond, an exocyclic disubstituted double bond, and a strongly shielded triplet carbon (19.0 ppm, C-13; shielded by the 7,17-ene) (Table I). The 13C NMR assignments are based on heteronuclear chemical shift correlation spectroscopy (CSCM).l' IH NMR (360 or 300 MHz) with COSY: 2D-NOE,8 sequential additions of lanthanide shift reagent [ E~(f o d)~-d ,~] , extensive homonuclear difference decoupling, and NOE measurements allowed for complete proton assignments (Table I) and also established the relative configurations 2. A possible biogenesis12 of cubugene 1 together with the co-occurring irregular cubitene 32 is shown in Figure 1.
Stereoselective Synthesis and Reactivity of Dienyl Zirconocene Derivatives
Synthesis, 2004
Stereoselective dienyl zirconocene derivatives have been prepared via a tandem allylic C-H bond activation isomerization-elimination reaction. These reagents can be either trapped directly with electrophiles or transmetalated to copper to participate in several carboncarbon bond formations. When the transmetalation is performed on allylic as well as vinylic zirconocene derivatives, the reaction occurs with inversion of stereochemistry of the dienyl system.
Organic Letters, 2001
A novel reaction between 1,3-dienes and selenium dioxide to give syn 1,2-and 1,4-diol cyclic selenites was studied in detail. This study indicates that an initial concerted [4 + 2] cycloaddition followed by a stereospecific carbon−selenium bond oxidation is involved in this unprecedented syn dihydroxy addition reaction mediated by selenium dioxide. The formation of a diol from an alkene is an important transformation in organic chemistry. 1 Usually, the syn addition of dihydroxy groups is achieved by OsO 4 , 2 RuO 4 , 3 and MnO 4 , 4 whereas anti addition is brought about by other oxides such as SeO 2 5 and WO 3. 6 In our investigations of the reaction between SeO 2 and 1,3-dienes, a novel reactivity profile of SeO 2 was observed, affording syn 1,2-and 1,4diols and selenophene. Treatment of diene 1, containing a carbonyl group at the C-1 position, with SeO 2 led to the formation of selenophene 2, whereas the analogous diene 3, containing a TBS-ether, yielded 1,4-diol cyclic selenite 4 and 1,2-diol cyclic selenite 5 (Scheme 1). Notably, the conversion of 3 to 4 involves an unprecedented direct oxidation of a C-Se bond to a CO bond, a transformation known to be difficult. 7 In this account, we elucidate the nature (1) (a) Haines, A. H. In ComprehensiVe Organic Synthesis; Trost, B.