Cyclopentane stereochemistry

It may be seen from the scheme that although //ww-substituted olefins lead essentially to tran.v-substituted cyclopentanes, stereochemistry is not maintained in the case of n s-olefins. Thus, dialkylmaleates form a cisjtrans mixture of products. This is a result of palladium(0)-me-diated isomerization prior to codimerization. By employing the above-mentioned pumping technique, the yield of the same addition is nearly quantitative after 2 h at 130°C with 90% predominance of the m-isomer41. As with the TMM-Pd procedure, cyclodimerization with 2-cycloalkenones can only be achieved with 2-cyclopentenone in satisfactory yields wiLh the unsubstituted methylenecyclopropane. 

Cyclopentene derivatives with carboxylic acid side-chains can be stereoselectively hydroxy-lated by the iodolactonization procedure (E.J. Corey, 1969, 1970). To the trisubstituted cyclopentene described on p. 210 a large iodine cation is added stereoselectively to the less hindered -side of the 9,10 double bond. Lactone formation occurs on the intermediate iod-onium ion specifically at C-9ot. Later the iodine is reductively removed with tri-n-butyltin hydride. The cyclopentane ring now bears all oxygen and carbon substituents in the right stereochemistry, and the carbon chains can be built starting from the C-8 and C-12 substit""" ... 

In one of their solutions to the problem, Corey and his coworkers solved the first problem by starting with a preconstructed cyclopentane the stereochemistry was steered by deriving the oxygen atoms from a rigid bicyclic molecule. Alkylation of... 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

Synthesis, Stereochemistry and Transformations of Cyclopentane-, Cyclohexane-, Cycloheptane-, and Cyclooctane-Fused 1,3-Oxazines, 1,3-Thiazines, and Pyrimidines... 

The final chapter in Volume 69 is concerned with the synthesis, stereochemistry, and transformation of cyclopentane-, cyclohexane-, cyclohep-tane-, and cyclooctane-fused 1,3-oxazines, 1,3-thiazines, and pyrimidines and is authored by Professors Ferenc Ftilop, Gabor Bernath, and Kalevi Pihlaja from the Universities of Szeged in Hungary and Turku in Finland. This is a field which has shown rapid development over the last dozen years because of the increased availability of spectroscopic and other analytical methods allowing definition of the precise steric chemistry of these compounds. 

Cyclohexene does not polymerize by either route except when it is part of a bicyclic structure as in norbornene. Stereochemistry in the ROMP of norbomene is complicated since the polymer, LXVI in Sec. 7-8, has possibilities of isomerism at both the ring and the double bond. Most polymerizations by the typical ROMP initiators yield cis stereochemistry at the cyclopentane ring with varying amounts of cis and trans placements at the double bond [Ivin, 1987]. Metallocene initiators yield predominantly double-bond polymerization with 1,2-placement [Janiak and Lassahn, 2001]. 

Our approach was to use the unsaturated bromodeoxylactones in an intramolecular radical reaction, since these compounds possess both the radical precursor and the radical trap within the same molecule. Thus, reacting the unsaturated bromodeoxyheptonolactone 20 (Scheme 14) with tributyltin hydride and a radical initiator, the bicyclic lactone 65 a was obtained in a quantitative yield within 1 h. The stereocontrol in the reaction was determined by the structure of the product, since the compound obtained has two fused cyclopentane rings which can only be cis anellated. The radical A, which is the intermediate, was trapped by the tin hydride. The stereochemistry of the newly formed chiral center is determined by the configuration at C-4 in the educt 20 [45]. 

Scheme 68 illustrates cyclopolymerization of 1,5-hexadiene catalyzed by a homogeneous chiral zirconocene complex to form optically active poly(methylenecyclopentane), whose chirality derives from configurational main-chain stereochemistry (757). This polymer is predominantly isotactic and contains predominantly trans cyclopentane rings. 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

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