Aliphatic compounds, stereochemistry

In the previous sections the main focus of the discussion of substituent effects was their dependence on structure (especially stereochemistry) and the underlying transmission mechanisms. Admittedly, it would still be veiy difficult to predict 13C chemical shifts in more complicated and highly substituted aliphatic compounds with sufficient precision even if our knowledge about the physical back-... 

Oxidation of Aliphatic Compounds. - A general review of the use of supra-molecular systems as microreactors for photochemical reactions contains a section dealing with the photosensitized oxidation of alkenes included in zeolites, nation membranes and vesicles. Particular consideration is given to the possibility of controlling the form and environment of the sensitizer and substrate so that the reaction selectively follows an energy-transfer or an ET pathway. The same authors have also provided a more substantial review on the same theme. Recent developments in relation to the stereochemistry and mechanism of the ene photooxygenation of alkenes by O2 have also been reviewed. ... 

An interesting example of regioselective CM with ethylene as a tool in natural product degradation was recently disclosed by Hawaiian authors [149]. Thus, CM using catalyst C and ethylene gas was used to degrade the plant polyacetylene oxylipin (+)-falcarindiol (342) with uncertain stereochemistry at C3. As the reaction provided a meso product (343) in 81% yield by regioselective attack at the aliphatic side chain, the natural compound 342, isolated from a Hawaiian endemic plant, had the 3R,8S configuration shown in Scheme 66. 

There is insufficient information on the stereochemistry of the experimentally less simple hydrodimerization of aliphatic carbonyl compounds in a protic... 

Like the parent compound (1), alkyl- and/or aryl-substituted 1,2,3-trioxolanes can be generated only at low temperatures by the cycloaddition of ozone to alkenes. The concerted nature of the cycloaddition is reflected in the stereochemistry of the products observed aliphatic cis alkenes afford only cis 4,5-dialkyl-1,2,3-trioxolanes, and the analogous trans alkenes yield only trans 4,5-dialkyl-... 

In the sulfur literature only a few examples of thioaldehyde S-oxides 99 (mono-substituted sulfines) have been reported due to the low stability of these compounds, and they were not prepared by oxidative methods since the starting thioaldehydes are also very unstable. These problems were circumvented by using silyl thioketones249,250 as precursors which, by controlled oxidation with mCPBA, afforded the corresponding thioacylsilane S-oxides 98. These sulfines were subsequently desilylated with BU4NF (equation 104) and the stereochemistry of this process has been studied in detail408. This indirect route has been applied to the preparation of aromatic and aliphatic, not enethi-olizable, thioaldehyde S-oxides. 

The title compounds, isolated from Berberis buxifolia Lam. (Berberidaceae), are the most recently reported of the eight known bisbenzylisoquinoline /V-oxide alkaloids. They are noteworthy in being the first N-oxides of this group to have their complete stereochemistry assigned by NMR NOEDS studies (see Section VI,A,2,b). Unfortunately, the first report (59) was based on an incorrect assignment due to overlap of the NMe and aliphatic signals of calafatine (88, Section... 

The asymmetric addition of diethylzinc to heptanal, a straight-chain aliphatic aldehyde, was catalyzed by compound 20 (Table 3-6). All R,S catalysts 20, except 20s and 20u, afforded (S)-3-nonanol in moderate enantiomeric excess, regardless of the stereochemistry of the asymmetric carbon bearing the hydroxyl group. Cyclo-hexanol derivative 20g, which is less bulky around the carbinol center than catalysts... 

The stereochemistry of hydrodimerization of aliphatic carbonyl compounds has very rarely been studied. It was reported that an aliphatic ketone gave an almost equal quantity of the isomeric hydrodimers at a mercury cathode [160]. 

The stereochemistry of the alkene formed by coupling of aliphatic carbonyl compounds is related to the energy difference between the E) and (Z)-alkene. If AE is larger than 4-5 kcal/mol, the ( )-isomer is formed selectively. 

More recently, the same catalyst was used to produce cyclic amines with retention of stereochemistry from a simple linear aliphatic azide [53]. Treatment of a substituted aliphatic azide by complex 66 afforded the cyclized compound 75, by insertion of the nitrene moiety in allylic, benzylic, and even in the less reactive tertiary C—H bonds. The catalyst is inhibited by coordination of the product to the metal center. However, that can be avoided by using an in situ protecting agent (Boep is preferred over Fmoc-OSuc which leads to catalyst decomposition). 

In the Knoevenagel reaction, the thermodynamically more stable compounds are usually formed. However, a closely related transformation provides access to compounds of opposite stereochemistry. Condensation of phosphonoacetate (40) with aliphatic and aromatic aldehydes in the presence of N-methylmorpholine/TiCU yields product (41) with the thermodynamically more stable ( )-configura-tion. In contrast, titanated (40), obtained from the reaction of the sodium salt of (40) with ClTi(OCHMe2)3 reacts with aldehydes to give preferentially the thermodynamically less stable (Z)-isomer (42). ... 

Pyrazolones (154) and isoxazolones (155) can also be used in the Knoevenagel reaction. Thus condensations with aliphatic aldehydes, aromatic aldehydes and ketones in the presence of ethylenediammon-ium diacetate or other typical catalysts provide the corresponding alkylidene and benzylidene compounds in good yields (for stereochemistry see Section 1.1A new method involves the use of dicyclohexylcarbodiimide at 20 C without an additional catalyst. ... 

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