Acyclic oxidative cleavage

Scheme 7.5 Formation of some aroma compounds after oxidative cleavage of a acyclic carotenoids (e.g., lycopene, phytofluene and phytoene) and b cyclic carotenoids (e.g. a-carotene and / -caro-tene)...

Oxidative cleavage of P-aminoacyl complexes can yield P-amino acid derivatives (320,321). The rhodium(I)-catalyzed carbonylation of substituted aziridines leads to P-lactams, presumably also via a P-aminoacyl—metal acyclic compound as intermediate. The substituent in the aziridine must have 71 or n electrons for coordination with the rhodium (322,323). 

Trans -substituted diarylalkenes undergo oxidative cleavage upon treatment by potassium permanganate in the presence of moist alumina as a solid support (equation 32)158. Under the same conditions, cyclic alkenes, with medium-sized rings, give acyclic dialdehydes (equation 33). 

Lead acetate azides, Pb(OAc)4 (N3)n, prepared in situ from lead tetraacetate and azidotrimethylsilane, react with alkenes to yield a variety of products, depending on the structure of the alkene 1,2-diazides. 1,2-acetoxy azides, a-azido ketones, allylic azides, and <5-oxo nitriles (by the oxidative cleavage of cyclohexene rings)97. The diazides and acetoxy azides are formed by preferential syn addition, but the diastereoselectivity (up to 3 1) is far from satisfactory with both acyclic and cyclic alkenes98,99. 

The (3-methyl homoallylic alcohol moiety of both anti- and 5yn-configurations is a characteristic structural element of a number of macrolides and polyether antibiotics. Reactions of crotylmetal (2-butenylmetal) reagents with carbonyl substrates provide access to acyclic stereo- and enantioselective syntheses of p-methyl homoallylic alcohols. The alkene moiety of these alcohols can be further elaborated into aldehydes by oxidative cleavage of the double bond, leading to aldol-type products. 

Synthesis of the acyclic portion began, as in the previous synthesis, with enantiomerically pure citronellol (25). Protection of the alcohol as the benzyl ether and oxidative cleavage of the olefin to the aldehyde gave 26 (85%). Chain extension via the masked acyl anion equivalent 27, alcohol protection, and concomitant -elimination and isomerization of the allene to die alkyne with butyl lithium gave 28. The resulting protected ketone must now be converted to the P-alcohol required for the completion of the synthesis. Thus hydrolysis to die ketone followed by enantioselective reduction with (—)-N-methylephedrine-... 

Oxidative cleavage of cycloalkanones The formation of acyclic keto acids from a-substituted cyclopentanones and cyclohexanones by Cu(NOj)2 -mediated autooxidation in aqueous acetic acid is quite efficient (60-96% yield). 

An acyclic precursor devised by Bogndr and Herczegh, prepared via oxidative cleavage of the furane nucleus, has been applied in the synthesis of racemic pentoses. 2-(2-Furyl)-4,4,S,S-tetramethyl-l,3-dioxolane 257 obtained in reaction of furfural with 2,3-dimethylbutane-2,3-diol was chosen as a substrate, owing to its stability in acidic media (at pH > 3). Oxidation of 257 with bromine water gave the unstable endialone derivative 258, which was reduced with sodium borohydride to 259 (isolated as crystalline dibenzoate). Epoxidation of 259 gave... 

Ochiai and coworkers have developed an efficient iodoarene-catalyzed oxidative cleavage of alkenes and alkynes using mCPBA as a terminal oxidant [40], Various cyclic and acyclic alkenes as well as aliphatic and aromatic alkynes are smoothly cleaved to carboxylic acids under these organocatalytic conditions (Scheme 4.15) [40]. 

Silicon has also been employed as a tether for directing ringclosing metathesis (RCM) reactions. The RCM precursors 38 were generated by attachment of chlorodimethylvinylsilane to alkenyl alcohols. An olefin metathesis reaction using Schrock s catalyst provided the cyclized products 39 in excellent yields (eq 18). Although Grubbs catalyst had been successfully employed in the case of allylsiloxy dienes, Schrock s catalyst resulted in better yields in the case of the vinylsiloxy dienes 39. Oxidative cleavage of the silicon tether was then used to provide acyclic hydroxyaldehydes. 

Preparation of the corresponding a isomers of these C-dimers requires only slight modification of the syntheses, as illustrated in Scheme 39 by simple reversion of the order of the synthetic steps in the preparation of the acyclic precursor. Hence, the isopropylidene in intermediate 141 was hydrolyzed, followed by diol cleavage and preparation of the anomeric />-nitrobenzoate 146. Introduction of the C6 carbon could be achieved by stereoselective allenylation resulting in formation of the manno derivative 147 after chair inversion. Oxidative cleavage then reduction of the allene moiety and subsequent debenzylation and methyl glycoside formation led to the a-D-Man(l-4)-D-Glc carbon analog 148. Access to methyl C-maltoside 149 was... 

A carbonyl group can be protected as a sulfur derivative—for example, a dithio acetal or ketal, 1,3-dithiane, or 1,3-dithiolane—by reaction of the carbonyl compound in the presence of an acid catalyst with a thiol or dithiol. The derivatives are in general cleaved by reaction with Hg(II) salts or oxidation acidic hydrolysis is unsatisfactory. The acyclic derivatives are formed and hydrolyzed much more readily than their cyclic counterparts. Representative examples of formation and cleavage are shown below. 

Iron(II) salts, usually in conjunction with catalytic amounts of copper(II) compounds, have also been used to mediate radical additions to dienes91,92. Radicals are initially generated in these cases by reductive cleavage of peroxyesters of hydroperoxides to yield, after rearrangement, alkyl radicals. Addition to dienes is then followed by oxidation of the allyl radical and trapping by solvent. Hydroperoxide 67, for example, is reduced by ferrous sulfate to acyclic radical 68, which adds to butadiene to form adduct radical 69. Oxidation of 69 by copper(H) and reaction of the resulting allyl cation 70 with methanol yield product 71 in 61% yield (equation 29). 

Amenates acylated at the exocydic nitrogen are stable as solid compounds but decompose in aqueous solutions releasing NO. This decomposition depends on the pH and most importantly on their chemical structure [147, 152]. The proposed mechanism of NO-release is shown in Scheme 6.24. It is related to that postulated for sydnonimines. The main difference is that here 5-substituted amenates 128 are able to react with water to form acyclic nitroso semicarbazides 129 directly without needing enzymatic cleavage, and these intermediates release NO by an oxidative or thiol mediated mechanism that is not fully understood [153]. 

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