Alkenes chain elongation

Alkenes, chain elongation, 56, 32 Alkenyl alcohols, (E J-disubstituted, 55, 66 A -Alkenylhydroxylamines, 58, 108... 

CHAIN ELONGATION OF ALKENES via gem-DIHALOCYCLOPROPANES l,l-DIPHENYL-2-BROMO-3-ACETOXY-l-PROPENE... 

Fluoroalkylations arc undesirable side-reactions of hydrogen fluoride catalyzed alkylations of alkenes.208 However, selective chain elongation can be achieved in moderate yields with ethene and its halogen derivatives. Thus, various alcohols (terl-butyl, 1,1-dimethylpropyl. benzyl) and alkenes, such as cyclohexene, have been used in fluoroalkylations with ethene derivatives in the presence of hydrogen fluoride (e.g., generation of 1 and 2, respectively).205,400... 

Irradiation of carbene complexes in CO atmosphere generates the ketene 305 and its [2+2] cycloaddition to alkene gives the cyclobutanone 306 [93], Total synthesis of (+)-cerulenin (310) has been carried out by the formation of cyclobutanone 309 by cycloaddition of 307 to the double bond of 308 as the key reaction without attacking the triple bond. Then cyclobutanone 309 was converted to (+)-cerulenin (310) via regioselective Bayer-Villiger reaction of 309, and side-chain elongation using n-methallylnickel bromide, epoxidation and hydrolysis [94],... 

Recently, most radical cyclizations of halides have been performed using iodides with intramolecular alkene or alkyne functionality. As with chain elongation reactions, both chain and nonchain processes are possible, with the former being the most common. The nonchain processes are not covered here and the reader is referred to the excellent reviews in the literature for details. 

Fig. 11.28. Aldehyde alkyne chain elongation via [1 -rearrangement of a vinyl carbene (Seyferth procedure). First, a Horner-Wadsworth-Emmons olefination of the aldehyde is carried out to prepare alkene A. Upon warming to room temperature, alkene A decomposes and gives the vinyl carbene B. From that, the alkyne is formed by way of a [1,2]-rearrangement.

The termination will be the loss of a proton to form an alkene (an El reaction), Providing that the tertiary carbocation is reasonably stable, this will be a slower process than chain elongation, especially as there are no good bases around, and long polymer chains may result. 

As was amply demonstrated in the preceding sections of this chapter, numerous C-C bond-forming reactions are applicable for the preparation of products with a terminal double bond. Thus the sequence (i) introduction of the terminal alkene moiety, (ii) isohypsic double bond transformation leading to the derivatives like 127 or 128, and (iii) the C-C bond-forming step, may be considered as a reliable operation for carbon chain elongation. 

The two-carbon chain elongation of protected D-glyceraldehyde (R)-24 and (R)-62 can be realized via ( -7-alkoxyallylboronate additions, followed by alkene ozonolysis. The allylboronate 162 generated in situ adds to (R)-62 giving a single stereoisomer 163, which can be converted to D-arabinose derivatives following ozonolysis (Scheme 13.57). Similarly, 164 added to (R)-24 gave a major adduct 165 that was then converted into 2-0-methyl-D-arabinose derivatives [105]. 

Two-carbon chain elongation of protected D-glyceraldebyde R)-37 and R)-99 can be realized via ( )-)/-alkoxyallylboronate additions, followed by alkene ozonolysis, furnishing D-arabi-nose derivatives [300]. 

The formation and further transformation of esters belongs to the fundamentals of organic chemistry. Moreover, some esters have enormous importance for example triglycerides (1), in the form of fats and oils, are produced in million ton quantities for a number of applications. Other esters, e.g. (2) and (3), are olifactory components waxes, e.g. (4), are used commercially to protect metallic surfaces against corrosion. Aspartame (5) is an important artificial sweetener, and pyrethrin (6) is the prototype of the pyre-throids, an unusually potent class of insecticides. Apart from these more applied considerations, esters are important synthetic intermediates in a number of multistep sequences. Striking examples are chain elongations via Homer alkenation or a-alkylations of ester enolates, in particular the ones stereocon-trolled by chiral auxiliaries. ... 

Lactones are normally stable compounds, which have found ample application as synthetic intermediates, and, quite recently, have been detected as the central structural unit in physiologically active natural products like obaflorin (123) and lipstatin (124). Characteristic applications of 3-lactones in synthesis are the stereospecific CO2 elimination to form di- and tri-substituted alkenes (e.g. from 125 equation 40) or Grignard addition to the carbonyl group e.g. equation 41). Particularly useful is the formation of 3-lactone enolates (126), which react with a variety of electrophiles (EX) wiA high stereocontrol (equation 42). Organocuprates may be used in chain elongations to form 3-branched carboxylic acids (equation 43). ... 

This method also allows a two-directional chain elongation with considerable structural flexibility. Thus, double Ireland-Claisen rearrangement of the symmetrical bis(propanoate) 21 gives meso, a]]-syn 23 in 86% d.r. together with complete control of the alkene geometry446. 

Displacement reactions. 1,3-Dibromo-l-trimethylsilylpropene is available from allyltrimethylsilane by reaction with NBS. The dibromo compound couples with Grignard reagents to effect chain elongation. With further displacement of the vinylic bromine and protodesilyation, it constitutes an intriguing approach to (Z)-alkenes. ... 

Unsaturated fatty compounds such as oleic acid [la], 10-undecenoic acid [2a], pet-roselinic acid [3a], erucic acid [4a], and the respective esters, alcohols, and native oils (Fig. 1) are alkenes and contain an electron-rich double bond that can be functionalized in many different ways by reactions with electrophilic reagents. It is therefore remarkable that >90% of oleochemical reactions have been focused on the carboxylic acid functionality and < 10% have been reactions of the alkyl chain and the C,C-dou-ble bond (1). A review on radical additions to unsaturated fatty compounds that appeared in 1989 (2) quoted only very few C,C-bond-forming reactions giving branched and chain-elongated fatty compounds. Since then, modem preparative radical chemistry has been developing and has been applied also to fat chemistry (3-5). We report here on radical additions of activated haloalkanes such as alkyl 2-haloalka-noates and 2-haloalkanenitriles to unsaturated fatty compounds [l]-[4] initiated by electron transfer from copper in solvent-free systems. These additions were also car-... 

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