Regio and stereoisomerization

Regio- and stereoisomeric cycloaddition products might be formed in such reactions. In this case the regioisomer 13 is formed as the major product in 98.5% yield. ... 

The effect of HMPA on the reactivity of cyclopentanone enolate has been examined.44 This enolate is primarily a dimer, even in the presence of excess HMPA, but the reactivity increases by a factor of 7500 for a tenfold excess of HMPA at -50° C. The kinetics of the reaction with CH3I are consistent with the dimer being the active nucleophile. It should be kept in mind that the reactivity of regio- and stereoisomeric enolates may be different and the alkylation product ratio may not reflect the enolate composition. This issue was studied with 2-heptanone.45 Although kinetic deprotonation in THF favors the 1-enolate, a nearly equal mixture of C(l) and C(3) alkylation was observed. The inclusion of HMPA improved the C(l) selectivity to 11 1 and also markedly accelerated the rate of the reaction. These results are presumably due to increased reactivity and less competition from enolate isomerization in the presence of HMPA. 

Trifluoroethene has been reported to undergo cycloadditions with conjugated dienes to give both [2 + 2] and [4 + 2] cycloadducts.25 Although mixtures of regio- and stereoisomeric cyclobutanes are obtained in these cases it still constitutes a synthetically useful reaction. 

Two features of the total synthesis of sugars based on noncarbohydrate substrates should be emphasized (1) dihydropyrans or oxabicycloheptanes can be transformed into a variety of regio- and stereoisomeric saccharides, and (2) from these substrates sugars belonging to unnatural configurational series can be readily obtained. 

Fig. 1. Regio- and stereoisomeric composition of the aldehydes arising from asymmetric hydro-formylation of (Z)-2-hexene catalyzed by Rh/(—)-DIOP15)...

An important and characteristic difference between reactions of N-alkylquinoxalinium salts with 1,3-dinucleophiles, giving rise to the formation of tetrahydro cycloadducts 46, and the annelation reactions with 2,3-dichloroquinoxaline is that cycloadduct formation is reversible. Due to the reversibility of the diaddition reactions and the ambident character of the 1,3-dinucleophiles used, regio- and stereoisomeric cyclization products can be formed. Indeed, regioisomeric adducts 50 or 51 can be obtained when quinoxalinium salts 37 react with ammonium dithiocarbamates. It depends on the reaction conditions which product is favored. When the reaction was carried out in aprotic solvents, such as dimethyl sulfoxide (DMSO) or chloroform, only thiazolo [4,5-6] quinoxalines 50 could be isolated in good yield. When the same reaction is carried out in an ethanol solution it results in the formation of the regioisomeric thiazoloquinoxalines 51 (Scheme 43) (84KGS680). 

A considerable number of chiral natural compounds has been investigated under photooxygenation conditions, mostly terpenoids and steroids. Of these compounds, ( + )-limonene (1) has been utilized as a popular probe for singlet oxygen in view of the fact that it gives a characteristic pattern of regio- and stereoisomeric allylic hydroperoxides which were reduced to the alcohols 2-4 as products22-24. 

Although the hydrolysis of the allyl Grignard reagents tends to yield a mixture of regio- and stereoisomeric olefins, reaction with carbon electrophiles such as carbonyl compounds and nitriles usually afforded a single regioisomer (Scheme 3.25). 

Ultraviolet irradiation of pyridines can prodnce highly strained species that can lead to isomerised pyridines or can be trapped. The three picolines and the three cyano-substituted pyridines constitute photochemical triads irradiation of any isomer, in the vaponr phase at 254 nm, results in the formation of all three isomers. From pyridines and from 2-pyridones 2-azabicyclo[2.2.0]-hexadienes and -hexenones can be obtained in the case of pyridines these are nsnally nnstable and revert thermally to the aromatic heterocycle. Pyridone-derived bicycles are relatively stable, 4-alkoxy- and -acyloxy-pyridones are converted in particnlarly good yields. Irradiation of iV-methyl-2-pyridone in aqueous solution prodnces a mixture of regio- and stereoisomeric 4n pins 4n photo-dimers. ... 

Phosphine and hydroxy groups emerge simultaneously and therefore subsequent reaction with the air-sensitive hydroxyphosphine can be avoided. The structure of the cyclic ether determines in each case the distance separating functional groups in the product this means epoxides form 1,2-hydroxyphosphines, 1,3-prod-ucts can be produced by the employment of oxetanes [5], etc. Problems may be caused by the formation of regio- and stereoisomeric byproducts during the ringopening step by application of internal nonsymmetric cyclic ethers [6],... 

Irradiation of A -methyl-2-pyridone in aqueous solution produces a mixture of regio- and stereoisomeric dimers such as the one shown above " such 47t plus 47t photo-cycloadditions of 2-pyridones" have also been conducted between two tethered pyridones as illustrated below" and between a side-chain 1,3-diene and a 2-pyridone. ... 

A detailed discussion of the earlier work on this can be found in CHEC-I. The cycloaddition of tetrachlorothiophene-1,1-dioxide with azulene takes place in the [4 + 2]mode rather than a [4 + 6]mode. A frontier orbital-based explanation has been provided for this <85TL2607). 2-Menthyl-3,4,5-trihalothiophene-1,1-dioxides have been found to be asymmetric diene equivalents <88T3I27>. Dienophiles such as styrene, allyl alcohol and acrylamide lead to regio- and stereoisomerically pure adducts, which then eliminate SO2 to form dihydrobenzene derivatives. 

Our own experience with 3-carbomethoxy-2-pyrone has shown that it can be made to react with vinyl ethers in the presence of Lewis acid catalysts, such as montmorillonite clays, silica, or zinc salts in methylene chloride at ambient pressure and room temperature or lower to give excellent yields of bicyclic lactones 38 and 39 as single regio- and stereoisomeric products." ... 

Different fragmentation pathways following CT (isobutane) made it possible to distinguish regio- and stereoisomeric cyclopentenone photoadducts (Equation 8.30). Head-to-head isomers (a) and (b) undergo facile cleavage of the cyclo-... 

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